Re: [PATCH v2 18/18] migration/ram: Add direct-io support to precopy file migration

[Fabiano Rosas]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jun 12, 2024 at 07:15:19PM +0100, Daniel P. Berrangé wrote:
>> IIUC, with the "fixed-ram" feature, the on-disk format of a saved VM
>> should end up the same whether we're using traditional migration, or
>> multifd migration. Use of multifd is simply an optimization that lets
>> us write RAM in parallel to the file, with direct-io further optimizing.
>> 
>> There's also a clear break with libvirt between the existing on-disk
>> format libvirt uses, and the new fixed-ram format. So we have no backwards
>> compatibilty concerns added from multifd, beyond what we already have to
>> figure out when deciding on use of 'fixed-ram'. 
>> 
>> Thus I believe there is no downside to always using multifd for save
>> images with fixed-ram, even if we only want nchannels=1.
>
> That sounds good.
>
> Just to double check with all of us: so we allow mapped-ram to be used in
> whatever case when !dio, however we restrict dio only when with multifd=on,
> am I right?

Yes. The restricting part is not yet in place. I'll add a multifd check
to migrate_direct_io():

bool migrate_direct_io(void)
{
    MigrationState *s = migrate_get_current();

    return s->parameters.direct_io &&
        s->capabilities[MIGRATION_CAPABILITY_MAPPED_RAM] &&
        s->capabilities[MIGRATION_CAPABILITY_MULTIFD];
}