On 2 jan 2010, at 13.10, Erik Trimble wrote: > Joerg Schilling wrote: >> Ragnar Sundblad <ra...@csc.kth.se> wrote: >> >> >>> On 1 jan 2010, at 17.28, David Magda wrote: >>> >> >> >>>> Don't really see how things are either hardware specific or dependent. >>>> >>> The inner workings of a SSD flash drive is pretty hardware (or >>> rather vendor) specific, and it may not be a good idea to move >>> any knowledge about that to the file system layer. >>> >> >> If ZFS likes to keep SSDs fast even after it was in use for a while, then >> even ZFS would need to tell the SSD which sectors are no longer in use. >> >> >> Such a mode may cause a noticable performance loss as ZFS for this reason >> may need to traverse freed outdated data trees but it will help the SSD >> to erase the needed space in advance. >> >> Jör > the TRIM command is what is intended for an OS to notify the SSD as to which > blocks are deleted/erased, so the SSD's internal free list can be updated > (that is, it allows formerly-in-use blocks to be moved to the free list). > This is necessary since only the OS has the information to determine which > previous-written-to blocks are actually no longer in-use. > > See the parallel discussion here titled "preview of new SSD based on > SandForce controller" for more about "smart" vs "dumb" SSD controllers. > > From ZFS's standpoint, the optimal configuration would be for the SSD to > inform ZFS as to it's PAGE size, and ZFS would use this as the fundamental > BLOCK size for that device (i.e. all writes are in integer multiples of the > SSD page size). Reads could be in smaller sections, though. Which would be > interesting: ZFS would write in Page Size increments, and read in Block Size > amounts.
Well, this could be useful if updates are larger than the block size, for example 512 K, as it is then possible to erase and rewrite without having to copy around other data from the page. If updates are smaller, zfs will have to reclaim erased space by itself, which if I am not mistaken it can not do today (but probably will in some future, I guess the BP Rewrite is what is needed). I am still not entirely convinced that it would be better to let the file system take care of that instead of a flash controller, there could be quite a lot of reading and writing going on for space reclamation (depending on the work load, of course). /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
