Trends and future capabilities of the SCSI protocol
If you want to know what innovations await the market for SCSI devices in the not-so-distant future, and you have not heard anything about SAS-3, SCSI Express (SCSI over PCIe) or Multilink SAS, then you are the same person that read this article to the end !
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We will start with a relatively old, but nevertheless relevant, “road map” of the development of the SAS protocol from the SCSI Equipment Association (SCSI Trade Association).
First of all, it tells us that this year there will be products using SAS-3 technology (12 Gbit / s), which will be able to use the capabilities of the PCI Express 3.0 interface. The new generation of devices is backward compatible with SAS-2 (6 Gbit / s) and SAS (3 Gbit / s). For SAS-3, the SFF 8680 connector is used. It looks no different from SFF 8482, which was used in SAS-2. It uses all the same 29 pins, which are also located in the same way (3 sets of contacts: 7 + 7 + 15).
However, this year it is not worth waiting for the fact that SSD and HDD manufacturers will release disks with SAS-3, the new interface will first appear on RAID controllers and expanders. In particular, LSI stated that its devices will support the multiplexing of two 6 Gbit / s streams into one 12 Gbit / s and called it the voiced word DataBolt . Adaptec by PMC in its product line already has SAS expanders (from 24 to 68 ports) and SAS-3 controllers. At least all the major manufacturers in one way or another are already on sale or are about to start selling SAS-3 devices.
Also in the very near future we will see a new type of slot - Multilink SAS. This standard extends the SAS slot to four ports (and thereby doubles the bandwidth). Before the usual SAS-3, it has the following advantages:
Multilink SAS uses the new SFF 8630 standard connector (43 pin). It is similar to the SAS-2 and SAS-3 connector, but has a group of 14 additional contacts (shown in gray in the figure).
In the first half of 2014, the appearance of the first devices using SCSI Express technology is possible. The concept is quite simple - a SCSI device will connect directly to the PCI Express bus, bypassing the SCSI controller. To do this, both (initiator and target) must support the SOP (SCSI over PCIe) protocol. In the first version of the standard, the device will use up to 4 channels of a PCIe bus. It makes sense to use this technology, for example, to connect devices that will be used for caching hot data. SCSI Express will have to provide an extremely small delay in accessing the storage device. SCSI Express will get its own slot (SFF-8639). The slot will support hot add / remove devices and will also be compatible with all SATA and SAS devices.
SCSI protocol is also actively expanding. From interesting things, for example, a set of commands can be noted to implement atomic data (atomic writes), which at the SCSI protocol level will allow merging a group of SCSI commands into transactions. In this case, the storage device must either execute all the commands combined into one transaction, or, if it is impossible to execute all the commands, return to the initial state. This feature should improve performance when using journaling file systems and databases.
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SAS 12 Gb / s
We will start with a relatively old, but nevertheless relevant, “road map” of the development of the SAS protocol from the SCSI Equipment Association (SCSI Trade Association).
First of all, it tells us that this year there will be products using SAS-3 technology (12 Gbit / s), which will be able to use the capabilities of the PCI Express 3.0 interface. The new generation of devices is backward compatible with SAS-2 (6 Gbit / s) and SAS (3 Gbit / s). For SAS-3, the SFF 8680 connector is used. It looks no different from SFF 8482, which was used in SAS-2. It uses all the same 29 pins, which are also located in the same way (3 sets of contacts: 7 + 7 + 15).
However, this year it is not worth waiting for the fact that SSD and HDD manufacturers will release disks with SAS-3, the new interface will first appear on RAID controllers and expanders. In particular, LSI stated that its devices will support the multiplexing of two 6 Gbit / s streams into one 12 Gbit / s and called it the voiced word DataBolt . Adaptec by PMC in its product line already has SAS expanders (from 24 to 68 ports) and SAS-3 controllers. At least all the major manufacturers in one way or another are already on sale or are about to start selling SAS-3 devices.
Multilink SAS
Also in the very near future we will see a new type of slot - Multilink SAS. This standard extends the SAS slot to four ports (and thereby doubles the bandwidth). Before the usual SAS-3, it has the following advantages:
- Greater bandwidth. For the Multilink SAS device inserted into the Multilink slot, we can achieve a throughput of 96 Gbit / s (4 ports * 12 Gbit / s SAS in full duplex mode).
- Devices connected to this slot will receive more power - up to 25 watts. Together with a huge bandwidth, this will allow you to create SSD disks consisting of a very large number of memory modules. In this slot, they will be able to reveal their full potential.
- Backward compatible with SATA and SAS of all generations.
Multilink SAS uses the new SFF 8630 standard connector (43 pin). It is similar to the SAS-2 and SAS-3 connector, but has a group of 14 additional contacts (shown in gray in the figure).
SCSI Express
In the first half of 2014, the appearance of the first devices using SCSI Express technology is possible. The concept is quite simple - a SCSI device will connect directly to the PCI Express bus, bypassing the SCSI controller. To do this, both (initiator and target) must support the SOP (SCSI over PCIe) protocol. In the first version of the standard, the device will use up to 4 channels of a PCIe bus. It makes sense to use this technology, for example, to connect devices that will be used for caching hot data. SCSI Express will have to provide an extremely small delay in accessing the storage device. SCSI Express will get its own slot (SFF-8639). The slot will support hot add / remove devices and will also be compatible with all SATA and SAS devices.
SCSI protocol innovations
SCSI protocol is also actively expanding. From interesting things, for example, a set of commands can be noted to implement atomic data (atomic writes), which at the SCSI protocol level will allow merging a group of SCSI commands into transactions. In this case, the storage device must either execute all the commands combined into one transaction, or, if it is impossible to execute all the commands, return to the initial state. This feature should improve performance when using journaling file systems and databases.
Source: https://habr.com/ru/post/174033/
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