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Re: [Qemu-block] [PATCH v6 11/42] block: Add bdrv_supports_compressed_wr

From: Max Reitz
Subject: Re: [Qemu-block] [PATCH v6 11/42] block: Add bdrv_supports_compressed_writes()
Date: Mon, 9 Sep 2019 10:09:53 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 05.09.19 15:11, Kevin Wolf wrote:
> Am 09.08.2019 um 18:13 hat Max Reitz geschrieben:
>> Filters cannot compress data themselves but they have to implement
>> .bdrv_co_pwritev_compressed() still (or they cannot forward compressed
>> writes).  Therefore, checking whether
>> bs->drv->bdrv_co_pwritev_compressed is non-NULL is not sufficient to
>> know whether the node can actually handle compressed writes.  This
>> function looks down the filter chain to see whether there is a
>> non-filter that can actually convert the compressed writes into
>> compressed data (and thus normal writes).
>> Signed-off-by: Max Reitz <address@hidden>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <address@hidden>
> Should patches 2 and 3 that add the .bdrv_co_pwritev_compressed()
> callback to filter drivers come only after this one?

Why not.

> And should we also
> support it in the mirror filter?

Hm.  AFAIU, compressed writes have very limited use.  You can basically
only use them when writing to a new image (where you’d never write
anywhere you’ve already written something to), i.e. for qemu-img convert
or the backup target.  It makes sense to blockdev-backup to throttle, so
that’s why it should be implemented there.  I don’t really see how it
would make sense for mirror.

OTOH, it doesn’t make sense for COR either.  And it isn’t that hard.
Now I don’t have a strong preference for either dropping the COR patch
or adding it to mirror as well...


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