← Back to team overview

maria-discuss team mailing list archive

Re: Is disabling doublewrite safe on ZFS?


2018-08-21 11:16 GMT+03:00 Gionatan Danti <g.danti@xxxxxxxxxx>:
> On 20/08/2018 21:40, Reinis Rozitis wrote:
>> 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

I believe that the Linux kernel can interrupt any write at 4096-byte
boundaries when a signal is delivered to the process.
I am curious: Where was it claimed that data=journal guarantees atomic
writes (other than [1])?
I would expect it to only guarantee that anything that was written to
the journal will be durable.
Whether the actual write request was honored in full is a separate matter.

>> 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/

Please report back any findings, whether or not you consider them to
be interesting.

I believe that it is technically possible for a copy-on-write
filesystem like ZFS
to support atomic writes, but for that to be possible in practice, the
inside the kernel must be implemented in an appropriate way.
Disclaimer: I have no knowledge of the implementation details of any kernel.
Marko Mäkelä, Lead Developer InnoDB
MariaDB Corporation

Follow ups