Hard drives sas sata. SAS and SATA drives are so similar and so different. Number of sectors available to the user

Hard disk for the server, features of choice

A hard drive is the most valuable component on any computer. After all, it stores information with which the computer and the user work, in the case when it comes to a personal computer. Every time a person sits down at a computer, he expects that he will now run through the boot screen of the operating system, and he will begin to work with his data, which will be produced by the hard drive from its bowels. If we are talking about a hard drive, or even about their array in the server, then there are tens, hundreds and thousands of such users who expect to gain access to personal or working data. And all their quiet work or rest and entertainment depends on these devices, which constantly store data in themselves. Already from this comparison it is clear that requests to the hard drives of the home and industrial class are presented unequally - in the first case, one user works with him, in the second - thousands. It turns out that the second hard drive must be more reliable, faster, more stable than the first many times over, because many users work with it, many people rely on it. This article will discuss the types of hard drives used in the corporate sector and the features of their design, allowing to achieve the highest reliability and performance.

SAS and SATA drives - so similar and so different

Until recently, the standards for industrial-grade and domestic hard drives differed significantly, and were incompatible - SCSI and IDE, now the situation has changed - the vast majority of hard drives in the market are SATA and SAS (Serial Attached SCSI) hard drives. The SAS connector is versatile and form factor compatible with SATA. This allows you to directly connect to the SAS system as high-speed, but at the same time small capacity (at the time of writing this article - up to 300 GB) SAS drives, as well as lower-speed, but at times more capacious, SATA drives (at the time of writing, up to 2 TB ) Thus, in one disk subsystem, you can combine vital applications that require high performance and fast access to data, and more economical applications with lower cost per gigabyte.

Such structural compatibility is beneficial both for rear panel manufacturers and end users, as it reduces equipment and design costs.

That is, both SAS devices and SATA can be connected to SAS connectors, and only SATA devices are connected to SATA connectors.

SAS and SATA - high speed and high capacity. What to choose?

SAS disks, which replaced the SCSI disks, completely inherited their main characteristics characterizing the hard drive: spindle speed (15000 rpm) and volume standards (36.74.147 and 300 GB). However, SAS technology itself is significantly different from SCSI. Briefly consider the main differences and features: The SAS interface uses a point-to-point connection - each device is connected to the controller by a dedicated channel, in contrast to it, SCSI works on a common bus.

SAS supports a large number of devices (\u003e 16384), while the SCSI interface supports 8, 16, or 32 devices on the bus.

SAS interface supports data transfer rate between devices at speeds of 1.5; 3; 6 Gb / s, while the SCSI interface does not have a bus speed allocated to each device, but is divided between them.

SAS supports connecting slower SATA devices.

SAS configurations are much easier to install. Such a system is easier to scale. In addition, SAS hard drives inherited the reliability of SCSI hard drives.

When choosing a disk subsystem - SAS or SATA, you need to be guided by what functions will be performed by the server or workstation. To do this, you need to decide on the following questions:

1. How many simultaneous diverse requests will the drive process? If it’s big, your unique choice is SAS drives. Also, if your system will serve a large number of users - choose SAS.

2. How much information will be stored on the disk subsystem of your server or workstation? If more than 1-1.5 Tb, you should pay attention to the system based on SATA hard drives.

3. What is the budget allocated for the purchase of a server or workstation? It should be remembered that in addition to SAS disks, a SAS controller is required, which also needs to be taken into account.

4. Do you plan, as a consequence, to increase the volume of data, increase productivity or increase system fault tolerance? If so, then you need a SAS-based disk subsystem, it is easier to scale and more reliable.

5. Your server will work with critical data and applications - your choice is SAS disks designed for severe operating conditions.

Reliable disk subsystem, it is not only high-quality hard drives of a renowned manufacturer, but also an external disk controller. They will be discussed in one of the following articles. Consider SATA drives, which varieties of these drives are and which should be used when building server systems.

SATA drives: household and industrial sector

SATA disks used everywhere, from consumer electronics and home computers to high-performance workstations and servers, differ in sub-types, there are disks for use in household appliances, with low heat, energy consumption, and as a result, low performance, there are disks - middle class, for home computers, and there are drives for high-performance systems. In this article we will consider the class of hard drives for productive systems and servers.

  Performance characteristics

The table shows the characteristics of the hard drives of one of the leading manufacturers, in one column the data are SATA of a server-class hard drive, in the other a regular SATA hard drive.

From the table we see that the disks differ not only in performance characteristics, but also in operational characteristics, which directly affect the life expectancy and successful operation of the hard drive. It should be noted that externally these hard drives differ insignificantly. Let's consider what technologies and features allow this to be done:

The reinforced shaft (spindle) of the hard drive, is fixed at two ends for some manufacturers, which reduces the effect of external vibration and contributes to the accurate positioning of the head unit during read and write operations.

The use of special intelligent technologies to take into account both linear and angular vibration, which reduces the time of head positioning and increases disk performance up to 60%

The function of eliminating errors in operating time in RAID arrays - prevents the loss of hard drives from RAID, which is a characteristic feature of conventional hard drives.

The adjustment of the flight altitude of the heads in conjunction with the technology of preventing contact with the surface of the plates, which leads to a significant increase in the life of the disk.

A wide range of self-diagnosis functions, allowing you to predict in advance when the hard drive will fail, and to notify the user about this, which allows you to manage to save information to a backup drive.

Features to reduce the rate of unrecoverable read errors, which increases the reliability of the server hard drive compared to conventional hard drives.

Speaking about the practical side of the issue, we can confidently say that specialized hard drives in servers “behave” much better. The technical service receives many times fewer calls for the instability of RAID arrays and hard drive failures. The manufacturer’s support for the server segment of the hard drives is much faster than conventional hard drives, due to the fact that the priority sector of any manufacturer of storage systems is the industrial sector. After all, it is in it that the most advanced technologies that guard your information find application.

Analog SAS drives:

Hard drives from Western Digital VelociRaptor. These drives with a disk rotation speed of 10 thousand rpm, equipped with a SATA 6 Gb / s interface and 64 MB of cache memory. The MTBF of these drives is 1.4 million hours.
  More details on the manufacturer's website www.wd.com

You can order a server assembly based on SAS or an analogue of SAS hard drives in our Status company in St. Petersburg, you can also buy or order SAS hard drives in St. Petersburg:

• call + 7-812-385-55-66 in St. Petersburg
• write to the address
• leave a request on our website on the page "Online Application"

In modern computer systems, SATA and SAS interfaces are used to connect the main hard drives. As a rule, the first option suits home workstations, the second - server ones, so the technologies do not compete with each other, meeting different requirements. A significant difference in cost and memory size makes users wonder how SAS differs from SATA and look for compromises. Let's see if this is appropriate.

SAS   (Serial Attached SCSI) - a serial interface for connecting storage devices, developed on the basis of parallel SCSI to execute the same set of commands. It is used mainly in server systems.

SATA   (Serial ATA) - a serial data exchange interface based on parallel PATA (IDE). It is used in home, office, multimedia PCs and laptops.

If we talk about the HDD, then, despite the varying technical specifications and connectors, there are no cardinal discrepancies between the devices. Backward one-way compatibility makes it possible to connect disks to the server board both one and a second interface.

It is worth noting that both connection options are real for SSDs, but the significant difference between SAS and SATA in this case will be in the cost of the drive: the first one can be ten times more expensive with a comparable amount. Therefore, today such a solution, if not rare, is well-balanced, and is intended for fast data centers of the enterprise level.

Comparison

As we already know, SAS is used in servers, SATA - in home systems. In practice, this means that the first one is accessed by many users and many tasks are being solved, while the second is dealt by one person. Accordingly, the server load is much higher, so the disks should be sufficiently fault tolerant and fast. SCSI protocols (SSP, SMP, STP) implemented in SAS allow you to process more I / O operations at the same time.

Directly to the HDD, the speed of rotation is determined primarily by the spindle speed. For desktop systems and laptops, 5400 - 7200 RPM is necessary and sufficient. Accordingly, it is almost impossible to find a SATA-drive with 10,000 RPM (unless you look at the WD VelociRaptor series, intended, again, for workstations), and everything above is absolutely unattainable. SAS HDD spins at least 7200 RPM, the standard can be considered 10000 RPM, and a sufficient maximum - 15000 RPM.

Serial SCSI drives are believed to be more reliable; they have higher MTBFs. In practice, stability is achieved more through the function of checking checksums. SATA drives suffer from “silent errors” when data is being written partially or damaged, which leads to the appearance of bad sectors.

The main advantage of SAS is the fault tolerance of the system - two duplex ports that allow you to connect one device via two channels. In this case, information will be exchanged simultaneously in both directions, and reliability is provided by Multipath I / O technology (two controllers insure each other and share the load). The queue of marked commands is lined up to a depth of 256. Most SATA drives have one half-duplex port, and the queue depth using NCQ technology is no more than 32.

The SAS interface assumes the use of cables up to 10 m long. Up to 255 devices can be connected to one port through expanders. SATA is limited to 1 m (2 m for eSATA), and supports connecting only one point-to-point device.

Prospects for further development - this is where the difference between SAS and SATA is also felt quite sharply. The bandwidth of the SAS interface reaches 12 Gb / s, and manufacturers are announcing support for the data transfer rate of 24 Gb / s. The latest SATA revision stopped at 6 Gb / s and will not evolve in this regard.

SATA drives in terms of cost of 1 GB have a very attractive price tag. In systems where the speed of data access is not critical, and the amount of information stored is large, it is advisable to use them.

Table

With the advent of a sufficiently large number of peripherals Serial Attached SCSI (SAS), we can state the beginning of the transition of the corporate environment to the rails of new technology. But SAS is not only the recognized successor of UltraSCSI technology, but also implements new areas of use, raising the ability to scale systems directly to unimaginable heights. We decided to demonstrate the potential of SAS by taking a close look at technology, host adapters, hard drives, and storage systems.

SAS cannot be called a completely new technology: it takes the best of both worlds. The first part of SAS concerns serial data transmission, which requires less physical wires and contacts. The transition from parallel to serial transmission allowed us to get rid of the bus. Although the current throughput of SAS specifies a bandwidth of 300 MB / s per port, which is less than 320 MB / s for UltraSCSI, replacing the shared bus with a point-to-point connection is a significant advantage. The second part of SAS is SCSI, which remains powerful and popular.

SAS can use a large set rAID varieties   . Giants such as Adaptec or LSI Logic, in their products offer an expanded set of functions for expansion, migration, creating "slots" and other features, including those related to distributed RAID arrays across multiple controllers and drives.

Finally, most of the actions mentioned today are already on the fly. Here we should note the excellent products. AMCC / 3Ware , Areca   and Broadcom / raidcore   , which allowed transferring enterprise-class functions to SATA spaces.

Compared to SATA, the traditional SCSI implementation is losing ground on all fronts, with the exception of high-end enterprise solutions. SATA offers suitable hard drives   It has a good price and a wide range of decisions   . And let's not forget about another “smart” feature of SAS: it easily gets along with existing SATA infrastructures, since SAS host adapters work easily with SATA drives as well. But here the SAS drive cannot be connected to the SATA adapter anymore.

Source: Adaptec.

First, we think we should turn to the history of SAS. The SCSI standard (stands for "small computer system interface") has always been regarded as a professional bus for connecting drives and some other devices to computers. Hard drives for servers and workstations still use SCSI technology. Unlike the mass ATA standard, which allows you to connect only two drives to a single port, SCSI allows you to connect up to 15 devices on one bus and offers a powerful command protocol. Devices must have a unique SCSI ID, which can be assigned either manually or through SCAM (SCSI Configuration Automatically). Since device IDs for buses of two or more SCSI adapters may not be unique, Logical Unit Numbers (LUNs) were added to identify devices in complex SCSI environments.

SCSI hardware is more flexible and reliable than ATA (this standard is also called IDE, Integrated Drive Electronics). Devices can be connected both inside the computer and externally, and the cable can be up to 12 m long if it is correctly terminated (in order to avoid signal reflections). As SCSI evolved, numerous standards appeared that stipulated different bus widths, clock speeds, connectors and signal voltage (Fast, Wide, Ultra, Ultra Wide, Ultra2, Ultra2 Wide, Ultra3, Ultra320 SCSI). Fortunately, they all use a single set of commands.

Any SCSI communication is established between the initiator (host adapter) sending the commands and the target drive responding to them. Immediately after receiving a set of commands, the target drive sends the so-called sense-code (state: busy, error or free), by which the initiator finds out whether he will receive the right answer or not.

SCSI protocol specifies almost 60 different commands. They are divided into four categories: non-data (non-data), bi-directional (bi-directional), read data (read data) and write data (write data).

SCSI restrictions begin to show up when you add drives to the bus. Today it is hardly possible to find a hard drive capable of fully loading the throughput of 320 MB / s with the Ultra320 SCSI. But five or more drives on one bus is a completely different matter. An option would be to add a second host adapter to balance the load, but it costs money. The problem is with cables: twisted 80-wire cables are very expensive. If you also want to get a “hot swap” drive, that is, easy replacement of a failed drive, then you need special equipment (backplane).

Of course, it is best to place the drives in separate snap-ins or modules that usually support hot-swap capabilities along with other nice control features. As a result, there are more professional SCSI solutions on the market. But they all cost a lot, which is why the SATA standard has been developing so rapidly in recent years. Although SATA will never satisfy the needs of high-end enterprise systems, this standard complements SAS perfectly when creating new scalable solutions for next-generation network environments.

SAS does not use a shared bus for multiple devices. Source: Adaptec.

SATA

On the left is the SATA connector for data transfer. On the right is the power connector. The contacts are sufficient to supply 3.3 V, 5 V and 12 V to each SATA drive.

The SATA standard has existed on the market for several years, and today it has reached the second generation. SATA I was characterized by a bandwidth of 1.5 Gbit / s with two serial connections using low-voltage differential signaling. At the physical level, 8/10 bits (10 bits of actual for 8 bits of data) are used, which explains the maximum interface bandwidth of 150 MB / s. After the transition of SATA to a speed of 300 MB / s, many began to call the new standard SATA II, although with standardization SATA-IO   (International Organization) was planned to first add more features, and then call SATA II. Hence the latest specification and is called SATA 2.5, it includes such SATA extensions as Native command queuing   (NCQ) and eSATA (external SATA), port multipliers (up to four drives per port), etc. But additional SATA functions are optional for both the controller and the hard drive itself.

Let's hope that in 2007 SATA III at 600 MB / s will come out.

If parallel ATA (UltraATA) cables were limited to 46 cm, then SATA cables can be up to 1 m long, and for eSATA it can be twice as long. Instead of 40 or 80 wires, serial transmission requires only a few contacts. Therefore, SATA cables are very narrow, they are easy to lay inside the computer case, and they do not interfere so much with the air flow. One device relies on the SATA port, which makes it possible to classify this interface as a point-to-point type.

SATA connectors for data and power provide separate plugs.

SAS

The signal protocol here is the same as that of SATA. Source: Adaptec.

A nice feature of Serial Attached SCSI is that the technology supports both SCSI and SATA, as a result of which SAS or SATA disks (or both standards at once) can be connected to SAS controllers. However, SAS drives cannot work with SATA controllers due to the use of the Serial SCSI Protocol (SSP). Like SATA, SAS follows the point-to-point connection principle for drives (today 300 MB / s), and thanks to SAS expanders (or expanders), you can connect more drives than SAS ports are available. SAS hard drives support two ports, each with its own unique SAS ID, so you can use two physical connections to provide redundancy - connect the drive to two different host nodes. Thanks to the STP (SATA Tunneling Protocol), SAS controllers can exchange data with SATA-drives connected to the expander.

Source: Adaptec.

Source: Adaptec.

Source: Adaptec.

Of course, the only physical connection of the SAS expander to the host controller can be considered a bottleneck, so the standard provides for wide SAS ports. The wide port groups several SAS connections into a single connection between any two SAS devices (usually between the host controller and expander / expander). The number of connections within the communication can be increased, it all depends on the requirements. But redundant connections are not supported, and no loops or rings should be allowed.

Source: Adaptec.

Future SAS implementations will add bandwidth of 600 and 1200 MB / s per port. Of course, the performance of hard drives in the same proportion will not grow, but it will be more convenient to use expanders on a small number of ports.

Devices called Fan Out and Edge are expanders. But only the main Fan Out expander can work with the SAS domain (see 4x link in the center of the diagram). Up to 128 physical connections are allowed per Edge Expander, and you can use wide ports and / or connect other expanders / drives. The topology can be very complex, but at the same time flexible and powerful. Source: Adaptec.

Source: Adaptec.

Equipment (backplane) - the main building block of any storage system that must support a "hot plug". Therefore, SAS expanders often mean powerful snap-ins (both in a single enclosure and not). Usually, a single connection is used to connect a simple snap-in to the host adapter. Expanders with built-in snap-ins, of course, rely on multi-channel connections.

Three types of cables and connectors have been developed for SAS. SFF-8484 is a multi-core internal cable connecting the host adapter with the snap-in. In principle, the same can be achieved by branching at one end of this cable into several separate SAS connectors (see illustration below). SFF-8482 - connector through which the drive is connected to a single SAS interface. Finally, the SFF-8470 is an external multi-core cable up to six meters long.

Source: Adaptec.

SFF-8470 cable for external multi-channel SAS connections.

Stranded cable SFF-8484. Four SAS channels / ports go through one connector.

SFF-8484 cable for connecting four SATA drives.

SAS as part of SAN solutions

Why do we need all this information? Most users will not come close to the SAS topology that we talked about above. But SAS is more than the next generation interface for professional hard drives, although it is ideal for building simple and complex RAID arrays based on one or more RAID controllers. SAS is capable of more. This is a sequential point-to-point interface that scales easily as you add the number of connections between any two SAS devices. SAS drives come with two ports, so you can connect one port through the expander to the host system, and then create a backup path to another host system (or another expander).

The connection between SAS adapters and expanders (as well as between two expanders) can be as wide as the number of SAS ports available. Expanders are usually rack-mount systems that can accommodate a large number of drives, and the possible connection of the SAS to a superior device in a hierarchy (for example, to a host controller) is limited only by the expander's capabilities.

Thanks to its rich and functional infrastructure, SAS allows you to create complex storage topologies, rather than dedicated hard drives or separate network attached storage. In this case, “complex” should not be understood that it is difficult to work with such a topology. SAS configurations consist of simple disk snap-ins or use expanders. Any SAS link can be expanded or narrowed, depending on bandwidth requirements. You can use both powerful SAS hard drives and capacious SATA models. Together with powerful RAID controllers, you can easily configure, expand or reconfigure data arrays - both in terms of RAID level and hardware.

All this becomes all the more important if we take into account how quickly corporate storages grow. Today, everyone has heard of the SAN - a storage area network. It implies a decentralized organization of the storage subsystem with traditional servers, using physically remote storage. Over existing Gigabit Ethernet or Fiber Channel networks, a slightly modified SCSI protocol is launched, which is encapsulated in Ethernet packets (iSCSI - Internet SCSI). A system that runs from one hard drive to complex nested RAID arrays becomes the so-called target (target) and binds to the initiator (host system, initiator), which treats the target as if it were just a physical element.

iSCSI, of course, allows you to create a strategy for storage development, data organization or access control. We get another level of flexibility by removing direct attached storage servers, allowing any storage subsystem to become an iSCSI target. The transition to remote storage makes the system independent of storage servers (a dangerous point of failure) and improves the manageability of hardware. From a software point of view, the storage is still "inside" the server. The iSCSI target and initiator can be nearby, on different floors, in different rooms or buildings - it all depends on the quality and speed of the IP connection between them. From this point of view, it is important to note that the SAN is poorly suited to the requirements of online applications such as databases.

2.5 "SAS Hard Drives

2.5 "hard drives for the professional sphere are still perceived as a novelty. We have long considered the first such drive from Seagate - 2.5 "Ultra320 Savvio   which left a good impression. All 2.5 "SCSI drives use a spindle speed of 10,000 rpm, but they do not reach the level of performance that 3.5" drives have with the same spindle speed. The fact is that the external tracks of 3.5 "models rotate with a higher linear speed, which provides a higher data transfer speed.

The advantage of small hard drives is not in capacity: today they still have a maximum of 73 GB for them, while for 3.5 "enterprise-class hard drives we already get 300 GB. In many areas, the ratio of performance to the occupied physical volume is very important or energy efficiency. The more hard drives you use, the more productivity you will reap - paired with the appropriate infrastructure, of course. At the same time, 2.5 "winchesters consume almost half as much energy as 3.5" onkurenty. If we consider the balance of performance in watts (watt number of I / O operations), the 2.5 "form factor gives very good results.

If you primarily need capacity, then 3.5 "drives at 10,000 rpm are unlikely to be the best choice. The fact is that 3.5" SATA drives provide 66% more capacity (500 instead of 300 GB for hard drive), leaving the performance level acceptable. Many manufacturers of hard drives offer SATA-models for 24/7 operation, and the price of drives is reduced to a minimum. Reliability problems can be solved by purchasing spare drives for immediate replacement in the array.

The current generation of 2.5 "Fujitsu drives for the professional sector is represented in the MAY line. The rotation speed is 10,025 rpm and the capacities are 36.7 and 73.5 GB. All drives come with 8 MB of cache and give an average read search time 4.0 ms and 4.5 ms recordings. As we have already mentioned, a nice feature of 2.5 "hard drives is reduced power consumption. Usually one 2.5 "hard drive saves at least 60% energy compared to a 3.5" drive.

3.5 "SAS Hard Drives

Under MAX lies the current line of high-performance Fujitsu hard drives with a rotation speed of 15,000 rpm. So the name is consistent. Unlike 2.5 "drives, here we get as much as 16 MB of cache and a short average search time of 3.3 ms for reading and 3.8 ms for writing. Fujitsu offers 36.7 GB, 73.4 GB and 146 models GB (with one, two and four plates).

Hydrodynamic bearings reached the enterprise-class hard drives, so the new models are significantly quieter than the previous ones by 15,000 rpm. Of course, such hard drives should be properly cooled, and this also provides equipment.

Hitachi Global Storage Technologies also offers its own line of high-performance solutions. The UltraStar 15K147 operates at a speed of 15,000 rpm and is equipped with a 16 MB cache, like Fujitsu drives, but the plate configuration is different. The 36.7 GB model uses two plates, not one, but 73.4 GB uses three plates, not two. This indicates a lower data recording density, but a similar design, in fact, allows not to use the internal, slowest areas of the plates. As a result, the heads have to move less, which gives the best average access time.

Hitachi also offers 36.7 GB, 73.4 GB, and 147 GB models with a limiting search time (read) of 3.7 ms.

Although Maxtor has already become part of Seagate, the company's product lines are still preserved. The manufacturer offers 36, 73 and 147 GB models, all of which differ in spindle speed of 15,000 rpm and 16 MB cache. The company claims an average search time of 3.4 ms for reading and 3.8 ms for writing.

Cheetah has long been associated with high-performance hard drives. A similar association with the release of Barracuda Seagate was able to instill in the desktop segment, offering the first desktop drive at 7200 rpm in 2000.

36.7 GB, 73.4 GB and 146.8 GB models are available. They all differ in spindle speeds of 15,000 rpm and a cache of 8 MB. The average search time for reading is 3.5 ms and for writing is 4.0 ms.

Host adapters

Unlike SATA controllers, SAS components can only be found on server-class motherboards or as expansion cards for PCI-X or PCI Express   . If you take a step forward and consider RAID-controllers (Redundant Array of Inexpensive Drives), then because of their complexity they are sold, for the most part, in the form of separate cards. RAID cards contain not only the controller itself, but also a redundancy information calculation acceleration chip (XOR engine), as well as a cache memory. Sometimes a small amount of memory is soldered to the card (most often 128 MB), but some cards allow you to expand the volume using DIMM or SO-DIMM.

When choosing a host adapter or RAID controller, you should clearly determine what you need. The range of new devices is growing just before our eyes. Simple multi-port host adapters will cost relatively cheaply, and you will have to seriously spend on powerful RAID cards. Think about where you will place the drives: external storage requires at least one external connector. Rack servers typically require low profile cards.

If you need RAID, decide whether you will use hardware acceleration. Some RAID cards consume CPU resources for XOR calculations for RAID 5 or 6; others use their own XOR hardware engine. RAID acceleration is recommended for those environments where the server is engaged not only in data storage, for example, for databases or web-servers.

All host adapter cards that we presented in our article support 300 MB / s per SAS port and allow for a very flexible storage infrastructure implementation. You will hardly be surprised by external ports today, and consider the support for both SAS and SATA hard drives. All three cards use the PCI-X interface, but versions for PCI Express are already under development.

In our article, we honored the card with eight ports, but the number of connected hard drives is not limited to this. Using the SAS expander (external) you can connect any storage. As long as a four-channel connection is enough, you can increase the number of hard drives up to 122. Due to the performance cost of calculating the parity information of RAID 5 or RAID 6, typical external RAID storage will not be able to sufficiently load the bandwidth of the four-channel connection, even if you use a large number of drives.

The 48300 is a SAS host adapter for the PCI-X bus. PCI-X continues to dominate the server market today, although more motherboards are equipped with PCI Express interfaces.

Adaptec SAS 48300 uses a PCI-X interface at a speed of 133 MHz, which gives a bandwidth of 1.06 GB / s. Fast enough if the PCI-X bus is not loaded with other devices. If you include a slower device in the bus, then all other PCI-X cards will reduce their speed to the same. For this purpose, several PCI-X controllers are sometimes installed on the board.

Adaptec positions the SAS 4800 for mid- and low-cost servers, as well as for workstations. The recommended retail price is $ 360, which is quite reasonable. Adaptec HostRAID is supported, allowing you to upgrade to the simplest RAID arrays. In this case, it is RAID levels 0, 1 and 10. The card supports an external four-channel connection SFF8470, as well as an internal connector SFF8484 paired with a cable for four SAS devices, that is, we get eight ports.

The card fits in a 2U rack server if you install a low-profile slot cover. The package also includes a driver CD, a quick installation guide and an internal SAS cable through which you can connect up to four system drives to the card.

A player in the SAS market LSI Logic sent us a host adapter SAS3442X PCI-X, a direct competitor to Adaptec SAS 48300. It comes with eight SAS ports that are shared between two four-channel interfaces. The "heart" of the card is the LSI SAS1068 chip. One of the interfaces is intended for internal devices, the second - for external DAS (Direct Attached Storage). The board uses the PCI-X 133 bus interface.

As usual, the 300 MB / s interface is supported for SATA and SAS drives. There are 16 LEDs on the controller board. Eight of them are simple activity LEDs, and eight more are designed to report a system malfunction.

The LSI SAS3442X is a low-profile card, so it fits easily in any 2U rack server.

Note driver support for Linux, Netware 5.1 and 6, Windows 2000 and Server 2003 (x64), Windows XP (x64) and Solaris up to 2.10. Unlike Adaptec, LSI decided not to add support for any RAID modes.

RAID adapters

SAS RAID4800SAS - Adaptec solution for more complex SAS environments, it can be used for application servers, streaming servers, etc. Before us, again, is a card with eight ports, with one external four-channel SAS connection and two internal four-channel interfaces. But if an external connection is used, then only one four-channel interface remains from the internal ones.

The card is also designed for the PCI-X 133 bus, which provides sufficient bandwidth for even the most demanding RAID configurations.

As for the RAID modes, here the SAS RAID 4800 easily overtakes the "little brother": by default, RAID levels 0, 1, 10, 5, 50 are supported if you have a sufficient number of drives. Unlike the 48300, Adaptec included two SAS cables, so you can immediately connect eight hard drives to the controller. Unlike the 48300, the card requires a full-sized PCI-X slot.

If you decide to upgrade the card to Adaptec Advanced Data Protection Suite   , you will get the opportunity to switch to RAID modes with double redundancy (6, 60), as well as a number of enterprise-class functions: striped mirror drive (RAID 1E), hot spacing (RAID 5EE) and copyback hot spare. The Adaptec Storage Manager utility has a browser-like interface, with its help you can manage all Adaptec adapters.

Adaptec offers drivers for Windows Server 2003 (and x64), Windows 2000 Server, Windows XP (x64), Novell Netware, Red Hat Enterprise Linux 3 and 4, SuSe Linux Enterprise Server 8 and 9, and FreeBSD.

SAS snap-ins

The 335SAS is a snap-in for four SAS or SATA drives, but it should be connected to the SAS controller. Thanks to the 120 mm fan, the drives will cool well. You will have to connect two Molex power plugs to the snap-in.

Adaptec included the I2C cable, which can be used to control equipment through the appropriate controller. But with SAS drives this is no longer possible. An additional LED cable is designed to signal drive activity, but, again, only for SATA drives. The package also includes an internal SAS cable for four drives, so an external four-channel cable will be enough to connect the drives. If you want to use SATA drives, you will have to use adapters from SAS to SATA.

The retail price of $ 369 cannot be called low. But you will get a solid and reliable solution.

SAS Repositories

SANbloc S50 - enterprise-class solution for 12 drives. You will receive a 2U rack-mount chassis that connects to SAS controllers. This is one of the best examples of scalable SAS solutions. 12 drives can be either SAS or SATA. Or represent a mixture of both types. The built-in expander can use one or two four-channel SAS interfaces to connect the S50 to a host adapter or RAID controller. Since this is clearly a professional solution, it is equipped with two power supplies (with redundancy).

If you have already purchased the Adaptec SAS host adapter, you can easily connect it to the S50 and manage the drives using the Adaptec Storage Manager. If you install SATA hard drives of 500 GB, then we get 6 TB of storage. If you take 300 GB SAS drives, then the capacity will be 3.6 TB. Since the expander is connected to the host controller with two four-channel interfaces, we get a throughput of 2.4 GB / s, which will be more than enough for an array of any type. If you install 12 drives in a RAID0 array, then the maximum throughput is only 1.1 GB / s. In the middle of this year, Adaptec promises to release a slightly modified version with two independent SAS I / O blocks.

SANbloc S50 contains the function of automatic monitoring and automatic control of fan speed. Yes, the device is too loud, so we were relieved to give it out of the lab after the tests were completed. A drive failure message is sent to the controller via SES-2 (SCSI Enclosure Services) or through the physical I2C interface.

Operating temperatures for drives are 5-55 ° C, and for accessories - from 0 to 40 ° C.

At the beginning of our tests, we got a peak throughput of only 610 MB / s. By changing the cable between the S50 and the Adaptec host controller, we were still able to reach 760 MB / s. To load the system in RAID 0 mode, we used seven hard drives. The increase in the number of hard drives did not lead to an increase in throughput.

Test configuration

We stop at the choice: what kind of hard drives to install. The most popular types of hard drives are: SAS, SATA, and NL-SAS. These three types belong to the fastest storage media, they store most of the data in the world. Our article is about the first type. We will consider what SAS disks are, what are their parameters, and what is the difference between the main types of devices mentioned.

Specifications

SAS drives replaced the SCSI type. They have become the new standard in enterprise-class information storage. Of the three types listed, SAS disks are considered the most reliable, they are able to maintain performance in very difficult operating conditions. SAS hard drives work much better than NL or SATA drives. An indicator of their reliability is such a parameter as the error rate. It determines the probability with which one bit of error can occur in media data. The error rate for SAS-type drives is usually one out of 10 16 bits. That is, this means that the probability of error can occur in one of ten quadrillion bits. For example, you can compare this indicator with the error value in SATA-type hard drives, where it is one out of 10 15 (or one quadrillion). As you can see, the protection of SATA hard drives is also quite high, however, when the question arises about the protection of stored information, the difference by one order of magnitude is very significant.

SAS disks are manufactured adhering to more stringent standards than when developing other types of hard drives. So, this technology is characterized by an average time between failures of 1.6 million hours, and SATA technology - 1.2 million. In addition to the parameters listed, controllers and drives of the type in question have many additional commands for diagnostics. These features make this technology more efficient than SATA. This is especially evident in force majeure situations.

NL Winchesters

This technology is the "new player" in the market. NL-drives are a hybrid: SATA-hard drive with a connector type SAS. That is, the speed, filling and head are taken from SATA technology, and the interface is fully compatible with SAS. NL technology is inferior to the disks in performance (due to the relatively low speed). However, it is fully consistent with them in the order of commands, as well as multi-threaded data transfer and support for multiple hosts.

Tagged order of sending commands and multi-threaded transmission

The simultaneous coordination of several sets of storage instructions, as well as the information ordered by the storage controller, is transmitted most efficiently. SAS-technology provides several full-duplex channels that provide quick access to stored information. One SAS-type hard drive can be controlled simultaneously from several personal computers without the use of switches.

Conclusion

In essence, SAS and SATA technologies are designed for various purposes: the first for fault tolerance and performance, and the second for capacity. Therefore, they should not compete with each other.

Why SAS?

The Serial Attached SCSI interface is not just a sequential implementation of the SCSI protocol. It implements much more than the simple transfer of SCSI functions, such as TCQ (Tagged Command Queuing) through a new connector. If we needed the greatest simplicity, then we would use the Serial ATA (SATA) interface, which is a simple point-to-point connection between a host and an end device, such as a hard disk.

But SAS is based on an object model defining a “SAS domain” - a data delivery system that can include optional expanders and SAS end devices, such as hard drives and host adapters (host bus adapters, HBAs). from SATA, SAS devices can have several ports, each of which can use several physical connections to provide faster (wider) SAS connections.In addition, several initiators can reach any specific target, and the cable length can be up to eight meters (for the first generation SAS) versus one meter for SATA. It is clear that this provides many opportunities for creating high-performance or redundant storage solutions. In addition, SAS supports the SATA Tunneling Protocol (STP), which allows you to connect SATA devices to the SAS controller .

The second generation SAS standard increases the connection speed from 3 to 6 Gb / s. This speed boost is very important for complex environments where high performance is required due to high-speed storage. The new version of SAS is also designed to reduce the complexity of cabling, as well as the number of connections per Gbit / s bandwidth, increasing the possible cable length and improving the work of expanders (zoning and automatic detection). Below we will talk about these changes in detail.

Up to 6 Gb / s SAS

To convey the benefits of SAS to a wider audience, the SCSI Trade Association (SCSI TA) introduced a SAS technology tutorial at the Storage Networking World Conference, which was held earlier this year in Orlando (USA, Florida). The so-called SAS Plugfest, which demonstrated the operation of SAS at 6 Gb / s, compatibility and functions, went even earlier in November 2008. LSI and Seagate were the first on the market to introduce hardware compatible with 6 Gb / s SAS, but other manufacturers should also catch up soon. In our article, we will review the current state of SAS technologies and some new devices.

SAS Features and Basics

Fundamentals of SAS

Unlike SATA, the SAS interface works on the basis of full duplex, providing full bandwidth in both directions. As mentioned earlier, SAS connections are always established through physical connections using unique device addresses. In contrast, SATA can only address port numbers.

Each SAS address can contain several physical layer interfaces (PHYs), which allows you to create wider connections through InfiniBand (SFF-8470) or mini-SAS cables (SFF-8087 and -8088). Typically, four SAS interfaces with one PHY on each are combined into one wide SAS interface that is already connected to the SAS device. Communication can also be through expanders, which operate more as switches than SAS devices.

Features such as zoning now allow administrators to bind specific SAS devices to initiators. This is where the increased 6 Gb / s SAS bandwidth will be useful, since a four-channel connection will now have twice the speed. Finally, SAS devices can even have multiple SAS addresses. Because SAS drives can use two ports, with one PHY on each, a drive can have two SAS addresses.

Connections and Interfaces

Click on the image to enlarge.

Addressing SAS connections occurs through SAS ports using SSP (Serial SCSI Protocol), but communication at the lower level from PHY to PHY is performed using one or more physical connections for reasons of increased throughput. SAS uses 8/10 bit encoding to convert 8 data bits to 10-character transmissions in order to restore synchronization, DC balance and error detection. As a result, we get an effective bandwidth of 300 MB / s for the transmission mode of 3 Gbit / s and 600 MB / s for connections of 6 Gbit / s. Fiber Channel, Gigabit Ethernet, FireWire, and others work in a similar coding scheme.

The SAS and SATA power and data interfaces are very similar to each other. But if SAS has data and power interfaces combined into one physical interface (SFF-8482 on the device side), then SATA requires two separate cables. The gap between the power and data contacts (see illustration above) is closed in the case of SAS, which does not allow connecting the SAS device to the SATA controller.

On the other hand, SATA devices can work perfectly on the SAS infrastructure thanks to STP or in the “native” mode if expanders are not used. STP adds an extra delay when passing through expanders, as they need to establish a connection, which is slower than direct SATA communication. However, the delays are still very small.

Domains, expanders

SAS domains can be represented as tree structures like complex Ethernet networks. SAS expanders can work with a large number of SAS devices, but they use the principle of circuit switching rather than the more common packet switching. Some expanders contain SAS devices, others do not.

SAS 1.1 recognizes edge expander, which allows the SAS initiator to communicate with up to 128 additional SAS addresses. Only two boundary expanders can be used in a SAS 1.1 domain. However, one fanout expander can connect up to 128 boundary expanders, which significantly increases the infrastructure capabilities of your SAS solution.

Click on the image to enlarge.

Compared to SATA, the SAS interface may seem complicated: different initiators access the target devices through expanders, which means laying the appropriate routes. SAS 2.0 simplifies and improves routing.

Keep in mind that SAS forbids loops or multiple paths. All connections must be point-to-point and exclusive, but the connection architecture itself scales well.

New SAS 2.0 Features: Expanders, Performance

Expander Zones and Auto Configuration

Boundary (edge) and expanding (fanout) expanders have almost remained in history. This is often associated with updates in SAS 2.0, but the reason is actually in the SAS zones that appeared in 2.0, which allow us to remove the separation between boundary and expanding expanders. Of course, zones are usually implemented specifically for each manufacturer, and not as a single industry standard.

In fact, several zones can now be located on the same information delivery infrastructure. This means that different initiators can access the targets (drives) in the storage through the same SAS expander. Domain segmentation is performed through zones, access is exclusive.