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

[Anthony Liguori <anthony@codemonkey.ws>]

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?

Regards,

Anthony Liguori