On 31 dec 2009, at 17.18, Bob Friesenhahn wrote: > On Thu, 31 Dec 2009, Ragnar Sundblad wrote: >> >> Also, currently, when the SSDs for some very strange reason is >> constructed from flash chips designed for firmware and slowly >> changing configuration data and can only erase in very large chunks, >> TRIMing is good for the housekeeping in the SSD drive. A typical >> use case for this would be a laptop. > > I have heard quite a few times that TRIM is "good" for SSD drives but I don't > see much actual use for it. Every responsible SSD drive maintains a reserve > of unused space (20-50%) since it is needed for wear leveling and to repair > failing spots. This means that even when a SSD is 100% full it still has > considerable space remaining.
(At least as long as those blocks aren't used up in place of bad/worn out) blocks...) > A very simple SSD design solution is that when a SSD block is "overwritten" > it is replaced with an already-erased block from the free pool and the old > block is submitted to the free pool for eventual erasure and re-use. This > approach avoids adding erase times to the write latency as long as the device > can erase as fast as the average date write rate. This is what they do, as far as I have understood, but more free space to play with makes the job easier and therefor faster, and gives you a larger burst headroom before you hit the erase-speed limit of the disk. > There are of course SSDs with hardly any (or no) reserve space, but while we > might be willing to sacrifice an image or two to SSD block failure in our > digital camera, that is just not acceptable for serious computer use. I think the idea is that with TRIM you can also use the file system's unused space for wear leveling and flash block filling. If your disk is completely full there is of course no gain. /ragge s _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
