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Re: Is disabling doublewrite safe on ZFS?


On 20/08/2018 21:40, Reinis Rozitis wrote:
I reread my previous mail and I have been a bit unclear - what I wanted to say that turning off doublewrite feels (at least for me) safe enough (compared to other disasters you could get into), but it's not 100% safe. (commenting on the initial "From my understanding, disabling doublebuffer is really 100% safe only when enabling atomic writes").

Ok, understood.

For the application (mysql) itself to perform any sort of checksum (after a crash boot) and possibility to recover you have to write (and keep until the second write is acknowledged) data twice anyways hence the doublewrite buffer.


Those atomic write hardware bits the same as ZFS [1] just ensure that any write either lands on the metal (which also tend to lie - write caches on drives/controllers, silent errors, bit rot etc) or not. If the application itself issues a 4KB write when it actually needed to write 8KB.

What puzzles me it that ext4 with data=journal *should* guarantee atomic writes also, but disabling doublewrites led to InnoDB corruption in case of mysqld crash/kill

It has been covered in some recent Percona blog comments [2] (by some authorities in mysql world)

[2] https://www.percona.com/blog/2017/12/07/hands-look-zfs-with-mysql/#comment-10968768

Thanks for the link. The problem I have with such blog is that, in the past, it gave incorrect information on doublewrite safety guarantees [1] Even in the comments section of the link you posted there are users warning against disabling checksum (which is another can of worms, for sure).

Well, it seems I just need to do some in-house testing... :p
I'll report back any interesting findings.


[1] https://www.percona.com/blog/2015/06/17/update-on-the-innodb-double-write-buffer-and-ext4-transactions/

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