On Fri, Mar 16, 2018 at 5:38 PM, Richard Elling < [email protected]> wrote:
> On Mar 15, 2018, at 9:48 PM, Mike Gerdts <[email protected]> wrote: > I had started down that route, then convinced myself that it may be there > for good reason. Can you explain how a refreservation greater than the > size calculated by zvol_volsize_to_reservation() is useful? I couldn't > come up with a way (aside from a potential bug in that copies is not always > accounted for). > > > Sure, raidz skip blocks are not accounted for. In part this is logically > due to skip blocks being assigned > at the SPA layer and reservations are at the DSL layer. The pathological > example is raidz2 on 4kn disks > with volblocksize=8k (default). The predicted reservation is 8k per block > (logical) plus 8k parity = 16k, but > the actual allocated space is 24k. The DSL "free" space assumes 16k so it > overestimates the usable space. > Thus you can run out of allocated space in the pool before hitting > refreservation -- a bad thing. > One way to innoculate is to increase refreservation to a value greater > than volsize. > > Thanks for that lesson. It seems as though it would be better to fix zvol_volsize_to_reservation() to account for this rather than to leave it to system operators to know about this and then each independently come up with the appropriate algorithm to reserve the right amount of space. Is the algorithm for performing the proper calculation written down somewhere that is publicly accessible? Thanks, Mike ------------------------------------------ openzfs: openzfs-developer Permalink: https://openzfs.topicbox.com/groups/developer/discussions/Te3d593ba00521b6d-Me3f4e917afd977dad1afa1a0 Delivery options: https://openzfs.topicbox.com/groups
