Re: domain: how long is new xml in saved file
On 4/24/20 7:37 AM, Daniel P. Berrangé wrote:
On Fri, Apr 24, 2020 at 02:33:13PM +0200, Michal Privoznik wrote:
> On 4/24/20 6:38 AM, Vincent Wu wrote:
>>
>
> The save format is fragile. At the beginning there is a header which
> describes the file, then there is libvirt section (which contains the domain
> XML and a cookie) and then there is QEMU section (where QEMU saved the guest
> memory). Because of this, we have to have the check you are hitting in place
> so that we don't accidentally overwrite the QEMU section.
BTW, does anyone recall why we were so restrictive on the XML length
in the first place ? I looked at history and didn't see why we did
it this way.
It occurrs to me that given guest typical RAM sizes measuring many
100's of MB, we could easily make the header section have 1 MB of
padding, and thus allow essentially arbitrary XML updates without
worry about hitting a size limit.
We've had guest XML reaching 1M before, but I agree that the initial
saved image creation should include padding to a nice boundary to make
future edits less likely to overflow the reserved heading.
On new enough Linux, some file systems support
fallocate(FALLOC_FL_INSERT_RANGE) which can splice in a hole (all later
file contents are shifted in offsets); maybe our save code could take
advantage of that to repair existing saved images with insufficient
header size in a more efficient manner than manually shifting the rest
of the file contents ourselves.
--
Eric Blake, Principal Software Engineer
Red Hat, Inc. +1-919-301-3226
Virtualization: qemu.org | libvirt.org