SSD versus Enterprise SAS and SATA disks
by Johan De Gelas on March 20, 2009 2:00 AM EST- Posted in
- IT Computing
RAID 5 in Action
RAID 0 is good way to see how adding more disks scales up your writing and reading performance. However, it is rarely if ever used for any serious application. What happens if we use RAID 5?
We see a picture very similar to what we saw with our RAID 0 numbers. The SAS protocol is more efficient than the SATA interface. Four SLC SSDs already come close to the highest bandwidth that our Adaptec HBA is capable of extracting out of SATA disks. Add four more and you bump into a 700 MB/s bottleneck. At first, our eight drive results look weird, as they are more than twice as fast as the four drives results. In the case of four drives, our controller will write three striped blocks of data and one parity block on our RAID array. With eight drives, we are striping seven blocks and we only have to write one parity block. So we can expect a maximum scaling of 2.3 (7/3), which explains the "the better than doubling" in performance.
Once we interleave some writing with reading, something remarkable happens: eight SLC SSDS are (slightly) slower than four are. We have no real explanation for this, other than that this might have something to do with the choices Adaptec made when writing the firmware. We tried out the Areca 1680 controller, which uses the same controller (Intel Dual-core IOP348 1.2GHz) but with different firmware.
The Areca 1680 controller is almost 20% slower with four SLC SSD drives in RAID 0, but scales well from four to eight drives (+78%). In comparison, the Adaptec controller is only 19% faster with an additional four drives. In RAID 5 the difference is even more dramatic. Adding four extra drives (for a total of eight) leads to a performance decrease on the Adaptec card, while there is a decent 47% scale up in performance in the same situation on the Areca 1680. Thus, we conclude that is likely that the Adaptec firmware is to blame. We used both the 1.44 (August 2008) and 1.46 (February 2009) firmware, and although the latest firmware gave a small boost, we made the same observations.
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marraco - Wednesday, March 25, 2009 - link
The comparison is not fair, but can be fairer:If the RAID of SATA/SAS disks is restricted to the same storage capacity than the SSD, limiting the partition to the fastest external tracks/cilynders, the latency is significantly reduced, and average read/write speed is significantly increased, so
PLEASE, PLEASE, PLEASE
Repeat the benchmarcks, but with short stroking for magnetic disks.
JohanAnandtech - Friday, March 27, 2009 - link
May I ask what the difference with the fact that we created a relatively small partition across our RAID-5 raidset? Also, you can imagine that our 23 GB database was at the outer tracks of the disks. I have to verify, but that seems logical.This kind of testing should give the same effects as short stroking. I personally think Short stroking can not be good for your actuator, while a small partition should be no problem.
marraco - Friday, March 27, 2009 - link
See this link.http://www.tomshardware.com/reviews/short-stroking...">http://www.tomshardware.com/reviews/short-stroking...
Clearly, you results are orders of magnitude than those showed on that benchmark.
As I understand, short stroking increase actuator health, because reduces physical acceleration on the actuator.
Anything necessary, is to use a small partition on the fastest external track.
you utilized a raid 0 of 16 disks, with less than 1000 gb/second.
On Tomshardware, a raid of only 4 disk achieved average (not maximun) 1400 to 1600 Mb/s. (of course, the test are not the same; for that reason, I ask for new test)
About the RAID 5: I would love to see RAID 0.
I are interesed on comparing a fast SSD as the intels, (or OCZ Vostro/Summit), with what can be achieved at the same cost, with magnetic media, if the partition size is restricted to the same total capacity than the SSD.
Anyway, thanks for the article. Good work.
So good, I want to see more :)
marraco - Sunday, April 5, 2009 - link
Please, tell me you are preparing such article :)JohanAnandtech - Tuesday, April 7, 2009 - link
We are investigating the issue. I like to have some second opinions before I start heavy benchmarking on THG article. They tend to be sensational...araczynski - Wednesday, March 25, 2009 - link
wow, color me impressed. all the more reason to upgrade everything to gigabit and fiber.BailoutBenny - Tuesday, March 24, 2009 - link
Can we get any updates on the future of chalcogenide glass (phase change) based drive technologies? IBM's Millipede and other MEMS probe storage devices? Any word about Intel and STMicroelectronics' shipments of PRAM samples to customers that happened last year? What do the rumor mills say? Are these technologies proving viable? It is difficult to formulate a coherent picture for these technologies without being an industry insider.Black Jacque - Tuesday, March 24, 2009 - link
RAID 5 in Action... However, it is rarely if ever used for any serious application.
You are obviously not a SAN Admin or know too much about enterprise level storage.
RAID 5 is the mainstay of block-level storage systems by companies like EMC.
In addition, the article mentions STEC EFDs used by EMC. On the EMC CLARiiON line, those EFDs are provisioned in RAID 5 groups.
spikespiegal - Wednesday, March 25, 2009 - link
[quote]RAID 5 is the mainstay of block-level storage systems by companies like EMC. [/quote]Which thus explains why in this day in age I see so many SANs blowing entire volumes and costing days of restoration when the room temp gets a few degrees above ambient.
Corrupted RAID 5 arrays have cost me more lost enterprise data than all the non-RAID client side disks I've ever replaced; iSeries, all brands of x386, etc. EMC has a great script to account for this in which they always blame the drives first, then only when cornered by an enraged CIO will they admit it's their controllers. Been there...done that...for over a decade in many different industries.
If you haven't been burned by RAID 5, or dare claim a drive controller in RAID 5 mode has a better MTBF than the drives it's hosting, then it's time to quite your day job at the call center in India. RAID 5 saves you the cost of one drive every four, which was logical in 1998 but not today. At least span across multiple redundant controllers in RAID 10 or something....
JohanAnandtech - Tuesday, March 24, 2009 - link
I fear you misread that sentence:"RAID 0 is good way to see how adding more disks scales up your writing and reading performance. However, it is rarely if ever used for any serious application."
So we are talking about RAID-0 not RAID-5.
http://it.anandtech.com/IT/showdoc.aspx?i=3532&...">http://it.anandtech.com/IT/showdoc.aspx?i=3532&...