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

[Christoph Hellwig <hch@lst.de>]

On Wed, Jan 19, 2011 at 11:21:07AM -0500, Chunqiang Tang wrote:
> Yes, a fundamental and optimal solution would be changing guest file 
> systems, but it would take a much much longer route to introduce 
> virtualization awareness into all guest file systems and its also requires 
> changing the interface between the guest and the host.

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.

> Preallocation is not a universal solution here, because it just defeats 
> the other goal: thin provisioning. Moreover, if preallocation is used, it 
> works best for RAW images and makes it unnecessary to use a compact image, 
> which is exactly one goal of FVD, i.e., allowing optionally disabling a 
> compact image data layout without giving up other features, e.g., 
> copy-on-write.

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.