On Tue, Apr 06, 2010 at 01:44:20PM -0400, Tony MacDoodle wrote: > I am trying to understand how "refreservation" works with snapshots. > > If I have a 100G zfs pool........ > > I have 4 20G volume groups in that pool. > > refreservation = 20G on all volume groups. > > Now when I want to do a snapshot will this snapshot need 20G + the amount > changed (REFER)? If not I get a "out of space"......... > > How does refreservation relate to snapshots?
The refreservation is a committment that X amount of space can be written to. When space currently in the volume (usedbydataset) is shared with snapshots, new writes to those blocks will need to allocate new space, and the original copy remains in the snapshot. Therefore, as a snapshot is taken, the "usedbyrefreservation" figure is increased from whatever its current value, back up to the size of the refreservation. This represents the committment in advance of space from the pool to hold the potential overwrite of the dataset, as well as the new snapshot. If there's not enough pool space for this increase, the snapshot is denied. You have reminded me.. I wrote some patches to the zfs manpage to help clarify this issue, while travelling, and never got around to posting them when I got back. I'll dig them up off my netbook later today. -- Dan.
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