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Re: [libvirt] [PATCH 1/n] dumpxml: add flag to virDomainGetXMLDesc



On 10/23/2014 07:52 AM, Adam Litke wrote:
> On 17/09/14 08:43 -0600, Eric Blake wrote:
>> + * If @flags contains VIR_DOMAIN_XML_BLOCK_INFO, the listing for each
>> + * <disk> device will contain additional information such as capacity
>> + * and allocation, similar to what is displayed by
>> virStorageVolGetXMLDesc(),
>> + * and avoiding the need to call virDomainGetBlockInfo() separately.
>> + *
> 
> Eric:  We decided to go with this extra flag since gathering the block
> info was seen as a potentially expensive operation which users would
> want to selectively enable.  Do you have any thoughts on how expensive
> this would be in practice?  We are considering enabling it
> unconditionally in oVirt and am weighing a decision between A)
> Simplicity and Cacheability vs. B) Performance.

I just posted a v2 patch.  The extra expense is that I have to obtain a
job lock in order to call into the qemu monitor.  Thankfully, it's only
a read lock (so it can be done even during a long-running operation like
a migration), but obtaining that lock is not instant.  It's still
probably not noticeable in the long run, but avoiding the flag avoids
the need for the job lock in the first place.

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

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