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



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).

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.

-- 
Eric Blake   eblake redhat com    +1-801-349-2682
Libvirt virtualization library http://libvirt.org

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