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Re: Is disabling doublewrite safe on ZFS?


> Hi all,
> anyone with some suggestion/insight on the matter?

While I can't comment on the intricacies or internals of MySQL being (un)able to recover after a crash without the doublewrite buffer, if you skim through the changelog between versions (be that upstream Oracle or downstream in Maria/Percona), nearly every second (even minor) version has some sort of dataloss/corruption/segfault type of bug. Just for example a recent comes into mind https://jira.mariadb.org/browse/MDEV-15764 

>From my experience I've been switching off doublewrite on MySQL (even on XFS and now on ZFS (because of compression)) for years and even in the few accidental powerloss/total crash cases I haven't seen a corruption caused by an unexpected reboot (possible write lost midflight). Most times mysql hasn't been unable to start just because of internal issues (which you solve by having slaves and backups). 

My point being - zfs in principle is the same as the "atomic write hardware" (eg either the block writes succeed fully or not at all) so if you turn off doublewrite on those fancy Fusionio cards, I don't see a reason why you can't do the same on zfs. Even if there are some edge cases where it could become "unsafe" most of the time you still run with better performance and considering the SSD wear level the hardware could fail (reach end of life) ~two times sooner ;)

p.s. sorry for the mail not being about the particular technical aspects rather than general thoughts


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