The SSD Relapse: Understanding and Choosing the Best SSD
by Anand Lal Shimpi on August 30, 2009 12:00 AM EST- Posted in
- Storage
The Instruction That Changes (almost) Everything: TRIM
TRIM is an interesting command. It lets the SSD prioritize blocks for cleaning. In the example I used before, a block is cleaned only when the drive runs out of places to write things and has to dip into its spare area. With TRIM, if you delete a file, the OS sends a TRIM command to the drive along with the associated LBAs that are no longer needed. The TRIM command tells the drive that it can schedule those blocks for cleaning and add them to the pool of replacement blocks.
A used SSD will only have its spare area to use as a scratch pad for moving data around; on most consumer drives that’s around 7%. Take a look at this graph from a study IBM did on SSD performance:
Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory
Note how dramatically write amplification goes down when you increase the percentage of spare area the drive has. In order to get down to a write amplification factor of 1 our spare area needs to be somewhere in the 10 - 30% range, depending on how much of the data on our drive is static.
Remember our pool of replacement blocks? This graph actually assumes that we have multiple pools of replacement blocks. One for frequently changing data (e.g. file tables, pagefile, other random writes) and one for static data (e.g. installed applications, data). If the SSD controller only implements a single pool of replacement blocks, the spare area requirements are much higher:
Write Amplification vs. Spare Area, courtesy of IBM Zurich Research Laboratory
We’re looking at a minimum of 30% spare area for this simpler algorithm. Some models don’t even drop down to 1.0x write amplification.
But remember, today’s consumer drives only ship with roughly 6 - 7% spare area on them. That’s under the 10% minimum even from our more sophisticated controller example. By comparison, the enterprise SSDs like Intel’s X25-E ship with more spare area - in this case 20%.
What TRIM does is help give well architected controllers like that in the X25-M more spare area. Space you’re not using on the drive, space that has been TRIMed, can now be used in the pool of replacement blocks. And as IBM’s study shows, that can go a long way to improving performance depending on your workload.
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sotoa - Friday, September 4, 2009 - link
Another great article. You making me drool over these SSD's!I can't wait till Win7 comes to my door so I can finally get an SSD for my laptop.
Hopefully prices will drop some more by then and Trim firmware will be available.
lordmetroid - Thursday, September 3, 2009 - link
I use them both because they are damn good and explanatory suffixes. It is 2009, soon 2010 I think we can at least get the suffixes correct, if someone doesn't know what they mean, wikipedia has answers.AnnonymousCoward - Saturday, September 5, 2009 - link
As someone who's particular about using SI and being correct, I think it's better to stick to GB for the sake of simplicity and consistency. The tiny inaccuracy is almost always irrelevant, and as long as all storage products advertise in GB, it wouldn't make sense to speak in terms of GiB.Touche - Thursday, September 3, 2009 - link
Both articles emphasize Intel's performance lead, but, looking at real world tests, the difference between it and Vertex is really small. Not hardly enough to justify the price difference. I feel like the articles are giving an impression that Intel is in a league of its own when in fact it's only marginally faster.smjohns - Tuesday, September 8, 2009 - link
This is where I struggle. It is all very well quoting lots of stats about all these drives but what I really want to know is if I went for Intel over the OCZ Vertex (non-turbo) where would I really notice the difference in performance on a laptop?Would it be slower start up / shut down?
Slower application response times?
Speed at opening large zipped files?
Copying / processing large video files?
If the difference is that slim then I guess it is down to just a personal preference....
morrie - Thursday, September 3, 2009 - link
I've made it a habit of securely deleting files by using "shred" like this: shred -fuvz, and accepting the default number of passes, 25. Looks like this security practice is now out, as the "wear" on the drive would be at least 25x faster, bringing the stated life cycles closer to having an impact on drive longevity. So what's the alternative solution for securely deleting a file? Got to "delete" and forget about security? Or "shred" with a lower number of passes, say 7 or 10, and be sure to purchase a non-Intel drive with the ten year warranty and hope that the company is still in business, and in the hard drive business, should you need warranty service in the outer years...Rasterman - Wednesday, September 16, 2009 - link
watching too much CSI, there is an article somewhere i read by a data repair tech who works in one of the multi-million dollar data recovery labs, basically he said writing over it once is all you should do and even that is overkill 99% of the time. theoretically it is possible to even recover that _sometimes_, but the expense required is so high that unless you are committing a billion dollar fraud or are the secretary to osama bin laden no one will ever try to recover such data. chances are if you are in such circles you can afford a new drive 25x more often. and if you have such information or knowledge wouldn't be far easier and cheaper to simply beat it out of you than trying to recover a deleted drive?iamezza - Friday, September 4, 2009 - link
1 pass should be sufficient for most purposes. Unless you happen to be working on some _extremely_ sensitive/important data.derkurt - Thursday, September 3, 2009 - link
I may be wrong on this, but I'd assume that once TRIM is enabled, a file is securely deleted if it has been deleted on the filesystem level. However, it might depend on the firmware when exactly the drive is going to actually delete the flash blocks which are marked as deletable by TRIM. For performance reasons the drive should do that as soon as possible after a TRIM command, but also preferably at a time when there is not much "action" going on - after all, the whole point of TRIM is to change the time of block erasing flash cells to a point where the drive is idle.
morrie - Thursday, September 3, 2009 - link
That's on a Linux system btwAs to aligning drives...how about an update to the article on what needs to be done/ensured, if anything, for using the drives with a Linux OS?