[libvirt-users] Problem with the use of domfsfreeze mountpoint option

[Eric Blake]

On 11/17/2014 07:37 AM, Payes Anand wrote:

> This patchset adds virDomainFSFreeze()/virDomainFSThaw() APIs and virsh
>> domfsfreeze/domfsthaw commands to enable the users to freeze and thaw
>> domain's filesystems cleanly.
>>
> 
> Do these API's ( commands ) also push the dirty buffer to the disk, or it
> has to be done separately before freezing, to take snapshots.

virDomainFSFreeze does whatever the guest agent is wired to do (and the
default implementation of qemu-guest-agent has hooks for you to add any
additional work around the envelope of freezing your guest's
filesystem).  There have been other threads on this list on how to add a
hook that prepares a database to be frozen, for example.  Passing the
--quiesce flag to taking a snapshot is just shorthand for triggering the
same actions as what you manually control by calling virDomainFSFreeze
yourself.

> 
> If we don't specify any mount points, how different is
> domfsfreeze/domfsthaw from suspend/resume. Which of them would be better to
> take live incremental external snapshots of iSCSI devices.

suspend does NOT flush pending guest I/O.  It just stops the guest CPUs
from making further changes.  Taking a snapshot of a suspended guest is
NOT going to be consistent in the same way as a snapshot of a guest that
has had its filesystems frozen (that is, booting the disks that were
copied from the time that a guest was frozen will likely need fsck
repair, and will not contain pending I/O transactions that were still
held in memory; while booting a disk that was copied while filesystems
were frozen should see a fully consistent file system that is not
lacking any pending I/O).

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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