Greg Mason wrote:
What is the downtime for doing a send/receive? What is the downtime
for zpool export, reconfigure LUN, zpool import?
We have a similar situation. Our home directory storage is based on
many X4540s. Currently, we use rsync to migrate volumes between
systems, but our process could very easily be switched over to zfs
send/receive (and very well may be in the near future).
What this looks like, if using zfs send/receive, is we perform an
initial send (get the bulk of the data over), and then at a planned
downtime, do an incremental send to "catch up" the destination. This
"catch up" phase is usually a very small fraction of the overall size
of the volume. The only downtime required is from just before the
final snapshot you send (the last incremental), and when the send
finishes, and turning up whatever service(s) on the destination
system. If the filesystem a lot of write activity, you can run
multiple incrementals to decrease the size of that last snapshot. As
far as backing out goes, you can simply destroy the destination
filesystem, and continue running on the original system, if all hell
breaks loose (of course that never happens, right? :)
That is how I migrate services (zones) and their data between hosts with
one of my clients. The big advantage of zfs send/receive over rsync is
the final replication is very fast. Run a send/receive just before the
migration than top up after the service shuts down. The last one we
moved was a mail server with 1TB of small files and the downtime was
under 2 minutes. The biggest delay was sending the "start" and "done"
text messages!
When everything checks out (which you can safely assume when the recv
finishes, thanks to how ZFS send/recv works), you then just have to
destroy the original fileystem. It is correct in that this doesn't
shrink the pool, but it's at least a workaround to be able to swing
filesystems around to different systems. If you had only one
filesystem in the pool, you could then safely destroy the original
pool. This does mean you'd need 2x the size of the LUN during the
transfer though.
For replication of ZFS filesystems, we a similar process, with just a
lot of incremental sends.
Same here.
--
Ian.
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