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Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration
From: |
Daniel P . Berrangé |
Subject: |
Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability |
Date: |
Fri, 31 Mar 2023 08:56:01 +0100 |
User-agent: |
Mutt/2.2.9 (2022-11-12) |
On Thu, Mar 30, 2023 at 06:01:51PM -0400, Peter Xu wrote:
> On Thu, Mar 30, 2023 at 03:03:20PM -0300, Fabiano Rosas wrote:
> > From: Nikolay Borisov <nborisov@suse.com>
> >
> > Implement 'fixed-ram' feature. The core of the feature is to ensure that
> > each ram page of the migration stream has a specific offset in the
> > resulting migration stream. The reason why we'd want such behavior are
> > two fold:
> >
> > - When doing a 'fixed-ram' migration the resulting file will have a
> > bounded size, since pages which are dirtied multiple times will
> > always go to a fixed location in the file, rather than constantly
> > being added to a sequential stream. This eliminates cases where a vm
> > with, say, 1G of ram can result in a migration file that's 10s of
> > GBs, provided that the workload constantly redirties memory.
> >
> > - It paves the way to implement DIO-enabled save/restore of the
> > migration stream as the pages are ensured to be written at aligned
> > offsets.
> >
> > The feature requires changing the stream format. First, a bitmap is
> > introduced which tracks which pages have been written (i.e are
> > dirtied) during migration and subsequently it's being written in the
> > resulting file, again at a fixed location for every ramblock. Zero
> > pages are ignored as they'd be zero in the destination migration as
> > well. With the changed format data would look like the following:
> >
> > |name len|name|used_len|pc*|bitmap_size|pages_offset|bitmap|pages|
>
> What happens with huge pages? Would page size matter here?
>
> I would assume it's fine it uses a constant (small) page size, assuming
> that should match with the granule that qemu tracks dirty (which IIUC is
> the host page size not guest's).
>
> But I didn't yet pay any further thoughts on that, maybe it would be
> worthwhile in all cases to record page sizes here to be explicit or the
> meaning of bitmap may not be clear (and then the bitmap_size will be a
> field just for sanity check too).
I think recording the page sizes is an anti-feature in this case.
The migration format / state needs to reflect the guest ABI, but
we need to be free to have different backend config behind that
either side of the save/restore.
IOW, if I start a QEMU with 2 GB of RAM, I should be free to use
small pages initially and after restore use 2 x 1 GB hugepages,
or vica-verca.
The important thing with the pages that are saved into the file
is that they are a 1:1 mapping guest RAM regions to file offsets.
IOW, the 2 GB of guest RAM is always a contiguous 2 GB region
in the file.
If the src VM used 1 GB pages, we would be writing a full 2 GB
of data assuming both pages were dirty.
If the src VM used 4k pages, we would be writing some subset of
the 2 GB of data, and the rest would be unwritten.
Either way, when reading back the data we restore it into either
1 GB pages of 4k pages, beause any places there were unwritten
orignally will read back as zeros.
> If postcopy might be an option, we'd want the page size to be the host page
> size because then looking up the bitmap will be straightforward, deciding
> whether we should copy over page (UFFDIO_COPY) or fill in with zeros
> (UFFDIO_ZEROPAGE).
This format is only intended for the case where we are migrating to
a random-access medium, aka a file, because the fixed RAM mappings
to disk mean that we need to seek back to the original location to
re-write pages that get dirtied. It isn't suitable for a live
migration stream, and thus postcopy is inherantly out of scope.
With regards,
Daniel
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- [RFC PATCH v1 04/26] tests/qtest: migration-test: Add tests for file-based migration, (continued)
- [RFC PATCH v1 04/26] tests/qtest: migration-test: Add tests for file-based migration, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 01/26] migration: Add support for 'file:' uri for source migration, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 03/26] tests/qtest: migration: Add migrate_incoming_qmp helper, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 05/26] migration: Initial support of fixed-ram feature for analyze-migration.py, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 06/26] io: add and implement QIO_CHANNEL_FEATURE_SEEKABLE for channel file, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 08/26] io: implement io_pwritev/preadv for QIOChannelFile, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 07/26] io: Add generic pwritev/preadv interface, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 09/26] migration/qemu-file: add utility methods for working with seekable channels, Fabiano Rosas, 2023/03/30
- [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability, Fabiano Rosas, 2023/03/30
- Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability, Fabiano Rosas, 2023/03/31
Re: [RFC PATCH v1 10/26] migration/ram: Introduce 'fixed-ram' migration stream capability, Markus Armbruster, 2023/03/31
[RFC PATCH v1 11/26] migration: Refactor precopy ram loading code, Fabiano Rosas, 2023/03/30
[RFC PATCH v1 12/26] migration: Add support for 'fixed-ram' migration restore, Fabiano Rosas, 2023/03/30
[RFC PATCH v1 13/26] tests/qtest: migration-test: Add tests for fixed-ram file-based migration, Fabiano Rosas, 2023/03/30
[RFC PATCH v1 15/26] migration/multifd: Remove direct "socket" references, Fabiano Rosas, 2023/03/30
[RFC PATCH v1 14/26] migration: Add completion tracepoint, Fabiano Rosas, 2023/03/30