Re: [Qemu-devel] [RFC] Propose the Fast Virtual Disk (FVD) image format that outperforms QCOW2 by 249%

[Chunqiang Tang]

> Actually current filesystems do pretty well on thinly provisioned
> storage, as long as your extent size is not too small.  Starting from
> extent size in the 64M to 256M range there's almost no difference to
> non-virtualized storage.
> 
> Again, sparse images with a large enough allocation size give you almost
> the same numbers as preallocated images.  I've been doing quite a lot of
> work on TP support in QEMU.  Using an XFS filesystem to back the image
> with an extent size hint in the above mentioned range gives performance
> withing 1% of fully preallocated images.  With the added benefit of
> allowing to deallocate the space again through the SCSI WRITE_SAME
> or ATA TRIM commands.

These  numbers are very interesting and I would like to read more. Are 
your detailed results accessible on the Internet? Do you  have numbers on 
the impact of using a large extent size (64-256MB) on thin provisioning 
(since a guest file system's metadata are written first and are scattered 
in the virtual disk)?