Re: [PATCH v2 28/29] migration: Add direct-io parameter

[Fabiano Rosas]

Daniel P. Berrangé <berrange@redhat.com> writes:

> On Mon, Oct 23, 2023 at 05:36:07PM -0300, Fabiano Rosas wrote:
>> Add the direct-io migration parameter that tells the migration code to
>> use O_DIRECT when opening the migration stream file whenever possible.
>> 
>> This is currently only used for the secondary channels of fixed-ram
>> migration, which can guarantee that writes are page aligned.
>
> When you say "secondary channels", I presume you're meaning that
> the bulk memory regions will be written with O_DIRECT, while
> general vmstate will use normal I/O on the main channel ?  If so,
> could we explain that a little more directly.

Yes, the main channel writes via QEMUFile, so no O_DIRECT. The channels
created via multifd_new_send_channel_create() have O_DIRECT enabled.

> Having a mixture of O_DIRECT and non-O_DIRECT I/O on the same
> file is a little bit of an unusual situation. It will work for
> us because we're writing to different regions of the file in
> each case.
>
> Still I wonder if it would be sane in the outgoing case to
> include a fsync() on the file in the main channel, to guarantee
> that the whole saved file is on-media at completion ? Or perhaps
> suggest in QAPI that mgmts might consider doing a fsync
> themselves ?

I think that should be present in QIOChannelFile in general. Not even
related to this series. I'll add it at qio_channel_file_close() unless
you object.