Re: [Qemu-devel] [PATCH 0/3] info blockstats (block-qcow2): show highest allocated offset (bytes)

[Uri Lublin <uril@redhat.com>]

Anthony Liguori wrote:
> Uri Lublin wrote:
>> From: Uri Lublin <uril@redhat.com>
>>
>> This patchset let the user know the highest allocated byte of qcow2 
>> images.
>> Actually it's the first unallocated byte after the highest byte written,
>> cluster-size aligned.
>>
>> The highest allocated byte gives a maximal limit (easy to calculate)
>> to the number of bytes allocated for that image, and may hint how many 
>> more allocations can be done before we reach end-of-file (end of host 
>> block device).
>> Although there may be many free blocks below that number (allocated 
>> and freed)
>> the file system can not deallocate those blocks, and they have to be 
>> reused
>> by qemu. Also note that due to fragmentation those free blocks may not
>> be used on next allocations.
>>
>> It can be useful for truncation of backing file images (ftruncate).
>> Also it may be useful for defragmentation later (although we'll need
>> the number of free blocks as well).
> 
> I'm having trouble seeing the utility of this as it seems to be not 
> really reliable.  Surely, after a lot of work, you'll have one block far 
> at the end of the file, no?  I don't see how knowing this location helps 
> practically speaking.  Can you explain a little more about what you want 
> to use this functionality for?
> 

Currently, qcow2 images can only grow, never shrink.
The main usage would be to trigger an appropriate operation when a threshold is 
reached. The threshold and operation are defined by a management application.
Basically we can do one of the following:
1. Defragment the qcow2 image (simplest way is to qemu-img convert it, the best 
is to do it online if possible).
2. Allocate more space (especially when using LVM)

I plan on adding another "blockstat" that shows the number of free 
bytes/blocks/clusters for a qcow2 image. This would make it easier to choose the 
appropriate operation above.

Thanks,
     Uri.