Conclusion

The fact that a SAS HBA and SAS storage rack can contain both SAS and SATA is revolutionary. It is not unlikely that SATA will push all SCSI based disks - SCSI-320, FC and SAS - into a niche market, more precisely the transactional database storage market. The high price premium for the 15000 RPM SCSI based disks can only be justified if they are used in a mission critical environment with OLTP or similar transactional workloads. In that case, the ultra low access times and slightly higher reliability pays off.

Western Digital even tries to convince us that even in that niche market, expensive SAS disks should be replaced by 10000 rpm enterprise SATA disks. It is not likely that the WD Raptors will replace the twice as expensive SAS disks, as the latter still perform better thanks to higher RPM and lower seek times. But of course, we'll give them the benefit of the doubt until we have performed some thorough testing. If the size of your storage rack is not really a concern, twice as many SATA drives might outperform a SAS drive configuration. If space, power consumption and performance are your most important concerns, the relatively expensive but small 2.5 inch SAS disks (10000 rpm) are the best option.

For all other storage needs, SATA in a SAS storage rack is the most interesting candidate. It is however unwise to use cheap desktop SATA disks in large disk arrays for enterprise use. Intensive use of those arrays will lead to very slow access times (as the seek time increase significantly with vibration) and high failure rates. At a small price premium "Nearline" or "Enterprise" SATA disks are available which are less sensitive to vibration and much more reliable. In a nutshell, SAS, FC and SCSI drives are still the only choice for OLTP database applications, but the cheaper "Nearline", "Enterprise" and "RE" disks are probably going to chase the SCSI based drives away in the e-mail, archive, file, FTP and backup servers.

Choosing a disk interface is only a small part of choosing the right storage solution. What about SAN, NAS, iSCSI, DAS, Switched Fabric? What influence will the SAS/SATA revolution have on the topology of storage? Watch out for our next server guide which will continue to guide you through the storage and server jungle!


Thanks and References

Special thanks to Steven Peeters, expert in Storage solutions at eSys distribution, for allowing us to test the Promise Vtrak J300s. I also like to thank Remy van Heugten, Aimée Boerrigter and Kenneth Heal of Promise EMEA for their support.

[1] "Evolution in Hard Disk Drive Technology: SAS and SATA", IDC, Dave Reinsel September 2005

[2] WinHEC 2005, "SATA in the Enterprise," and Seagate Market Research

Enterprise SATA
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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
    Reply
  • 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?
    Reply
  • Fantec - Thursday, October 19, 2006 - link

    quote:

    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?

    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).
    Reply
  • 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. Reply
  • 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. Reply
  • 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. Reply
  • 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. Reply
  • 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.
    Reply
  • 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?)
    Reply
  • 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.
    Reply

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