Talk:Serial Attached SCSI
|This is the talk page for discussing improvements to the Serial Attached SCSI article.
This is not a forum for general discussion of the article's subject.
|WikiProject Computing / Hardware||(Rated Start-class, Mid-importance)|
- 1 Q + A
- 2 SAS vs parallel SCSI
- 3 Cleanup
- 4 SATA 1 and SATA 2?
- 5 Photo
- 6 is it ok?
- 7 drive identification
- 8 error in SAS layers diagram?
- 9 Usage / deployment?
- 10 Is it compatible with SATA 6.0 Gbit/s?
- 11 Slower than Parallel SCSI?
- 12 Number of devices
- 13 port identifiers
- 14 SAS signal pins relative to connectors used in marketplace
- 15 SAS vs. SSA
- 16 SFF-8470 connector pin count
- 17 Requested move 09 April 2016
Q + A
Q: It would be interesting to see even a short list of points about how SAS compares to SATA - what limitations each one has the the other doesn't?
A: As noted in the article, SATA is limited to direct-attached connection of ATA drives only and does not support SAS drives. SAS supports SATA drives (in fact, it uses the same internal narrow cables), a complex topology, a SCSI command set, and for SAS-native drives, a globally unique drive identifier. 18.104.22.168 02:46, 6 October 2006 (UTC) A: Sata does actually support port multipliers but only one level of them and only one initiator and relatively few ports. SAS allows for multiple initiators and networks of expanders. 21:46, 13 April 2008 (UTC)
Q: I'd like to see something about performance of SAS relative to SCSI or SATA or IDE. Probably best as a link out than directly here. Leodirac 23:46, 5 January 2007 (UTC)
Q: It would be nice if SAS Expanders/Fanouts were explained more thoroughly. IE What the devices look like and how they interact with the controller. More of a broad explanation and implementation rather then ultra technical. Diagram, pictures, etc.
Q: The article says that SAS is backward compatible with SATA. What does that mean exactly? A: Sata drives can be connected to a SAS system but not vice-versa. Plugwash (talk) 21:46, 13 April 2008 (UTC)
SAS vs parallel SCSI
Since parallel SCSI also support hot swap. I've remove the following line
- SAS supports hot swapping.
--Sltan 05:48, 23 January 2007 (UTC)
Parallel SCSI does not really support hot swapping..You an attach / unattach devices without powering down the computer, but you need to manually do a bus-rescan every single time the bus is changed. If you do not do a bus rescan, the changes will not show.
-- comments by ard >: Hotswap with parallel scsi comes from the enclose controller. That one knows when a new disk is inserted, and hence a bus scan can be targeted to one id, and initiated automatically. Sata/Sas practice the same thing except that the hot-swap is speced as part of the bus, and hence is supported in the controller, and hence the *driver* knows when to scan instead of some enclosure daemon that polls the enclosure on events. Furthermore: currently it says that parallel scsi is multidrop, but it's just a bus which can host multiple initiators and targets. Initiators can be targets as well (that has been supported in some of the better operating systems). —Preceding unsigned comment added by 22.214.171.124 (talk) 11:34, 27 August 2010 (UTC)
SATA 1 and SATA 2?
In the article about SATA it can be read that SATA II shouldn't be confused with SATA/300. Which is said on www.sata-io.org (the ones who make the standard). So the comparsion between SATA and SAS maybe should be changed. Like NCQ is an "additional capability" as they say, nothing about that it should be in SATA/300. Aqualize 22:55, 5 February 2007 (UTC)
A Hershey's kiss for size comparison?
- but they "are one of the most popular candies in the world". Yep, never heard of those before :)
Any chance someone could make a new photo and include a ruler in the picture to show the size/scale? One with metric and imperial would be nice :) It would be a little more useful than using a chocolate as the reference object. —Preceding unsigned comment added by 126.96.36.199 (talk) 05:18, 20 November 2007 (UTC)
is it ok?
List of SAS & SAS RAID controllers - in fact it is a commercial ad with prices, "buy" button etc., not a list. Is it ok in wikipedia.org? —Preceding unsigned comment added by 188.8.131.52 (talk) 12:33, 25 November 2007 (UTC)
"SATA devices are uniquely identified by their port number connected to the Host bus adapter while SAS devices are uniquely identified by their World Wide Name (WWN)." so how are SATA drives within a SAS system identified? Plugwash (talk) 18:18, 12 April 2008 (UTC)
error in SAS layers diagram?
In the frame definition layer, I think "SPP transport layer" is a typo; it should be "SSP transport layer". At least I can't find a reasonable definition for SPP, whereas SSP is "Serial SCSI Protocol" :-). --klode (talk) 16:20, 20 May 2008 (UTC)
Usage / deployment?
I'd like to know what the common usage for SAS drives would be (mission-critical servers, mid-range servers, low-demand servers, etc.) I'd also like to see some numbers as to how many are out there, how many will be out there, etc. Will this technology eventually dominate all server applications? ---Ransom (--184.108.40.206 (talk) 15:56, 3 June 2008 (UTC))
- Please read WP:FORUM, but the short answer is that SAS already has taken over all of the above. -- KelleyCook (talk) 16:22, 3 June 2008 (UTC)
Is it compatible with SATA 6.0 Gbit/s?
Yes, they are all (and will be) backwards compatible. Even 6 gig devices when plugged into a 3 gig hba should work... SAS/SATA regardless. It's kind of like PCIexpress (1 & 2) where as either card will work in either slot. 220.127.116.11 (talk) 21:15, 8 December 2008 (UTC) surge
Slower than Parallel SCSI?
In the first paragraph... "At present it is slightly slower than the final parallel SCSI implementation, but in 2009 it will double its present speed to 6 Gbit/s"
I don't think you undestand...it's 3 Gb/sec PER LANE...most SAS HBAs have 2 ports, 4 lanes per port (8 total) -- That's 2400 MB/sec half duplex, 4800 MB/sec full duplex...
Compare that to a Parallel SCSI HBA which has two independent U320 Parallel SCSI Busses (Not full duplex)...640 MB / sec.
Number of devices
Within the SAS domain section it is stated that a SAS domain can have up to 16,256 devices, while under the comparison to parallel scsi it is stated that sas supports up to 16,384 devices (the latter being 2^14, and thus sounding more correct to me, though I don't know). Are these numbers correct? If so it might be clearer if a note stating this were added. Peter Law (talk) 17:45, 13 January 2009 (UTC)
The article compares sas, scsi and FC port identifiers. It states that the "port identifier" in FC is the WWPN. This seems slightly confusing as the word "Port-ID" is usually referring to the identifier assigned in a fabric or loop to communicate with the port. The WWPN is not used for the communication itself. I think the comparison is fair as long as the "port identifier" is removed. Instead one could say something like HW address.. — Preceding unsigned comment added by Qualle14 (talk • contribs) 22:53, 14 March 2012 (UTC)
SAS signal pins relative to connectors used in marketplace
1. i read that SAS, being differential, and having 4 signals per channel, uses eight signal lines per drive. i read on this wiki page how the sff-8086 (26 pin) connector may be used to transmit four channels. if each drive requires 8 lines, and if four drives are to be supported: 8 * 4 = 32 lines. how is it physically possible to send signals to four drives using only 26 pins? an explanation here would be welcome
2. similarly at the actual drive, the sff-8482 connector is used. the wiki article mentions how on the 'underside' or 'backside' of the connector, additional pins are to be found - without any information as to the pin counts and/or mapping of pin to SAS signal.
3. finally, i realize this page is focused to speak towards the 'standards' side of SAS. however in practice, the market place sometimes offers implementations that might not be fully standards compliant. this is of course then confusing for a consumer. 'sideband' signalling found on many SAS implementations would be one topic that should be included here, so that it is at least discussed. which SAS signal pins are to be used for 'sideband' signal? and how does this need for extra signal relate to the pinouts of the standard connectors sff-8086,8087,8088?
- 1.+2. SAS uses one pair of contacts for each direction, so 4 electrical lines per channel. Check www.t10.org/ftp/t10/document.05/05-084r1.pdf for details.
- 3. Sideband signalling is present to allow for application specific signals without having to use additional cabling (e.g. SGPIO). Sideband pins are present on internal connectors only. Zac67 (talk) 18:51, 27 September 2012 (UTC)
SAS vs. SSA
SAS is reinventing the wheel. All the perks praised in this article were available 20 years ago in IBM's Serial Storage Architecture (SSA):
- point-to-point links
- loop topology
- dual-ported drives
- simultaneous multi-link transfers
- multiple initiators (up to 2 in RAID and up to 8 in JBOD configurations)
- "target mode SSA" communication between initiators
It even had optical extenders which allowed to extend the SSA loop between datacentres at distances up to 10 km.
The only difference being the electronic circuitry at the time was capable of 20 MB/s per link, upgraded in late 90s to 40 MB/s per link. 15 years later the only improvement is frequency increase mere 15-fold. SSA adapters allowed port 1 on adapter A to participate in a loop with port 2 on adapter B with the remaining two ports not sharing a loop. SAS adapters I've seen so far demand port 1 to be joined with port 1 which makes the implementation more error prone.
SFF-8470 connector pin count
I believe the SFF-8470 connector has 34 pins, not 32. I have already changed the article.
Unfortunately, I couldn't find this clearly mentioned in the SFF-8470 standard. It references figure 9, but the references to this figure in the PDF are not links or they do not work. And the figure 9 seems to describe a particular connector variant, not the pin numbering as referenced. But the values (
n) in the tables correspond as do other verification methods I tried (none of them significantly authoritative though).
The pin counts should be as follows:
- 16 signal pins,
- 18 ground pins.
The ground pins are actually 9 dual-sided pairs and could therefore be counted as two per pair, giving 18 pins altogether. Together with the signal pins we get the total count of 34 pins. This is also close to the original number of 32, so the original author might have used the same counting method.