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

[Daniel P . Berrangé]

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.

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 ?


With regards,
Daniel
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