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-
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