Hi Bob,
On Dec 15, 2009, at 6:41 PM, Bob Friesenhahn wrote:
On Tue, 15 Dec 2009, Bill Sprouse wrote:
Hi Everyone,
I hope this is the right forum for this question. A customer is
using a Thumper as an NFS file server to provide the mail store for
multiple email servers (Dovecot). They find that when a zpool is
freshly created and
It seems that Dovecot's speed optimizations for mbox format are
specially designed to break zfs
"http://wiki.dovecot.org/MailboxFormat/mbox#Dovecot.27s_Speed_Optimizations
"
and explains why using a tiny 8k recordsize temporarily "improved"
performance. Tiny updates seem to be abnormal for a mail server.
The many tiny updates combined with zfs COW conspire to spread the
data around the disk, requiring a seek for each 8k of data. If more
data was written at once, and much larger blocks were used, then the
filesystem would continue to perform much better, although perhaps
less well initially. If the system has sufficient RAM, or a large
enough L2ARC, then Dovecot's optimizations to diminish reads become
meaningless.
I think one of the reasons they went to small recordsizes was an issue
where they were getting killed with reads of small messages and having
to pull in 128K records each time. The smaller recordsizes seem to
have improved that aspect at least. Thanks for the pointer to the
Dovecot notes.
Is this expected behavior given the application (email - small,
random writes/reads)? Are there recommendations for system/ZFS/NFS
configurations to improve this sort of thing? Are there best
practices for structuring backups to avoid a directory walk?
Zfs works best when whole files are re-written rather than updated
in place as Dovecot seems to want to do. Either the user mailboxes
should be re-written entirely when they are "expunged" or else a
different mail storage format which writes entire files, or much
larger records, should be used.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
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