[libvirt] [PATCHv9 4/9] blockjob: support pivot operation on cancel

[Peter Krempa]

On 10/23/12 04:10, Eric Blake wrote:
> This is the bare minimum to end a copy job (of course, until a
> later patch adds the ability to start a copy job, this patch
> doesn't do much in isolation; I've just split the patches to
> ease the review).
>
> This patch intentionally avoids SELinux, lock manager, and audit
> actions.  Also, if libvirtd restarts at the exact moment that a
> 'block-job-complete' is in flight, the proposed proper way to
> detect the outcome of that would be with a persistent bitmap and
> some additional query commands for qemu 1.3 when libvirtd
> restarts (RHEL 6.3 is out of luck).  This patch is enough to
> test the common case of success when used correctly, while saving
> the subtleties of proper cleanup for worst-case errors for later.
>
> When a mirror job is started, cancelling the job safely reverts back
> to the source disk, regardless of whether the destination is in
> phase 1 (streaming, in which case the destination is worthless) or
> phase 2 (mirroring, in which case the destination is synced up to
> the source at the time of the cancel).  Our existing code does just
> fine in either phase, other than some bookkeeping cleanup; this
> implements live block copy.
>
> Pivoting the job requires the qemu 1.3 'block-job-complete' (safe)
> or the RHEL 6.3 '__com.redhat_drive-reopen' command (where failure
> of the command is potentially catastrophic to the domain, since
> it rips out the old disk before attempting to open the new one).
>
> Ideas for future enhancements via new flags:
>
> Since qemu 1.3 is safer than RHEL 6.3, it may be worth adding a
> VIR_DOMAIN_REBASE_COPY_ATOMIC flag that fails up front if we
> detect an older qemu with the risky pivot operation.
>
> Interesting side note: while snapshot-create --disk-only creates a
> copy of the disk at a point in time by moving the domain on to a
> new file (the copy is the file now in the just-extended backing
> chain), blockjob --abort of a copy job creates a copy of the disk
> while keeping the domain on the original file.  There may be
> potential improvements to the snapshot code to exploit block copy
> over multiple disks all at one point in time.  And, if
> 'block-job-cancel' were made part of 'transaction', you could
> copy multiple disks at the same point in time without pausing
> the domain.  This also implies we may want to add a --quiesce flag
> to virDomainBlockJobAbort, so that when breaking a mirror (whether
> by cancel or pivot), the side of the mirror that we are abandoning
> is at least in a stable state with regards to guest I/O.
>
> * src/qemu/qemu_driver.c (qemuDomainBlockJobAbort): Accept new flag.
> (qemuDomainBlockPivot): New helper function.
> (qemuDomainBlockJobImpl): Implement it.
> ---
>   src/qemu/qemu_driver.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 107 insertions(+), 1 deletion(-)
>

ACK if the RHEL stuff will be pulled in, otherwise it will require a few 
changes.

Peter