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Re: [Qemu-devel] [PATCH] qemu-img: make convert async
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH] qemu-img: make convert async |
Date: |
Wed, 22 Feb 2017 15:31:11 +0000 |
User-agent: |
Mutt/1.7.1 (2016-10-04) |
On Tue, Feb 21, 2017 at 01:29:51PM +0100, Peter Lieven wrote:
> 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
> paremeter 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 <address@hidden>
> ---
> RFC->V1: - add documentation
> - add missing coroutine_fn annotation [Stefan]
> - add a comment why it is safe to call coroutine_enter [Stefan]
> - check -m paramater for values < 1 [Stefan]
> - disallow -W parameter with compression [Stefan]
>
> RFC V3->V4: - avoid to prepare a request queue upfront [Kevin]
> - do not ignore the BLK_BACKING_FILE status [Kevin]
> - redesign the interface to the read and write routines [Kevin]
>
> RFC V2->V3: - updated stats in the commit msg from a host with a better
> network card
> - only wake up the coroutine that is acutally waiting for a write
> to complete.
> this was not only overhead, but also breaking at least linux
> AIO.
> - fix coding style complaints
> - rename some variables and structs
>
> RFC V1->V2: - using coroutine as worker "threads". [Max]
> - keeping the request queue as otherwise it happens
> that we wait on BLK_ZERO chunks while keeping the write order.
> it also avoids redundant calls to get_block_status and helps
> to skip some conditions for fully allocated imaged
> (!s->min_sparse)
>
> qemu-img-cmds.hx | 4 +-
> qemu-img.c | 276
> ++++++++++++++++++++++++++++++++++++++++---------------
> qemu-img.texi | 16 +++-
> 3 files changed, 220 insertions(+), 76 deletions(-)
Reviewed-by: Stefan Hajnoczi <address@hidden>
> @@ -326,6 +332,14 @@ skipped. This is useful for formats such as @code{rbd}
> if the target
> volume has already been created with site specific options that cannot
> be supplied through qemu-img.
>
> +With option @code{-W} specified, it is allowed to write out of order to the
> target.
> +This is option improves performance, but is only recommened for preallocated
> devices
Minor nit. Suggested rewording:
Out of order writes can be enabled with @code{-W} to improve
performance. This is only recommended for preallocated devices
> +like host devices or other raw block devices. Out of order write does not
> work
> +in combination with creating compressed images.
> +
> address@hidden specifies how many coroutines work in parallel during
> +the convert process (defaults to 8).
> +
> @item dd [-f @var{fmt}] [-O @var{output_fmt}] address@hidden address@hidden
> address@hidden address@hidden address@hidden
>
> Dd copies from @var{input} file to @var{output} file converting it from
> --
> 1.9.1
>
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