On Mon, Jan 16, 2012 at 09:13:03AM -0600, Bob Friesenhahn wrote: > On Mon, 16 Jan 2012, Jim Klimov wrote: > > > >I think that in order to create a truly fragmented ZFS layout, > >Edward needs to do sync writes (without a ZIL?) so that every > >block and its metadata go to disk (coalesced as they may be) > >and no two blocks of the file would be sequenced on disk together. > >Although creating snapshots should give that effect... > > In my experience, most files on Unix systems are re-written from > scatch. For example, when one edits a file in an editor, the editor > loads the file into memory, performs the edit, and then writes out > the whole file. Given sufficient free disk space, these files are > unlikely to be fragmented. > > The case of slowly written log files or random-access databases are > the worse cases for causing fragmentation.
The case I've seen was with an IMAP server with many users. E-mail folders were represented as ZFS directories, and e-mail messages as files within those directories. New messages arrived randomly in the INBOX folder, so that those files were written all over the place on the storage. Users also deleted many messages from their INBOX folder, but the files were retained in snapshots for two weeks. On IMAP session startup, the server typically had to read all of the messages in the INBOX folder, making this portion slow. The server also had to refresh the folder whenever new messages arrived, making that portion slow as well. Performance degraded when the storage became 50% full. It would increase markedly when the oldest snapshot was deleted. -- -Gary Mills- -refurb- -Winnipeg, Manitoba, Canada- _______________________________________________ zfs-discuss mailing list email@example.com http://mail.opensolaris.org/mailman/listinfo/zfs-discuss