Re: [Qemu-devel] [PATCH V18 0/6] add-cow file format

[Stefan Hajnoczi]

On Wed, Apr 10, 2013 at 04:11:47PM +0800, Dong Xu Wang wrote:
> It will introduce a new file format: add-cow.
> 
> The add-cow file format makes it possible to perform copy-on-write on top of
> a raw disk image.  When we know that no backing file clusters remain visible
> (e.g. we have streamed the entire image and copied all data from the backing
> file), then it is possible to discard the add-cow file and use the raw image
> file directly.
> 
> This feature adds the copy-on-write feature to raw files (which cannot support
> it natively) while allowing us to get full performance again later when we no
> longer need copy-on-write.
> 
> add-cow can benefit from other available functions, such as path_has_protocol
> and qed_read_string, so we will make them public.
> 
> snapshot_blkdev are not supported now for add-cow. Will add it in futher 
> patches.
> 
> These patches are using QemuOpts parser, former patches could be found here:
> http://patchwork.ozlabs.org/patch/235300/
> 
> 
> v17 -> v18:
> 1) remove version field.
> 2) header size is maximum value and cluster size value.
> 3) fix type.
> 4) move struct to source file.
> 5) cluster_size->table_size.
> 6) use error_report, not fprintf.
> 7) remove version field from header.
> 8) header_size is MAX(cluster_size, 4096).
> 9) introduce s->cluster_sectors.
> 10) use BLKDBG_L2_LOAD/UPDATE.
> 11) add 037 and 038 tests.

Left a few comments but the series is close.

The biggest practical issue is serialized allocating writes.
Installation or multi-threaded write workloads may be quite slow since
only one read request is processed at a time.  This can be solved later.

Stefan