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.
Facebook
Twitter
RSS
250 Comments
View All Comments
VaultDweller - Wednesday, March 18, 2009 - link
I love it when people critique someone's critique of grammar... and get it wrong.It's an SSD, not a SSD.
gwolfman - Wednesday, March 18, 2009 - link
lolzsidex - Wednesday, March 18, 2009 - link
I would like to know the firmware version of Vertex used in your review. To me sounds the old 0112kensiko - Wednesday, March 18, 2009 - link
Yes that is important to know.I'm sure this is not done with the latest firmware available which is 1199. This version got better performance.
Firmware 1275 is coming also.
Anand, will you update your benchmarks with the latest firmware?
If not, then the benchmarks are obsolete.
Anand Lal Shimpi - Wednesday, March 18, 2009 - link
I tested with the shipping firmware for this article (0122). I've been playing around with 1199 in the lab and will most likely have an update in a couple of weeks once I've done a thorough evaluation of it. By then I should also have the final version of the new Samsung drive and maybe even some other interesting things.For now, I've got to get to work on the new Mac Pro and the updated Ion article :) I need a small break from SSDs por favor :)
Take care,
Anand
VaultDweller - Wednesday, March 18, 2009 - link
Awww, don't you have some underlings to do your SSD-related will?Would love to see an update, and would love to see Corsair's SSD drive tested as well (it's based on Samsung's last generation MLC controller, and doesn't seem to emphasize sequential like the Summit does).
Anand Lal Shimpi - Wednesday, March 18, 2009 - link
I will do an update on the new firmware, I just want to do it right so it'll take some time :)I'll put in a request for the Corsair drive as well :)
-A
Slash3 - Wednesday, March 18, 2009 - link
Page 29: "Not all applications will launch faster than a VelociRaptor on a SSD, but let's not forget that the VelociRaptor is the world's fastest hard drive."Really? What about the nice and speedy enterprise-level 15k SAS/SCSI drives everyone neglects to acknowledge? :)
George Powell - Wednesday, March 18, 2009 - link
I believe it refers to consumer drives. While SAS drives are beginning to be a possibility on the desktop with newer motherboards supporting them natively, the drives themselves are too expensive and too noisy for most consumers to actually want them.FishTankX - Wednesday, March 18, 2009 - link
Good info. However, I noticed one mistake.Second page
Samsung had a MLC controller at the time but it was too expensive than what SuperTalent was shooting for.