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
The Blind SSD
Modern OSes talk to hard drives using logical block addressing. While hard drives are rotational media, logical block addressing organizes sectors on a hard drive linearly. When you go to save a file, Windows simply issues a write command for your file at a specific logical block address, say LBA 15 for example.
Your OS knows what LBAs are available and which ones are occupied. When you delete a file, the LBAs that point to that file on your hard disk are listed as available. The data you’ve deleted hasn’t actually been removed and it doesn’t get wiped until those sectors on the drive are actually overwritten.
Believe it or not, SSDs actually work the same way.
The flash translation layer in a SSD controller maps LBAs to pages on the drive. The table below explains what happens to the data on the SSD depending on the action in the OS:
| Action in the OS | Reaction on a HDD | Reaction on an SSD |
| File Create | Write to a Sector | Write to a Page |
| File Overwrite | Write new data to the same Sector | Write to a Different Page if possible, else Erase Block and Write to the Same Page |
| File Delete | Nothing | Nothing |
When you delete a file in your OS, there is no reaction from either a hard drive or SSD. It isn’t until you overwrite the sector (on a hard drive) or page (on a SSD) that you actually lose the data. File recovery programs use this property to their advantage and that’s how they help you recover deleted files.
The key distinction between HDDs and SSDs however is what happens when you overwrite a file. While a HDD can simply write the new data to the same sector, a SSD will allocate a new (or previously used) page for the overwritten data. The page that contains the now invalid data will simply be marked as invalid and at some point it’ll get erased.
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sbuckler - Wednesday, March 18, 2009 - link
Depends on how smart the controller is? Shuffling around the data now and again in the background would make sense.Frallan - Wednesday, March 18, 2009 - link
@AT
This is why i come to AT to read up on the developments.
@OCZ
Well played :0)
The ruler of the roost are the Intels however I will be able to afford one of those when there are cows enjoying themselfs by dancing on the moon. My next upgrade will be a Vertex - not only bc its Valu for money but equally much bc. OCZ obviously takes care of thier customers and listens to reason.
pmonti80 - Wednesday, March 18, 2009 - link
This is the kind of article that makes me come back here.nowayout99 - Wednesday, March 18, 2009 - link
OCZ, you should listen to Uncle Anand. ;) Hopefully Mr. Petersen understands that it's tough love.And the final product seems perfectly cool -- great performance at a better price than Intel. It's the first SSD I'd be able to reasonably consider.
SOLIDNecro - Wednesday, March 18, 2009 - link
Thx for this article Anand, I have been in a hotly contested debate over OCZ vs Samsung with my "Asperger Enhanced" nemisis/close friend...(In all fairness, I should mention I use the BiPolar SSE instruction set myself)
He was only looking at Samsung, I said he should look into what OCZ has now.
His reply was "I don't know them, and don't want to be disapointed"
(Long story behind that...He's from the Server/Workstain/HPC crowd, I am from the hardcore OC/Gamer/Desktop group, so he is not familiar with OCZ)
Looks like the Samsung (And alot of others) has "Issues" with performance degrading over time that are somewhat solved by Intel and OCZ (Plus maybe a few other companies that use the Rev B JMicron controller on there low cost SSD's)
I agree the OCZ Vertex offers the best bang for low buck SSD today, and I am tempted to grab one. But a year from now, anyone that bought a current gen MLC SSD will be saying "I coulda had a V-8" if that TRIM technology does what it promises!!!
James5mith - Wednesday, March 18, 2009 - link
As people continue to try and push the envelope of storage performance in a variety of ways, and as 6gbps SATA becomes available, the performance of SSD's will only go up.As always, I wanted to say thanks for the great article and keep them coming. It's the only way the rest of us can keep pace with what's happening out there in the world of performance storage.
vailr - Wednesday, March 18, 2009 - link
Is there any benefit in using 2 SSD's in a Raid 0 configuration?And: any differences between motherboard Intel Raid vs. a Raid controller card from Areca, for example. Also: can the "Trim" command work while in Raid mode? Probably not, I'm guessing...
7Enigma - Thursday, March 19, 2009 - link
Raid0 is really the holy grail for SSD's. The low risk of failure of SSD's which normally makes Raid0 with typical mechanical HD's more dangerous is very appealing. My personal storage-size goal is ~120-160gigs. Once they reach that size for under $300 I think I'm going to jump in. But I'm more likely to grab 2 60's or 2 80's and Raid0 them than get a single large SSD. The added performance will outweigh the higher power draw of 2 drives, and should make them extremely competitive with Intel's offerings (or whatever holds the crown at the time).I figure it will be about a year or so until the prices are in that range, as 2 60gig Vertex drives will currently run you about $400 after rebate.
I can't wait to jump on that upgrade and will then put my current 250gig mechanical drive as the storage drive (I don't use a ton of space in general as I have a 320gig external backup).
Rasterman - Thursday, March 19, 2009 - link
The problem with doing that is if you want to move your drives to another system they won't work, so upgrading is a pain. You could image them I guess, but plugging one drive in is much simpler. I had an older XP install that made it through 3-4 different systems.I would also question real world results, if you're going at 250MB/s or 500MB/s its not even going to be noticeable unless you are doing some massive video editing or some other huge file operations, and as Anand says, SSDs don't fill this role right now as they are super expensive per GB. So if you really are editing video a lot, you are going to need a hell of a lot more space than SSDs can offer you.
Gasaraki88 - Friday, March 20, 2009 - link
RAID is a universal standard so if you take two RAID0 drives out and move them to another computer with a RAID controller, it SHOULD just work if the original RAID was doing it correctly.