Bob Friesenhahn wrote: > Something else came to mind which is a negative regarding > deduplication. When zfs writes new sequential files, it > should try to > allocate blocks in a way which minimizes "fragmentation" > (disk seeks).
It should, but because of its copy-on-write nature, fragmentation is a significant part of the ZFS data lifecycle. There was a discussion of this on this list at the beginning of the year... http://mail.opensolaris.org/pipermail/zfs-discuss/2007-November/044077.h tml > Disk seeks are the bane of existing storage systems since they come > out of the available IOPS budget, which is only a couple hundred > ops/second per drive. The deduplication algorithm will surely result > in increasing effective fragmentation (decreasing sequential > performance) since duplicated blocks will result in a seek to the > master copy of the block followed by a seek to the next block. Disk > seeks will remain an issue until rotating media goes away, which (in > spite of popular opinion) is likely quite a while from now. On ZFS, sequential files are rarely sequential anyway. The SPA tries to keep blocks nearby, but when dealing with snapshotted sequential files being rewritten, there is no way to keep everything in order. But if you read through the thread referenced above, you'll see that there's no clear data about just how that impacts performance (I still owe Mr. Elling a filebench run on one of my spare servers) --Joe _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
