Re: [Qemu-devel] kvm / virsh snapshot management

[Gary Dale]

On 2019-06-10 6:07 p.m., Eric Blake wrote:
> On 6/10/19 4:27 PM, Gary Dale wrote:
>
> > Trying this against a test VM, I ran into a roadblock. My command line
> > and the results are:
> >
> > # virsh blockcommit stretch "/home/secure/virtual/stretch.qcow2" --top
> > stretchS3 --delete --wait
> > error: unsupported flags (0x2) in function qemuDomainBlockCommit
> >
> > I get the same thing when the path to the qcow2 file isn't quoted.
> That's a libvirt limitation - the --delete flag is documented from the
> generic API standpoint, but not (yet) implemented for the qemu driver
> within libvirt. For now, you have to omit --delete from your virsh
> command line, and then manually 'rm' the unused external file after the
> fact.
Which is not possible since I'm using internal snapshots.
> > I noted in
> > https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/6/html/virtualization_administration_guide/sub-sect-domain_commands-using_blockcommit_to_shorten_a_backing_chain
> > that the options use a single "-".
> Sounds like a bug in that documentation.

Yes, and the man page also seems to be wrong. The section on blockcommit 
begins:

blockcommit domain path [bandwidth] [--bytes] [base] [--shallow] [top] 
[--delete]
       [--keep-relative] [--wait [--async] [--verbose]] [--timeout 
seconds] [--active]
       [{--pivot | --keep-overlay}]
           Reduce the length of a backing image chain, by committing 
changes at the top of the
           chain (snapshot or delta files) into backing images. By 
default, this command
           attempts to flatten the entire chain.

In addition to "[base]" actually being "[--base base]" and "[top]" being 
"[--top top]", the description of what it does only applies to external 
snapshots. Similar things are wrong in the blockpull section.

> > However the results for that were:
> > # virsh blockcommit stretch /home/secure/virtual/stretch.qcow2 -top
> > stretchS3 -delete -wait
> > error: Scaled numeric value '-top' for <--bandwidth> option is malformed
> > or out of range
> >
> > which looks like virsh doesn't like the single dashes and is trying to
> > interpret them as positional options.
> >
> > I also did a
> >
> > # virsh domblklist stretch
> > Target     Source
> > ------------------------------------------------
> > vda        /home/secure/virtual/stretch.qcow2
> > hda        -
> >
> > and tried using vda instead of the full path in the blockcommit but got
> > the same error.
> >
> > Any ideas on what I'm doing wrong?
> Do you know for sure whether you have internal or external snapshots?
> And at this point, your questions are starting to wander more into
> libvirt territory.
Yes. I'm using internal snapshots. From your other e-mail, I gather that 
the (only) benefit to blockcommit with internal snapshots would be to 
reduce the size of the various tables recording changed blocks. Without 
a blockcommit, the L1 tables get progressively larger over time since 
they record all changes to the base file. Eventually the snapshots could 
become larger than the base image if I don't do a blockcommit.