Migrating from NetApp to ZFS

[Frido Otten]

Hi All,

We're currently in the process of migrating from a NetApp storage to a
ZFS storage, both shared over NFS. We've ran into some problems which
were solved by switching to dotlocks instead of flock. But now there's
one problem left with the dotlock method. When connecting to
Dovecot/IMAP, and moving messages from one folder to another, it is very
slow and I'm suspecting this has all to do with the dotlock method.
During this process of moving mail, the dotlock file is created and
stays there for about 90 seconds. The mail client is unresponsive in
this time period. This happens also when a new message is delivered to
the mailbox.
The dovecot example config talks about that dotlocking is using some
tricks which may create more disk I/O than other locking methods. What
are these tricks and can it be the cause of the slowness?
We've tested it on different ZFS storages and the problem seems to only
exist on ZFS storages which have their ZIL on SSD's
Just creating and removing a file from the CLI is fast though.

Regards,
Frido

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