Re: [Qemu-devel] [PATCH V3] qemu-img: make convert async

[Kevin Wolf <kwolf@redhat.com>]

Am 28.02.2017 um 13:40 hat Peter Lieven geschrieben:
> the convert process is currently completely implemented with sync operations.
> That means it reads one buffer and then writes it. No parallelism and each sync
> request takes as long as it takes until it is completed.
> 
> This can be a big performance hit when the convert process reads and writes
> to devices which do not benefit from kernel readahead or pagecache.
> In our environment we heavily have the following two use cases when using
> qemu-img convert.
> 
> a) reading from NFS and writing to iSCSI for deploying templates
> b) reading from iSCSI and writing to NFS for backups
> 
> In both processes we use libiscsi and libnfs so we have no kernel cache.
> 
> This patch changes the convert process to work with parallel running coroutines
> which can significantly improve performance for network storage devices:
> 
> qemu-img (master)
>  nfs -> iscsi 22.8 secs
>  nfs -> ram   11.7 secs
>  ram -> iscsi 12.3 secs
> 
> qemu-img-async (8 coroutines, in-order write disabled)
>  nfs -> iscsi 11.0 secs
>  nfs -> ram   10.4 secs
>  ram -> iscsi  9.0 secs
> 
> This patches introduces 2 new cmdline parameters. The -m parameter to specify
> the number of coroutines running in parallel (defaults to 8). And the -W parameter to
> allow qemu-img to write to the target out of order rather than sequential. This improves
> performance as the writes do not have to wait for each other to complete.
> 
> Signed-off-by: Peter Lieven <pl@kamp.de>

Thanks, updated in the block branch.

Kevin