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

[Fabiano Rosas <farosas@suse.de>]

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

> On Wed, Oct 25, 2023 at 02:30:01PM -0300, Fabiano Rosas wrote:
>> Daniel P. Berrangé <berrange@redhat.com> writes:
>> 
>> > On Wed, Oct 25, 2023 at 11:32:00AM -0300, Fabiano Rosas wrote:
>> >> Daniel P. Berrangé <berrange@redhat.com> writes:
>> >> 
>> >> > On Tue, Oct 24, 2023 at 04:32:10PM -0300, Fabiano Rosas wrote:
>> >> >> Markus Armbruster <armbru@redhat.com> writes:
>> >> >> 
>> >> >> > Fabiano Rosas <farosas@suse.de> writes:
>> >> >> >
>> >> >> >> 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.
>> >> >> >>
>> >> >> >> However the parameter could be made to affect other types of
>> >> >> >> file-based migrations in the future.
>> >> >> >>
>> >> >> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
>> >> >> >
>> >> >> > When would you want to enable @direct-io, and when would you want to
>> >> >> > leave it disabled?
>> >> >> 
>> >> >> That depends on a performance analysis. You'd generally leave it
>> >> >> disabled unless there's some indication that the operating system is
>> >> >> having trouble draining the page cache.
>> >> >
>> >> > That's not the usage model I would suggest.
>> >> >
>> >> 
>> >> Hehe I took a shot at answering but I really wanted to say "ask Daniel".
>> >> 
>> >> > The biggest value of the page cache comes when it holds data that
>> >> > will be repeatedly accessed.
>> >> >
>> >> > When you are saving/restoring a guest to file, that data is used
>> >> > once only (assuming there's a large gap between save & restore).
>> >> > By using the page cache to save a big guest we essentially purge
>> >> > the page cache of most of its existing data that is likely to be
>> >> > reaccessed, to fill it up with data never to be reaccessed.
>> >> >
>> >> > I usually describe save/restore operations as trashing the page
>> >> > cache.
>> >> >
>> >> > IMHO, mgmt apps should request O_DIRECT always unless they expect
>> >> > the save/restore operation to run in quick succession, or if they
>> >> > know that the host has oodles of free RAM such that existing data
>> >> > in the page cache won't be trashed, or
>> >> 
>> >> Thanks, I'll try to incorporate this to some kind of doc in the next
>> >> version.
>> >> 
>> >> > if the host FS does not support O_DIRECT of course.
>> >> 
>> >> Should we try to probe for this and inform the user?
>> >
>> > qemu_open_internall will already do a nice error message. If it gets
>> > EINVAL when using O_DIRECT, it'll retry without O_DIRECT and if that
>> > works, it'll reoprt "filesystem does not support O_DIRECT"
>> >
>> > Having said that I see a problem with /dev/fdset handling, because
>> > we're only validating O_ACCMODE and that excludes O_DIRECT.
>> >
>> > If the mgmt apps passes an FD with O_DIRECT already set, then it
>> > won't work for VMstate saving which is unaligned.
>> >
>> > If the mgmt app passes an FD without O_DIRECT set, then we are
>> > not setting O_DIRECT for the multifd RAM threads.
>> 
>> Worse, the fds get dup'ed so even without O_DIRECT, we we enable it for
>> the secondary channels the main channel will break on unaligned writes.
>> 
>> For now I can only think of requiring two fds. One for the main channel
>> and a second one for the rest of the channels. And validating the fd
>> flags to make sure O_DIRECT is only allowed to be set in the second fd.
>
> In this new model I think there's no reason for libvirt to set O_DIRECT
> for its own initial I/O. So we could just totally ignore O_DIRECT when
> initially opening the QIOCHannelFile.
>

Yes. I still have to disallow setting it on the main channel just to be
safe.

> Then provide a method on QIOCHannelFile to enable O_DIRECT on the fly
> which can be called for the multifd threads setup ?

Sure, but there's not really an "on the fly" here, after
file_send_channel_create() returns the channel should be ready to
use. It would go from:

 flag |= O_DIRECT;
 qio_channel_file_new_path(...);

to:

 qio_channel_file_new_path(...);
 qio_channel_file_set_direct_io();

Which could be cleaner since the migration code doesn't have to check
for O_DIRECT support.