Server Guide part 2: Affordable and Manageable Storage
by Johan De Gelas on October 18, 2006 6:15 PM EST- Posted in
- IT Computing
Enterprise Disks: all about SCSI
There are currently only two kinds of hard disks: those which work with SCSI commands and those which work with (S)ATA commands. However, those SCSI commands can be sent over three disk interfaces:
Will SATA kill off SCSI?
One look at the price of a typical "enterprise disk" -- whether it be a SCSI, FC or SAS disk -- will tell you that you have to pay at least 5 times and up to 10 times more per GB. Look at the specification sheets and you will see that the advantage you get for paying this enormous price premium seems to be only a 2 to 2.5 times lower access time (seek + latency) and a maximum transfer rate that is perhaps 20 to 50% better.
In the past, the enormous price difference between the disks which use ATA commands and the disks which use SCSI commands could easily be explained by the fact that a PATA disk would simply choke when you sent a lot of random concurrent requests. As quite a few of reviews here at AnandTech have shown, thanks to Native Command Queuing the current SATA drives handle enterprise workloads quite well. The number of concurrent I/O operations per second is easily increased by 50% thanks to NCQ. So while PATA disks were simply pathetically slow, the current SATA disks are - in the worst case - about half as fast as their SCSI counterparts when it comes to typical I/O intensive file serving.
There is more. The few roadblocks that kept SATA out of the enterprise world have also been cleared. One of the biggest problems was the point to point nature of SATA: each disks needs its own cable to the controller. This results in a lot of cable clutter which made SATA-I undesirable for enterprise servers or storage rack enclosures.
This roadblock can be removed in two ways. The first way is to use a backplane with SATA port multipliers. A port multiplier can be compared to a switch. One Host to SATA connection is multiplexed to multiple SATA connectors. At most 15 disks can make us of one SATA point to point connection. In reality, port multipliers connect 4 to 8 disks per port. As the best SATA disks are only able to sustain about 60 to 80 MB/s in the outer zones, a four disk port multiplier make sense even for streaming applications. For more random applications, even 8 or 15 disks on one 300 MB/s SATA connection would not result in a bottleneck.
Port multipliers are mostly used on the back panel of a server or the backplane of a storage rack. The second way is to use your SATA disks in a SAS enclosure. We will discuss this later.
There are currently only two kinds of hard disks: those which work with SCSI commands and those which work with (S)ATA commands. However, those SCSI commands can be sent over three disk interfaces:
- SCSI-320 or 16 bit Parallel SCSI
- SAS or Serial Attached SCSI
- FC or Fibre Channel
Will SATA kill off SCSI?
One look at the price of a typical "enterprise disk" -- whether it be a SCSI, FC or SAS disk -- will tell you that you have to pay at least 5 times and up to 10 times more per GB. Look at the specification sheets and you will see that the advantage you get for paying this enormous price premium seems to be only a 2 to 2.5 times lower access time (seek + latency) and a maximum transfer rate that is perhaps 20 to 50% better.
In the past, the enormous price difference between the disks which use ATA commands and the disks which use SCSI commands could easily be explained by the fact that a PATA disk would simply choke when you sent a lot of random concurrent requests. As quite a few of reviews here at AnandTech have shown, thanks to Native Command Queuing the current SATA drives handle enterprise workloads quite well. The number of concurrent I/O operations per second is easily increased by 50% thanks to NCQ. So while PATA disks were simply pathetically slow, the current SATA disks are - in the worst case - about half as fast as their SCSI counterparts when it comes to typical I/O intensive file serving.
There is more. The few roadblocks that kept SATA out of the enterprise world have also been cleared. One of the biggest problems was the point to point nature of SATA: each disks needs its own cable to the controller. This results in a lot of cable clutter which made SATA-I undesirable for enterprise servers or storage rack enclosures.
This roadblock can be removed in two ways. The first way is to use a backplane with SATA port multipliers. A port multiplier can be compared to a switch. One Host to SATA connection is multiplexed to multiple SATA connectors. At most 15 disks can make us of one SATA point to point connection. In reality, port multipliers connect 4 to 8 disks per port. As the best SATA disks are only able to sustain about 60 to 80 MB/s in the outer zones, a four disk port multiplier make sense even for streaming applications. For more random applications, even 8 or 15 disks on one 300 MB/s SATA connection would not result in a bottleneck.
Port multipliers are mostly used on the back panel of a server or the backplane of a storage rack. The second way is to use your SATA disks in a SAS enclosure. We will discuss this later.
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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.