On Mon, Apr 13, 2009 at 07:03, Robert Milkowski <mi...@task.gda.pl> wrote: > Hello Daniel, > > Thursday, April 9, 2009, 3:35:07 PM, you wrote: > > DR> Jonathan schrieb: >>> OpenSolaris Forums wrote: >>>> if you have a snapshot of your files and rsync the same files again, >>>> you need to use "--inplace" rsync option , otherwise completely new >>>> blocks will be allocated for the new files. that`s because rsync will >>>> write entirely new file and rename it over the old one. >>> >>> ZFS will allocate new blocks either way > > DR> No it won't. --inplace doesn't rewrite blocks identical on source and > DR> target but only blocks which have been changed. > > Yes, it will. Inplace in rsync has nothing to do with how ZFS works. But it has big consequences on how rsync uses the file system, and thus big consequences on how ZFS behaves. --inplace means rsync walks through the file on the receiving end until it finds a mismatch, and only then does it write new blocks to disk.
> Now with inplce you're telling rsync to overwrite any changed blocks > directly over the original file instead of making a full copy of a > file. Everytime you overwrite some data zfs will allocate new blocks > only for those blocks and keep the original blocks as long as they are > referenced by at least one snapshot. Exactly. But the consequence of this is that with no --inplace, rsync+snapshots balloon space usage under ZFS, and with --inplace you don't get that behavior. Perhaps ZFS could do some very simplistic de-dup here: it has the B-tree entry for the file in question when it goes to overwrite a piece of it, so it could calculate the checksum of the new block and see if it matches the checksum for the block it is overwriting. This is a much simpler case than the global de-dup that's in the works: only one checksum needs to be compared to one other, compared to the needle-in-haystack problem of dedup on the whole pool. Will _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
