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Solid State Drives Tested With TRIM Support
Vigile writes "Despite the rising excitement over SSDs, some of it has been tempered by performance degradation issues. The promised land is supposed to be the mighty TRIM command — a way for the OS to indicate to the SSD a range of blocks that are no longer needed because of deleted files. Apparently Windows 7 will implement TRIM of some kind but for now you can use a proprietary TRIM tool on a few select SSDs using Indilinx controllers. A new article at PC Perspective evaluates performance on a pair of Indilinx drives as well as the TRIM utility and its efficacy."
I finally got the opportunity to test out SSDs this year. There may be the odd teething problem to get over, but in my mind there is no market in the future for mechanical drives except maybe as cheap low-speed devices for storing non-critical information... in much the same way as tape drives were used a few years ago.
Disclaimer: I am not a SSD firmware author, although I've spoken to a few.*
As best I can understand it, the problem is that writes are scattered across the physical media by wear-leveling firmware on the disk. In order to do this, the firmware must have a "free list" of sorts that allows it to find an un-worn area for the next write. Of course, this unworn area also needs to not currently be storing any relevant data.
Now, consider a SSD in use. Initially, the whole disk is free, and writes can go anywhere at all. They do, too - you end up with meaningful (at some point) data covering the entirety of the physical memory cells pretty quickly (consider things like logfiles, pagefiles, hibernation data, temporary data, and so forth). Obviously, most of that data doesn't mean anything anymore - to the filesystem, only perhaps 20% of the SSD is actually used, after 6 months. However, the SSD's firmware things that every single part has now been used.
Obviously, the firmware needs to be able to detect when data on disk gets obsoleted, and can safely be deleted. The problems with this are that this leads to *very* complicated translation tables - logical disk blocks end up having no relation at all to physical ones, and the SSD needs to track those mappings. The other problem is that these tables get *huge* - a typical home system might have between 100K and 1M files on it after a few months of usage, but probably generates and deletes many thousands per day (consider web site cookies, for example - each time they get updated, the wear leveling will write that data to a new portion of the physical storage).
Maintaining the tables themselves is possible, and when a logical block gets overwritten to a new physical location, the old location can be freed. The problem is that this freeing comes at the same time that the SSD needs to find a new location to write to, and the only knowledge it has about physical blocks which can safely be overwritten is ones where the logical block has been overwritten already (to a different physical location). Obviously, the lookup into the table of active blocks has to be indexed by logical block, which may make it difficult to locate the oldest "free" physical blocks. This could lead to searches that, even with near-instant IO, result in noticeable slowdowns.
Enter the TRIM command, whereby an OS can tell the SSD that a given range of logical blocks (which haven't been overwritten yet) are now able to be recycled. This command allows the SSD to identify physical blocks which can safely be overwritten, and place them in its physical write queue, before the next write command comes down from the disk controller. It's unlikely to be a magic bullet, but should improve things substantially.
* As stated above, I don't personally write this stuff, so I may be mis-remembering or mis-interpreting. If anybody can explain it better, please do.
There's no place I could be, since I've found Serenity...
If you go to use that free space later, you find that area, and drop shit into it.
Knock it off with all the fancy jargon!
Don't blame me, I voted for Baltar.
For a thorough (RE: long) primer on SSDs and long term performance woes, Anand's overview is a must read.
No sig for you!!
Once upon a time, a technical subject on /. gave insightful and informative responses that were modded up. Time changes, I guess.
The "fragmentation" that SSD drive have don't really come from wear leveling, or from having to find some place to write things, but from the following properties:
* Filesystems read and write 4KiB pages.
* SSD can read many time 4KiB pages FAST, can write ONCE 4KiB pages FAST, but can only erase a whole 512KiB blocks SLOWLY.
When the drive is mostly empty, the SSD have no trouble finding blanks area to store the 4KiB write from the OS (he can even cheat with wear leveling to re-locate 4K pages to blank spaces when the OS re-write the same block). After some usage, ALL THE DRIVE HAVE BEEN WRITTEN TO ONCE. From the point of view of the SSD all the disk is full. From the point of view of the filesystem, there is unallocated space (for instance, space occupied for files that have been deleted).
At this point, when the OS send a write command to a specific page, the SSD is forced to to the following:
* read the 512KiB block that contain the page
* erase the block (SLOW)
* modify the page
* write back the 512KiB block
Of course, various kludges/caches are used to limit the issue, but the end result is here: writes are getting slow, and small writes are getting very slow.
The TRIM command is a command that tell the SSD drive that some 4KiB page can be safely erased (because it contains data from a delete file, for instance), and the SSD stores a map of the TRIM status of each page.
Then the SSD can do one of the following two things:
* If all the pages of a block are TRIMed, it can asynchronously erase the block. So, the next 4KiB write can be relocated to that block with free space, and also the 127 next 4KiB writes.
* If a write request come and there is no space to write data to, the drive can READ/ERASE/MODIFY/WRITE the block with most TRIMed space, which will speed up the next few writes.
(of course, you can have more complex algorithms to pre-erase at the cost of additional wear)