Re: [RFC PATCH v3 00/30] migration: File based migration with multifd and fixed-ram

[Fabiano Rosas <farosas@suse.de>]

Peter Xu <peterx@redhat.com> writes:

> On Thu, Jan 11, 2024 at 03:38:31PM -0300, Fabiano Rosas wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> 
>> > On Mon, Nov 27, 2023 at 05:25:42PM -0300, Fabiano Rosas wrote:
>> >> Hi,
>> >> 
>> >> In this v3:
>> >> 
>> >> Added support for the "file:/dev/fdset/" syntax to receive multiple
>> >> file descriptors. This allows the management layer to open the
>> >> migration file beforehand and pass the file descriptors to QEMU. We
>> >> need more than one fd to be able to use O_DIRECT concurrently with
>> >> unaligned writes.
>> >> 
>> >> Dropped the auto-pause capability. That discussion was kind of
>> >> stuck. We can revisit optimizations for non-live scenarios once the
>> >> series is more mature/merged.
>> >> 
>> >> Changed the multifd incoming side to use a more generic data structure
>> >> instead of MultiFDPages_t. This allows multifd to restore the ram
>> >> using larger chunks.
>> >> 
>> >> The rest are minor changes, I have noted them in the patches
>> >> themselves.
>> >
>> > Fabiano,
>> >
>> > Could you always keep a section around in the cover letter (and also in the
>> > upcoming doc file fixed-ram.rst) on the benefits of this feature?
>> >
>> > Please bare with me - I can start to ask silly questions.
>> >
>> 
>> That's fine. Ask away!
>> 
>> > I thought it was about "keeping the snapshot file small".  But then when I
>> > was thinking the use case, iiuc fixed-ram migration should always suggest
>> > the user to stop the VM first before migration starts, then if the VM is
>> > stopped the ultimate image shouldn't be large either.
>> >
>> > Or is it about performance only?  Where did I miss?
>> 
>> Performance is the main benefit because fixed-ram enables the use of
>> multifd for file migration which would otherwise not be
>> parallelizable. To use multifd has been the direction for a while as you
>> know, so it makes sense.
>> 
>> A fast file migration is desirable because it could be used for
>> snapshots with a stopped vm and also to replace the "exec:cat" hack
>> (this last one I found out about recently, Juan mentioned it in this
>> thread: https://lore.kernel.org/r/87cyx5ty26.fsf@secure.mitica).
>
> I digged again the history, and started to remember the "live" migration
> case for fixed-ram. IIUC that is what Dan mentioned in below email
> regarding to the "virDomainSnapshotXXX" use case:
>
> https://lore.kernel.org/all/ZD7MRGQ+4QsDBtKR@redhat.com/
>
> So IIUC "stopped VM" is not always the use case?
>
> If you agree with this, we need to document these two use cases clearly in
> the doc update:
>
>   - "Migrate a VM to file, then destroy the VM"
>
>     It should be suggested to stop the VM first before triggering such
>     migration in this use case in the documents.
>
>   - "Take a live snapshot of the VM"
>
>     It'll be ideal if there is a portable interface to synchronously track
>     dirtying of guest pages, but we don't...
>
>     So fixed-ram seems to be the solution for such a portable solution for
>     taking live snapshot across-platforms as long as async dirty tracking
>     is still supported on that OS (aka KVM_GET_DIRTY_LOG).  If async
>     tracking is not supported, snapshot cannot be done live on the OS then,
>     and one needs to use "snapshot-save".
>
>     For this one, IMHO it would be good to mention (from QEMU perspective)
>     the existance of background-snapshot even though libvirt didn't support
>     it for some reason.  Currently background-snapshot lacks multi-thread
>     feature (nor O_DIRECT), though, so it may be less performant than
>     fixed-ram.  However if with all features there I believe that's even
>     more performant.  Please consider mention to a degree of detail on
>     this.
>

I'll include these in some form in the docs update.

>> 
>> The size aspect is just an interesting property, not necessarily a
>> reason.
>
> See above on the 2nd "live" use case of fixed-ram. I think in that case,
> size is still a matter, then, because that one cannot stop the VM vcpus.
>
>> It's about having the file bounded to the RAM size. So a running
>> guest would not produce a continuously growing file. This is in contrast
>> with previous experiments (libvirt code) in using a proxy to put
>> multifd-produced data into a file.
>> 
>> I'll add this^ information in a more organized matter to the docs and
>> cover letter. Let me know what else I need to clarify.
>
> Thanks.
>
>> 
>> Some notes about fixed-ram by itself:
>> 
>> This series also enables fixed-ram without multifd, which would only
>> take benefit of the size property. That is not part of our end goal
>> which is to have multifd + fixed-ram, but I kept it nonetheless because
>> it helps to debug/reason about the fixed-ram format without conflating
>> matters with multifd.
>
> Yes, makes sense.
>
>> 
>> Fixed-ram without multifd also allows the file migration to take benefit
>> of direct io because the data portion of the file (pages) will be
>> written with alignment. This version of the series does not yet support
>> it, but I have a simple patch for the next version.
>> 
>> I also had a - perhaps naive - idea that we could merge the io code +
>> fixed-ram first, to expedite things and later bring in the multifd and
>> directio enhancements, but the review process ended up not being that
>> modular.
>
> What's the review process issue you're talking about?

No issue per-se. I'm just mentioning that I split the series in a
certain way and no one seemed to notice. =)

Basically everything up until patch 10/30 is one chunk that is mostly
separate from multifd support (patches 11-22/30) and direct-io + fdset
(32-30/30).

>
> If you can split the series that'll help merging for sure to me.  IIRC
> there's complexity on passing the o-direct fds around, and not sure whether
> that chunk can be put at the last, similarly to split the multifd bits.
>

The logical sequence for merging in my view would be:

1 - file: support - Steven already did that
2 - file: + fixed-ram
  2a- file: + fixed-ram + direct-io (optional, I will send a patch in v4)
3 - file: + fixed-ram + multifd
4 - file: + fixed-ram + multifd + direct-io (here we get the full perf. benefits)
5 - file:/dev/fdset + fixed-ram + multifd + direct-io (here we can go
    enable libvirt support)

> One thing I just noticed is fixed-ram seems to be always preferred for
> "file:" migrations.  Then can we already imply fixed-ram for "file" URIs?
>

The file URI alone is good to replace the exec:cat trick. We'll need it
once we deprecate exec: to be able to do debugging of the stream.

> I'm even thinking whether we can make it the default and drop the fixed-ram
> capability: fixed-ram won't work besides file, and file won't make sense if
> not using offsets / fixed-ram.  There's at least one problem, where we have
> released 8.2 with "file:", so it means it could break users already using
> "file:" there.  I'm wondering whether that'll be worthwhile considering if
> we can drop the (seems redundant..) capability.  What do you think?