On Sep 12, 2010, at 8:27 PM, Edward Ned Harvey wrote: >> From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- >> boun...@opensolaris.org] On Behalf Of Orvar Korvar >> >> I am not really worried about fragmentation. I was just wondering if I >> attach new drives and zfs send recieve to a new zpool, would count as >> defrag. But apparently, not. > > "Apparently not in all situations" would be more appropriate. > > The understanding I had was: If you send a single zfs send | receive, then > it does effectively get defragmented, because the receiving filesystem is > going to re-layout the received filesystem, and there is nothing > pre-existing to make the receiving filesystem dance around... But if you're > sending some initial, plus incrementals, then you're actually repeating the > same operations that probably caused the original filesystem to become > fragmented in the first place. And in fact, it seems unavoidable... > > Suppose you have a large file, which is all sequential on disk. You make a > snapshot of it. Which means all the individual blocks must not be > overwritten. And then you overwrite a few bytes scattered randomly in the > middle of the file. The nature of copy on write is such that of course, the > latest version of the filesystem is impossible to remain contiguous. Your > only choices are: To read & write copies of the whole file, including > multiple copies of what didn't change, or you leave the existing data in > place where it is on disk, and you instead write your new random bytes to > other non-contiguous locations on disk. Hence fragmentation.
This operational definition of "fragmentation" comes from the single-user, single-tasking world (PeeCees). In that world, only one thread writes files from one application at one time. In those cases, there is a reasonable expectation that a single file's blocks might be contiguous on a single disk. That isn't the world we live in, where have RAID, multi-user, or multi-threaded environments. -- richard -- OpenStorage Summit, October 25-27, Palo Alto, CA http://nexenta-summit2010.eventbrite.com Richard Elling rich...@nexenta.com +1-760-896-4422 Enterprise class storage for everyone www.nexenta.com _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
