Re: [Qemu-devel] [PATCH 1/2] Add virtagent file system freeze/thaw

[Stefan Hajnoczi <stefanha@gmail.com>]

On Tue, Feb 1, 2011 at 2:26 PM, Jes Sorensen <Jes.Sorensen@redhat.com> wrote:
> On 02/01/11 15:12, Stefan Hajnoczi wrote:
>> On Tue, Feb 1, 2011 at 10:58 AM,  <Jes.Sorensen@redhat.com> wrote:
>>> From: Jes Sorensen <Jes.Sorensen@redhat.com>
>>>
>>> Implement freeze/thaw support in the guest, allowing the host to
>>> request the guest freezes all it's file systems before a live snapshot
>>> is performed.
>>>  - fsfreeze(): Walk the list of mounted local real file systems,
>>>               and freeze them.
>>>  - fsthaw():   Walk the list of previously frozen file systems and
>>>               thaw them.
>>>  - fsstatus(): Return the current status of freeze/thaw. The host must
>>>               poll this function, in case fsfreeze() returned with a
>>>               timeout, to wait for the operation to finish.
>>
>> It is desirable to minimize the freeze time, which may interrupt or
>> degrade the service that applications inside the VM can provide.
>> Polling means we have to choose a fixed value (500 ms?) at which to
>> check for freeze completion.  In this example we could have up to 500
>> ms extra time spent in freeze because it completed right after we
>> polled.  Any thoughts on this?
>
> I have to admit you lost me here, where do you get that 500ms time from?
> Is that the XMLRPC polling time or? I just used the example code from
> other agent calls.

500 ms is made up.  I was thinking, "what would a reasonable polling
interval be?" and picked a sub-second number.

Can you explain how the timeout in fsfreeze can happen?  It's probably
because I don't know the virtagent details.

Stefan