The SSD Anthology: Understanding SSDs and New Drives from OCZ
by Anand Lal Shimpi on March 18, 2009 12:00 AM EST- Posted in
- Storage
Free Space to the Rescue
There’s not much we can do about the scenario I just described; you can’t erase individual pages, that’s the reality of NAND-flash. There are some things we can do to make it better though.
The most frequently used approach is to under provision the drive. Let’s say we only shipped our drive with 20KB of space to the end user, but we actually had 24KB of flash on the drive. The remaining 4KB could be used by our controller; how, you say?
In the scenario from the last page we had to write 12KB of data to our drive, but we only had 8KB in free pages and a 4KB invalid page. In order to write the 12KB we had to perform a read-modify-write which took over twice as long as a 12KB write should take.
If we had an extra 4KB of space our 12KB write from earlier could’ve proceeded without a problem. Take a look at how it would’ve worked:
We’d write 8KB to the user-facing flash, and then the remaining 4KB would get written to the overflow flash. Our write speed would still be 12KB/s and everything would be right in the world.
Now if we deleted and tried to write 4KB of data however, we’d run into the same problem again. We’re simply delaying the inevitable by shipping our drive with an extra 4KB of space.
The more spare-area we ship with, the longer our performance will remain at its peak level. But again, you have to pay the piper at some point.
Intel ships its X25-M with 7.5 - 8% more area than is actually reported to the OS. The more expensive enterprise version ships with the same amount of flash, but even more spare area. Random writes all over the drive are more likely in a server environment so Intel keeps more of the flash on the X25-E as spare area. You’re able to do this yourself if you own an X25-M; simply perform a secure erase and immediately partition the drive smaller than its actual capacity. The controller will use the unpartitioned space as spare area.
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Franco1 - Wednesday, March 18, 2009 - link
I've been waiting a long time for this review. It was certainly worth the wait! I would love to see some benchmarks with 2+ drives in RAID configurations via onboard and add-on controller cards. Maybe another follow up?Howard - Wednesday, March 18, 2009 - link
Looks like the Vertex is the drive to get, especially once the user base expands a bit.MagicalMule - Wednesday, March 18, 2009 - link
Thanks for the article. Everyone is critiquing grammar and all this nonsense it seems, but I really enjoyed the article.It was very thorough and very informative.
Keep up the good work. =).
futrtrubl - Wednesday, March 18, 2009 - link
You missed out a VERY significant step that causes the greater part of the slowdown associated with your scenario. After the block is read out to cache the block has to be erased before it can be written to again and as you pointed out earlier an erase cycle, and thus the entire read/modify+erase/write cycle, takes a relatively LONG time, much longer than a simple read/modify/write.Edward
DrKlahn - Wednesday, March 18, 2009 - link
I've worked in IT for 15 years and have played with very fast arrays and know a fair amount about storage. 2 months ago I replaced my Raptor boot/gaming drive with a GSkill Titan. In day to day use I have no stuttering. The only stutter I have seen was while installing a large patch, surfing with multiple windows/tabs open and using Outlook. It wasn't even a second. I did align the partition, turned off drive indexing and defragmentation, and turned on caching. In day to day use it simply kills the Raptor. Games and applications load in a fraction of the time. Vista boot time has decreased dramatically.This isn't a case of purchase justification. If the drive was a dud I would have moved it to a secondary machine, reinstalled the Raptor, and chalked it up as a bad decision. I simply have not run into any scenario in daily use that it performs worse than the drive it replaced and I have not seen any real stuttering in daily use.
Gary Key - Wednesday, March 18, 2009 - link
I have a GSkill Titan drive also and really like it. However, my experiences while positive overall, do not compare with yours when it comes to stuttering (yes, all optimizations have been done to the drive and OS). I still have significant stuttering problems when using multiple IM programs and having multiple windows/tabs open at the same time. I literally have to wait a few seconds when texting colleagues if more than two conversations are occurring at the same time as the system pauses, hitches, and stutters in this scenario. It is especially aggravating when on Skype and trying to text, speak, and transfer files at the same time. This does not occur on the Intel drive in my testing. Apparently, it is no longer a problem on the OCZ Vertex or Summit drives. Except for my example above, I would certainly use the Titan drive over my Raptor any day of the week.druc0017 - Wednesday, March 18, 2009 - link
great article, keep up the good work, cant wait to see more updates, thxmikeblas - Wednesday, March 18, 2009 - link
Is the Velociraptor really "World's fastest hard drive", as this article states? Faster than the Hitachi SAS drives?Gary Key - Wednesday, March 18, 2009 - link
We have changed those statements to "...fastest consumer desktop hard drive...", that was the original intent of the statement, just clarified now. :)7Enigma - Wednesday, March 18, 2009 - link
I think the majority of us understood that. People just like to nit-pick.