Server Guide part 2: Affordable and Manageable Storage
by Johan De Gelas on October 18, 2006 6:15 PM EST- Posted in
- IT Computing
Parallel SCSI in trouble
Just like P-ATA, which was limited to 133 MB/s, all kinds of skew and crosstalk problems kept parallel SCSI from using a 160 MHz clock. That 160 MHz clock was necessary to reach the 640 MB/s for the next evolution of parallel SCSI, SCSI-640. The result is that SCSI-640 died a silent death.
As you can use up to 14 disk drives on one shared SCSI bus, and with the fastest SCSI hard disk reaching up to 100 MB/s, the 320 MB/s transfer rate was starting to become a bottleneck in many fileserver related applications. By design, SCSI is not very efficient when it comes to raw bandwidth. Tests show that 2 Gb/s or 200 MB/s FC offers more raw bandwidth than SCSI 320. Of course, fourteen SCSI disk still do not need to transfer several hundreds of megabytes per second when running a transactional OLTP database workload, but maximum wire speed wasn't the only concern with parallel SCSI.
As SCSI-320 was still backwards compatible with the early SCSI versions, commands were sent at a "back to the eighties" pace: 5 MB/s. This slow rate of sending commands wastes up to 30% of the performance of the SCSI bus. Another big problem was the fact that you could only attach 14 devices on one host bus adapter. This limited the possibilities to expand your storage in directly attached storage configurations.
The SAS/SATA revolution
SAS is much more than a serialized version of SCSI-320. Instead of writing a - probably boring - essay about the new features in the SAS protocol, let us see what new functionality is available by just looking at real SAS implementations and products. First, we take a look at the LSI Logic SAS3442X-R.
LSI Logic SAS3442X-R: a 8-port 3Gb/s SAS, PCI-X HBA
The first things that you will notice is that you can attach two cables to the internal SAS connector: one for SATA and one for SAS drives. The male connector on our SAS HBA card can connect to both the "female" connectors of SATA and the SAS cables which are slightly different. As you can see below, it is not possible to use the SAS male connector (the lower drawing in blue and green) of the disk with SATA cables. Basically, SAS HBA supports both SAS and SATA drives whereas SATA HBA only support SATA drives.
The connector on top enables the ability to connect - internally - a SATA drive to our HBA, and the connector at the bottom allows us to connect SAS drives.
SAS is just like TCP/IP or Fibre Channel in that it's a layered protocol. There are three transport protocols that use the same SAS underlying physical and link-layer protocols:
The Serial Management Protocol supports expanders, which give SAS a very high degree of flexibility and scalability. The "Port layer" allows wide port which enables much more bandwidth than would have been possible with SCSI-320 and 640. Notice that the SCSI Protocol (SSP) and the Serial ATA Tunnelling Protocol use the same link and physical layers. This allows SATA and SAS drives on the same expander. Let us bring the theory into actual practice....
Just like P-ATA, which was limited to 133 MB/s, all kinds of skew and crosstalk problems kept parallel SCSI from using a 160 MHz clock. That 160 MHz clock was necessary to reach the 640 MB/s for the next evolution of parallel SCSI, SCSI-640. The result is that SCSI-640 died a silent death.
As you can use up to 14 disk drives on one shared SCSI bus, and with the fastest SCSI hard disk reaching up to 100 MB/s, the 320 MB/s transfer rate was starting to become a bottleneck in many fileserver related applications. By design, SCSI is not very efficient when it comes to raw bandwidth. Tests show that 2 Gb/s or 200 MB/s FC offers more raw bandwidth than SCSI 320. Of course, fourteen SCSI disk still do not need to transfer several hundreds of megabytes per second when running a transactional OLTP database workload, but maximum wire speed wasn't the only concern with parallel SCSI.
As SCSI-320 was still backwards compatible with the early SCSI versions, commands were sent at a "back to the eighties" pace: 5 MB/s. This slow rate of sending commands wastes up to 30% of the performance of the SCSI bus. Another big problem was the fact that you could only attach 14 devices on one host bus adapter. This limited the possibilities to expand your storage in directly attached storage configurations.
The SAS/SATA revolution
SAS is much more than a serialized version of SCSI-320. Instead of writing a - probably boring - essay about the new features in the SAS protocol, let us see what new functionality is available by just looking at real SAS implementations and products. First, we take a look at the LSI Logic SAS3442X-R.
LSI Logic SAS3442X-R: a 8-port 3Gb/s SAS, PCI-X HBA
The first things that you will notice is that you can attach two cables to the internal SAS connector: one for SATA and one for SAS drives. The male connector on our SAS HBA card can connect to both the "female" connectors of SATA and the SAS cables which are slightly different. As you can see below, it is not possible to use the SAS male connector (the lower drawing in blue and green) of the disk with SATA cables. Basically, SAS HBA supports both SAS and SATA drives whereas SATA HBA only support SATA drives.
The connector on top enables the ability to connect - internally - a SATA drive to our HBA, and the connector at the bottom allows us to connect SAS drives.
SAS is just like TCP/IP or Fibre Channel in that it's a layered protocol. There are three transport protocols that use the same SAS underlying physical and link-layer protocols:
- The Serial SCSI Protocol (SSP), which transports the SCSI commands over the link layer (similar to the Fibre Channel Protocol)
- The Serial ATA Tunnelling Protocol (STP), which transports the SATA frames to the SATA drives
- The Serial Management Protocol (SMP), the protocol which makes it possible to use expanders and to get diagnostic information (does the disk work, is it plugged in?)
The Serial Management Protocol supports expanders, which give SAS a very high degree of flexibility and scalability. The "Port layer" allows wide port which enables much more bandwidth than would have been possible with SCSI-320 and 640. Notice that the SCSI Protocol (SSP) and the Serial ATA Tunnelling Protocol use the same link and physical layers. This allows SATA and SAS drives on the same expander. Let us bring the theory into actual practice....
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Bill Todd - Saturday, October 28, 2006 - link
It's quite possible that the reason you are seeing far fewer unrecoverable errors than the specs would suggest is that you're reading all or at least a large percentage of your data far more frequently than the specs assume. Background 'scrubbing' of data - using a disk's idle periods to scan its surface and detect any sectors which are unreadable (in which case they can be restored from a mirror or parity-generated copy if one exists) or becoming hard to read (in which case they can just be restored to better health, possibly in a different location, from their own contents) - decreases the incidence of unreadable sectors by several orders of magnitude compared to the value specified, and the amount of reading that you're doing may be largely equivalent to such scrubbing (or, for that matter, perhaps you're actively scrubbing as well).While Johan's article is one of the best that I've seen on storage technology in years, in this respect I think he may have been a bit overly influenced by Seagate marketing, which conveniently all but ignores (and certainly makes no attempt to quantify) the effect of scrubbing on potential data loss from using those alleged-risky SATA upstarts. Seagate, after all, has a lucrative high-end drive franchise to protect; we see a similar bias in their emphasis on the lack of variable sector sizes in SATA, with no mention of new approaches such as Sun's ZFS that attain more comprehensive end-to-end integrity checks without needing them, and while higher susceptibility to rotational vibration is a legitimate knock, it's worst in precisely those situations where conventional SATA drives are inappropriate for other reasons (intense, continuous-duty-cycle small-random-access workloads: I'd really have liked to have seen more information on just how well WD SATA Raptors match enterprise drives in that area, because if one can believe their specs they would seem to be considerably more cost-effective solutions in most such instances).
- bill
JohanAnandtech - Thursday, October 19, 2006 - link
"Nonetheless, something bugs me in your article on Seagate test. I manage a cluster of servers whose total throughoutput is around 110 TB a day (using around 2400 SATA disks). With Seagate figure (an Unrecoverable Error every 12.5 terabytes written or read), I would get 10 Unrecoverable Errors every day. Which, as far as I know, is far away from what I may see (a very few per week/month). "1. The EUR number is worst case, so the 10 Unrec errors you expect to see are really the worst situation that you would get.
2. Cached reads are not included as you do not access the magnetic media. So if on average the servers are able to cache rather well, you are probably seeing half of that throughtput.
And it also depends on how you measured that. Is that throughput on your network or is that really measured like bi/bo of Vmstat or another tool?
Fantec - Thursday, October 19, 2006 - link
There is no cache (for two reason, first the data is accessed quite randomly while there is only 4 GB of memory for 6 TB of data, second data are stored/accessed on block device in raw mode). And, indeed, throughoutput is mesured on network but figures from servers match (iostat).
Sunrise089 - Thursday, October 19, 2006 - link
I liked this story, but I finished feeling informed but not satisfied. I love AT's focus on real-world performance, so I think an excellent addition would be more info into actually building a storage system, or at least some sort of a buyers guide to let us know how the tech theory translates over to the marketplace. The best idea would be a tour of AT's own equipment and a discussion of why it was chosen.JohanAnandtech - Thursday, October 19, 2006 - link
If you are feeling informed and not satisfied, we have reached our goal :-). The next article will go in through the more complex stuff: when do I use NAS, when do I use DAS and SAN. What about iSCSI and so on. We are also working to having different storage solutions in our lab.stelleg151 - Wednesday, October 18, 2006 - link
In the table the cheetah decodes 1000block of 4KB faster than the raptor decodes 100 blocks of 4KB. Guessing this is a typo. Liked the article.JarredWalton - Wednesday, October 18, 2006 - link
Yeah, I notified Johan of the error but figured it wasn't big enough problem to hold back releasing the article. I guess I can Photoshop the image myself... I probably should have just done that, but I was thinking it would be more difficult than it is. The error is corrected now.slashbinslashbash - Wednesday, October 18, 2006 - link
I appreciate the theory and the mentioning of some specific products and the general recommendations in this article, but you started off mentioning that you were building a system for AT's own use (at the lowest reasonable cost) without fully going into exactly what you ended up using or how much it cost.So now I know something about SAS, SATA, and other technologies, but I have no idea what it will actually cost me to get (say) 1TB of highly-reliable storage suitable for use in a demanding database environment. I would love to see a line-item breakdown of the system that you ended up buying, along with prices and links to stores where I can buy everything. I'm talking about the cables, cards, drives, enclosures, backplanes, port multipliers, everything.
Of course my needs aren't the same as AnandTech's needs, but I just need to get an idea of what a typical "total solution" costs and then scale it to my needs. Also it'd be cool to have a price/performance comparison with vendor solutions like Apple, Sun, HP, Dell, etc.
BikeDude - Friday, October 20, 2006 - link
What if you face a bunch of servers with modest disk I/O that require high availability? We typically use SATA drives in RAID-1 configurations, but I've seen some disturbing issues with the onboard SATA RAID controller on a SuperMicro server which leads me to believe that SCSI is the right way to go for us. (the issue was that the original Adaptec driver caused Windows to eventually freeze given a certain workload pattern -- I've also seen mirrors that refuse to rebuild after replacing a drive; we've now stopped buying Maxtor SATA drives completely)More to the point: Seagate has shown that massive amount of IO requires enterprise class drives, but do they say anything about how enterprise class drives behave with a modest desktop-type load? (I wish the article linked directly to the document on Seagate's site, instead it links to a powerpoint presentation hosted by microsoft?)
JohanAnandtech - Thursday, October 19, 2006 - link
Definitely... When I started writing this series I start to think about what I was asking myself years ago. For starters, what the weird I/O per second benchmarking. If you are coming from the workstation world, you expect all storage benchmarks to be in MB/s and ms.Secondly, one has to know the interfaces available. The features of SAS for example could make you decide to go for a simple DAS instead of an expensive SAN. Not always but in some cases. So I had to make sure that before I start talking iSCSI, FC SAN, DAS that can be turned in to SAN etc., all my readers know what SAS is all about.
So I hope to address the things you brought up in the second storage article.