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.
Exactly.
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.
Thanks.
[1]
https://www.percona.com/blog/2015/06/17/update-on-the-innodb-double-write-buffer-and-ext4-transactions/
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: [email protected] - [email protected]
GPG public key ID: FF5F32A8
_______________________________________________
Mailing list: https://launchpad.net/~maria-discuss
Post to : [email protected]
Unsubscribe : https://launchpad.net/~maria-discuss
More help : https://help.launchpad.net/ListHelp
