On 31 dec 2009, at 22.53, David Magda wrote: > On Dec 31, 2009, at 13:44, Joerg Schilling wrote: > >> ZFS is COW, but does the SSD know which block is "in use" and which is not? >> >> If the SSD did know whether a block is in use, it could erase unused blocks >> in advance. But what is an "unused block" on a filesystem that supports >> snapshots?
Snapshots make no difference - when you delete the last dataset/snapshot that references a file you also delete the data. Snapshots is a way to keep more files around, it is not a really way to keep the disk entirely full or anything like that. There is obviously no problem to distinguish between used and unused blocks, and zfs (or btrfs or similar) make no difference. > Personally, I think that at some point in the future there will need to be a > command telling SSDs that the file system will take care of handling blocks, > as new FS designs will be COW. ZFS is the first "mainstream" one to do it, > but Btrfs is there as well, and it looks like Apple will be making its own FS. That could be an idea, but there still will be holes after deleted files that need to be reclaimed. Do you mean it would be a major win to have the file system take care of the space reclaiming instead of the drive? > Just as the first 4096-byte block disks are silently emulating 4096 -to-512 > blocks, SSDs are currently re-mapping LBAs behind the scenes. Perhaps in the > future there will be a setting to say "no really, I'm talking about the > /actual/ LBA 123456". A typical flash page size is 512 KB. You probably don't want to use all the physical pages, since those could be worn out or bad, so those need to be remapped (or otherwise avoided) at some level anyway. These days, typically disks do the remapping without the host computer knowing (both SSDs and rotating rust). I see the possible win that you could always use all the working blocks on the disk, and when blocks goes bad your disk will shrink. I am not sure that is really what people expect, though. Apart from that, I am not sure what the gain would be. Could you elaborate on why this would be called for? /ragge _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
