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Re: [libvirt] [PATCH 1/6] Add new API virDomainStreamDisk[Info] to header and drivers



On Fri, 2011-04-08 at 11:13 -0600, Eric Blake wrote:
> On 04/08/2011 07:31 AM, Daniel P. Berrange wrote:
> > On Thu, Apr 07, 2011 at 04:31:59PM -0500, Adam Litke wrote:
> >>  /*
> >> + * Disk Streaming
> >> + */
> >> +typedef enum {
> >> +    VIR_STREAM_DISK_ONE   = 1,  /* Stream a single disk unit */
> >> +    VIR_STREAM_DISK_START = 2,  /* Stream the entire disk */
> >> +    VIR_STREAM_DISK_STOP  = 4,  /* Stop streaming a disk */
> >> +} virDomainStreamDiskFlags;
> > 
> > Using flags to combine two separate tasks into one single API
> > is rather unpleasant. As raised in the previous patch, the API
> > should also be taking a offset+length in bytes, then there is
> > no need for a special case transfer of an individual sector.
> 
> > Taking all the previous points together, I think the API needs to
> > look like this:
> > 
> >     typedef enum {
> >         /* If set, virDomainBlockAllocate() will return immediately
> >          * allowing polling for operation completion & status
> >          */
> >         VIR_DOMAIN_DISK_ALLOCATE_NONBLOCK,
> >     } virDomainBlockAllocateFlags;
> 
> It seems like adding one more flag would also make this API useful for
> supporting the converse operation: if we have a disk that is currently
> allocated, but we either know that a block is all 0s (or don't care
> about the data in that block if it was not all 0s), it would be nice to
> request punching a hole (for file-backed images residing on a file
> system and kernels new enough to do that) and/or truncate back to a
> smaller (thinly-provisioned) allocated size (which should work for both
> file-backed and lvm-backed disk images).

I agree that this could be a good future extension of the API and
further justifies the use of offset _and_ length parameters.  However,
I'd prefer not to consider this part at the moment since I am not aware
of a hypervisor that plans to implement it. 

> Meanwhile, I know that GNU coreutils has been working on an API for
> efficiently getting FIEMAP data from files; part of this effort needs to
> involve migrating that code into gnulib so that libvirt can indeed
> provide an API that enumerates sections of a disk image that are
> allocated vs. holes.

Why not just ask the hypervisor?  Qemu's image format code is probably
the most efficient place from which to gather this information.

-- 
Thanks,
Adam


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