GIT – SAS Viết tắt của Serial Attached SCSI, một tiến trình phát triển song song SCSI vào một điểm-đến-điểm giao tiếp nối ngoại vi, trong đó các bộ điều khiển được liên kết trực tiếp vào ổ đĩa.
SAS là một cải tiến hiệu suất hơn bởi vì truyền thống SCSI SAS cho phép nhiều thiết bị (up to 128) của các kích cỡ khác nhau và các loại được kết nối đồng thời với cáp mỏng hơn và lâu hơn, đầy đủ truyền tín hiệu hỗ trợ song 3.0Gb / s. Ngoài ra, ổ đĩa SAS có thể cắm nóng.
Serial Attached SCSI (SAS) hỗ trợ cả SCSI và ATA Ba giao thức vận chuyển
– Serial SCSI Protocol (SSP)
• Hỗ trợ SAS (SCSI) ổ đĩa, ổ băng…
– Serial ATA Tunneling Protocol (STP)
• Hỗ trợ các ổ đĩa Serial ATA
– Serial Management Protocol (SMP)
• Hỗ trợ SAS expanders
* SFF 8482 – SATA compatible, 29 Drive Connector
* SFF 8484 –32 Multi-Lane 4i, up to four devices
* SFF 8470 – External connector (InfiniBand connector), up to four devices
* SFF 8087 –Internal 36 position Mini SAS , up to four devices
* SFF 8088 –External 26 Mini SAS with Universal Key to fit 2, 4 & 6, up to four devices
Từ lâu Seagate đã công bố ổ cứng Barracuda ES.2 với chuẩn kết nối SAS (Serial Attached SCSI) 1Tb.
Các ứng dụng truyền thông đa phương tiện (như phim, nhạc..), email được tăng tốc 135% qua chuẩn SATA cùng các tương thích riêng và đặc điểm mạnh mẽ cho các doanh nghiệp.
Với Barracuda ES.2 1Tb của Seagate, lượng dữ liệu số của các doanh nghiệp ngày càng đồ sộ đã có thể dễ dàng truy xuất nhanh chóng với chuẩn SAS. Seagate là công ty đầu tiên giới thiệu thêm chuẩn kết nối SAS mới cho người tiêu dùng bên cạnh chuẩn SATA. Đối với các nhà cung cấp như HP, SAS mang đến cho họ các cấp độ bảo toàn dữ liệu đa dạng và hiệu năng cao.
Công nghệ PowerTrim Seagate được tích hợp trong dòng Barracuda ES.2, quản lý linh hoạt lượng điện năng tiêu thụ tùy vào mức độ sử dụng, tiết kiệm 20% lượng điện tiêu thụ chung của ổ đĩa và giảm 55% điện năng hơn so với các ổ đĩa cùng loại của hãng khác.
Những tính năng vượt trội của dòng Barracuda ES.2 thể hiện ở hiệu năng truy xuất dữ liệu, độ tin cậy, khả năng lưu trữ và tiết kiệm năng lượng, với tỷ lệ lỗi không thể phục hồi ít hơn 10 lần so với các ổ cứng máy để bàn, khả năng bị lỗi rất thấp ngay cả đối với các hệ thống hoạt động theo cơ chế 24/7 (cứ mỗi 1,2 triệu giờ hoạt động liên tục mới gặp 1 lỗi). Tốc độ dòng Barracuda ES.2 khoảng 7.200-rpm, thời gian tìm kiếm khoảng 8,5ms, có dung lượng 500Gb, 750Gb và 1Tb. Các sản phẩm được bảo hành 5 năm.
*SAS HDD ADAPTERS 1 port SAS and SATA Backplane . (Extra-Slim Type)
*FCI SAS and SATA Connectors Enable Port Selection in Hard Drive Carriers
*Với 4 port ra , nhưng có thể gắn được 16 HDD SAS/SATA
*32Pin SAS Controller to 4 x 7 Pin SATA Hard Drive HDD Splitter Cable
Serial attached SCSI
|Performance||Full-duplex with link aggregation (4-ports wide at 24 Gbit/s)|
|3.0 Gbit/s at introduction, 6.0 Gbit/s available February 2009, 12.0 Gbit/s in development|
|Connectivity||6 m passive copper cable
20 m active copper cable
100 m optical cable
|255 device port expanders (>65k total devices)|
|Driver||Software-transparent with SCSI|
The T10 technical committee of the International Committee for Information Technology Standards (INCITS) develops and maintains the SAS protocol; the SCSI Trade Association (SCSITA) promotes the technology.
A typical Serial Attached SCSI system consists of the following basic components:
- An Initiator: a device that originates device-service and task-management requests for processing by a target device and receives responses for the same requests from other target devices. Initiators may be provided as an on-board component on the motherboard (as is the case with many server-oriented motherboards) or as an add-on host bus adapter.
- A Target: a device containing logical units and target ports that receives device service and task management requests for processing and sends responses for the same requests to initiator devices. A target device could be a hard disk or a disk array system.
- A Service Delivery Subsystem: the part of an I/O system that transmits information between an initiator and a target. Typically cables connecting an initiator and target with or without expanders and backplanes constitute a service delivery subsystem.
- Expanders: devices that form part of a service delivery subsystem and facilitate communication between SAS devices. Expanders facilitate the connection of multiple SAS End devices to a single initiator port.
Identification and addressing
A SAS Domain is the SAS version of a SCSI domain—it consists of a set of SAS devices that communicate with one another by means of a service delivery subsystem. Each SAS port in a SAS domain has a SCSI port identifier that identifies the port uniquely within the SAS domain. It is assigned by the device manufacturer, like an Ethernet device’s MAC address, and is typically world-wide unique as well. SAS devices use these port identifiers to address communications to each other.
In addition, every SAS device has a SCSI device name, which identifies the SAS device uniquely in the world. One doesn’t often see these device names because the port identifiers tend to identify the device sufficiently.
For comparison, in parallel SCSI, the SCSI ID is the port identifier and device name. In Fibre Channel, the port identifier is a WWPN and the device name is a WWNN.
In SAS, both SCSI port identifiers and SCSI device names take the form of a SAS address, which is a 64 bit value, normally in the NAA IEEE Registered format. People sometimes call a SAS address a World Wide Name or WWN, because it is essentially the same thing as a WWN in Fibre Channel.
Comparison with parallel SCSI
- Comparison with SATAThe SAS bus operates point-to-point while the SCSI bus is multidrop. Each SAS device is connected by a dedicated link to the initiator, unless an expander is used. If one initiator is connected to one target, there is no opportunity for contention; with parallel SCSI, even this situation could cause contention.
- SAS has no termination issues and does not require terminator packs like parallel SCSI.
- SAS eliminates clock skew.
- SAS allows up to 65,535 devices through the use of expanders, while Parallel SCSI has a limit of 8 or 16 devices on a single channel.
- SAS allows a higher transfer speed (3 or 6 Gbit/s) than most parallel SCSI standards. SAS achieves these speeds on each initiator-target connection, hence getting higher throughput, whereas parallel SCSI shares the speed across the entire multidrop bus.
- SAS controllers may connect to SATA devices, either directly connected using native SATA protocol or through SAS expanders using SATA Tunneled Protocol (STP).
- Both SAS and parallel SCSI use the SCSI command-set.
There is little physical difference between SAS and SATA.
- Systems identify SATA devices by their port number connected to the host bus adapter, while SAS devices are uniquely identified by their World Wide Name (WWN).
- SAS protocol provides for multiple initiators in a SAS domain, while SATA has no analogous provision.
- Most SAS drives provide tagged command queuing, while most newer SATA drives provide native command queuing, each of which has its pros and cons.
- SATA uses the ATA command set; SAS uses the SCSI command set. Basic ATA has commands only for direct-access storage. However SCSI commands may be tunneled through ATA for devices such as CD/DVD drives.
- SAS hardware allows multipath I/O to devices while SATA (prior to SATA 3Gb/s) does not. Per specification, SATA 3Gb/s makes use of port multipliers to achieve port expansion. Some port multiplier manufacturers have implemented multipath I/O using port multiplier hardware.
- SATA is marketed as a general-purpose successor to parallel ATA and has become common in the consumer market, whereas the more-expensive SAS targets critical server applications.
- SAS error-recovery and error-reporting use SCSI commands which have more functionality than the ATA SMART commands used by SATA drives.
- SAS uses higher signaling voltages (800–1600 mV TX, 275–1600 mV RX) than SATA (400–600 mV TX, 325–600 mV RX). The higher voltage offers (among other features) the ability to use SAS in server backplanes.
- Because of its higher signaling voltages, SAS can use cables up to 10 m (33 ft) long, SATA has a cable-length limit of 1 m (3 ft) or 2 m (6.6 ft) for eSATA.
The Serial Attached SCSI standard defines several layers (in order from highest to lowest):
Serial Attached SCSI comprises three transport protocols:
- Serial SCSI Protocol (SSP) — for command-level communication with SCSI devices.
- Serial ATA Tunneling Protocol (STP) — for command-level communication with SATA devices.
- Serial Management Protocol (SMP) — for managing the SAS fabric.
For the Link and PHY layers, SAS defines its own unique protocol.
At the physical layer, the SAS standard defines connectors and voltage levels. The physical characteristics of the SAS wiring and signaling are compatible with and have loosely tracked that of SATA up to the present 6 Gbit/s rate, although SAS defines more rigorous physical signaling specifications as well as a wider allowable differential voltage swing intended to allow longer cabling. While SAS-1.0/SAS-1.1 adopted the physical signaling characteristics of SATA at the 1.5 Gbit/s and 3 Gbit/s rates, SAS-2.0 development of a 6 Gbit/s physical rate led the development of an equivalent SATA speed. According to the SCSI Trade Association, 12 Gbit/s is slated to follow 6 Gbit/s in a 2012/13 SAS-3.0 specification.
Architecture of SAS layers
SAS architecture consists of six layers
- Physical layer:
- defines electrical and physical characteristics
- differential signaling transmission
- Three connector types:
- SFF 8482 – SATA compatible
- SFF 8484 – up to four devices
- SFF 8470 – external connector (InfiniBand connector), up to four devices
- PHY Layer:
- 8b/10b data encoding
- Link initialization, speed negotiation and reset sequences
- Link capabilities negotiation (SAS-2)
- Link layer:
- Insertion and deletion of primitives for clock-speed disparity matching
- Primitive encoding
- Data scrambling for reduced EMI
- Establish and tear down native connections between SAS targets and initiators
- Establish and tear down tunneled connections between SAS initiators and SATA targets connected to SAS expanders
- Power management (proposed for SAS-2.1)
- Port layer:
- Combining multiple PHYs with the same addresses into wide ports
- Transport layer:
- Contains three transport protocols:
- Serial SCSI Protocol (SSP): for command-level communication with SCSI devices
- Serial ATA Tunneled Protocol (STP): for command-level communication with SATA devices
- Serial Management Protocol (SMP): for managing the SAS fabric
- Contains three transport protocols:
- Application layer
An initiator may connect directly to a target via one or more PHYs (such a connection is called a port whether it uses one or more PHYs, although the term wide port is sometimes used for a multi-PHY connection).
The components known as Serial Attached SCSI Expanders (SAS Expanders) facilitate communication between large numbers of SAS devices. Expanders contain two or more external expander-ports. Each expander device contains at least one SAS Management Protocol target port for management and may contain SAS devices itself. For example, an expander may include a Serial SCSI Protocol target port for access to a peripheral device. An expander is not necessary to interface a SAS initiator and target but allows a single initiator to communicate with more SAS/SATA targets. A useful analogy: one can regard an expander as akin to a network switch in a network which allows multiple systems to be connected using a single switch port.
SAS 1 defined two different types of expander; however, the SAS-2.0 standard has dropped the distinction between the two, as it created unnecessary topological limitations with no realized benefit:
- An edge expander allows for communication with up to 255 SAS addresses, allowing the SAS initiator to communicate with these additional devices. Edge expanders can do direct table routing and subtractive routing. (For a brief discussion of these routing mechanisms, see below). Without a fanout expander, you can use at most two edge expanders in your delivery subsystem (because you will connect the subtractive routing port of those edge expanders together, and you can’t connect any more expanders). To solve this bottleneck, you would use fanout expanders.
- A fanout expander can connect up to 255 sets of edge expanders, known as an edge expander device set, allowing for even more SAS devices to be addressed. The subtractive routing port of each edge expanders will be connected to the phys of fanout expander. A fanout expander cannot do subtractive routing, it can only forward subtractive routing requests to the connected edge expanders.
Direct routing allows a device to identify devices directly connected to it. Table routing identifies devices connected to the expanders connected to a device’s own PHY. Subtractive routing is used when you are not able to find the devices in the sub-branch you belong to. This will pass the request to a different branch altogether.
Expanders exist to allow more complex interconnect topologies. Expanders assist in link-switching (as opposed to packet-switching) end-devices (initiators or targets). They may locate an end-device either directly (when the end-device is connected to it), via a routing table (a mapping of end-device IDs and the expander the link should be switched to downstream to route towards that ID), or when those methods fail, via subtractive routing: the link is routed to a single expander connected to a subtractive routing port. If there is no expander connected to a subtractive port, the end-device cannot be reached.
Expanders with no PHYs configured as subtractive act as fanout expanders and can connect to any number of other expanders. Expanders with subtractive PHYs may only connect to two other expanders at a maximum, and in that case they must connect to one expander via a subtractive port and the other via a non-subtractive port.
SAS-1.1 topologies built with expanders will generally contain one root node in a SAS domain with the one exception case being topologies that contain two expanders connected via a subtractive-to-subtractive port. If it exists, the root node is the expander which is not connected to another expander via a subtractive port. Therefore, if a fanout expander exists in the configuration, it must be the domain’s root node. The root node contains routes for all end devices connected to the domain. Note that with the advent in SAS-2.0 of table-to-table routing and new rules for end-to-end zoning, more complex topologies built upon SAS-2.0 rules will not contain a single root node.
The SAS connector is much smaller than traditional parallel SCSI connectors, allowing for the small 2.5-inch (64 mm) drives. SAS currently provides for point data transfer speeds up to 6 Gbit/s, but is expected to reach 12 Gbit/s by the year 2012.[dated info]
The physical SAS connector comes in several different variants:
|Image||Codename||Other names||Ext./int.||No of pins||No of devices||Comment|
|SFF-8482||Internal||29||1||This form factor is designed for compatibility with SATA. The socket is compatible with SATA drives; however, the SATA socket is not compatible with SFF-8482 (SAS) drives. The pictured connector is a drive-side connector.|
|SFF-8484||Internal||32 (19)||4 (2)||Hi-density internal connector, 2 and 4 lane versions are defined by the SFF standard.|
|SFF-8485||Defines SGPIO (extension of SFF 8484), a serial link protocol used usually for LED indicators.|
|SFF-8470||Infiniband CX4 connector, Molex LaneLink™||External||32||4||Hi-density external connector (also used as an internal connector).|
|SFF-8086||Internal mini-SAS, internal mSAS||Internal||26||4||This is actually just a particular implementation of SFF-8086, which covers a variety of connectors, including SFF-8087 and SFF-8088 (below). This unnamed 26-circuit unshielded connector type is uncommon, as SFF-8087 is usually used instead.|
|SFF-8087||Internal mini-SAS, internal mSAS, internal iSAS||Internal||36||4||Unshielded 36-circuit implementation of SFF-8086. Molex iPass™ reduced width internal 4× connector with future 10 Gbit/s capability.|
|SFF-8088||External mini-SAS, external mSAS, external iSAS||External||26||4||Shielded 26-circuit implementation of SFF-8086. Molex iPass™ reduced width external 4× connector with future 10 Gbit/s capability.|
Nearline SAS or NL-SAS drives are enterprise SATA drives with a SAS interface, head, media, and rotational speed of traditional enterprise-class SATA drives with the fully capable SAS interface typical for classic SAS drives. System and storage vendors like Dell, EMC, Fujitsu, and IBM are offering these disks for SAN arrays, NAS solutions, and server systems.
They feature the following benefits compared to SATA:
- Dual ports allowing redundant paths
- Ability to connect a device to multiple computers
- Full SCSI command set
- Faster interface compared to SATA, up to 20%, no STP (Serial ATA Tunneling Protocol) overhead
- No need for SATA interposer cards (for high availability of SATA drives SATA interposer cards are needed)
To summarize: Nearline SAS drives are simply big, cheap, and slow SAS drives targeted toward nearline storage.