On 11/27/23 11:50, Daniel P. Berrangé wrote:

On Mon, Nov 27, 2023 at 11:40:29AM +0100, Claudio Fontana wrote:

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On 11/27/23 11:18, Daniel P. Berrangé wrote:

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On Mon, Nov 27, 2023 at 10:43:58AM +0100, Claudio Fontana wrote:

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Hi all,

I understand there has been some movement in this topic as the fixed-offset ram and multifd code evolves.

I think I understood that now the idea is to pass from libvirt to QEMU two file descriptors, one for writing metadata, and a separate one for the actual memory pages, which is the one that can potentially be O_DIRECT.

We determined that O_DIRECT changes propagate across dup'd file descriptors, so we have only two choices

* 1 FD, and QEMU has to take care to toggle O_DIRECT on/off repeatedly depending on what phase it is in * 2 FDs, one with and one without O_DIRECT

Either is viable for libvirt. I have a mild preference for having 1 FD, but not enough to call it a design blocker. So at the discretion of whomever implements the QEMU part.

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1) I would assume that libvirt would then need to check if the user requested --parallel / --parallel-connections to enable QEMU multifd.

Yes

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2) I would also assume that libvirt would check the presence of --bypass-cache as the condition to set O_DIRECT on the second (memory pages fd), and to enable QEMU "io-direct" feature.

Yes

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3) I would tentatively suggest that when it comes to fixed-ram-offset, the condition to enable that one is a check like the one currently in libvirt:

src/util/virfile.c::virFileDiskCopy()

ie checking that we are writing to a seekable file that is not S_ISBLK .

Does this match your understanding/reasoning?

Both the io-direct and fixed-ram-offset features are dependent on new QEMU impls, so there is a mild backwards compatibility concern.

ie, lets say if we are running QEMU 9.0.0, but with old machine type pc-i440fx-8.0.0, and we save the state, but want to then restore with QEMU 8.2.0.

Essentially we must NOT use io-direct/fixed-ram-offset if we want the ability to migrate to older QEMU. At the same time I would like us to be able to take advantage of this new QEMU support to the greatest extent possible, even if not doing the --parallel stuff which was the original motivator.

Thus, we need some way to decide whether to use the new or the old on disk format.

I wonder if having a setting in '/etc/libvirt/qemu.conf' is sufficient, or whether we must expose a flag via the API.

With regards, Daniel

Thanks Daniel,

that's an interesting point. The new fixed-ram-offset format is a QEMU format, and as such I presume that in theory there is a

qemu_saveimage.h:#define QEMU_SAVE_VERSION 2

that could be bumped to 3 if this new format is used?

But then again, libvirt would need to decide whether to save in "old QEMU compatibility mode" or in the new QEMU_SAVE_VERSION 3 mode that allows for fixed-ram-offset.

Maybe a new libvirt option for controlling which QEMU_SAVE_VERSION format to use for the save, with the default being v2 for backward compatibility reasons?

What makes me reluctant to add public API, is that generally I feel that things like file format versions are internal/private impl details that mgmt apps should not need to care about.

Even the compatibility problem should only be a short term issue, since there's only limited version combinations that end up getting used in practice, and so called "backwards" version compat usage is even rarer.

I guess if we stick with a qemu.conf setting initially, we can revisit later if we find an API control knob is requird.

With regards, Daniel

I think qemu.conf also works; I'd still default to v2 for a while, we can document that in order to get these additional features setting v3 in qemu.conf is required to save in the new format. Thanks, Claudio