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From: Kevin Wolf <kwolf@redhat.com>
To: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Cc: qemu-block@nongnu.org, armbru@redhat.com, qemu-devel@nongnu.org,
	mreitz@redhat.com, den@openvz.org
Subject: Re: [PATCH 0/7] qcow2: compressed write cache
Date: Wed, 10 Feb 2021 13:35:38 +0100	[thread overview]
Message-ID: <20210210123538.GB5144@merkur.fritz.box> (raw)
In-Reply-To: <20210129165030.640169-1-vsementsov@virtuozzo.com>

Am 29.01.2021 um 17:50 hat Vladimir Sementsov-Ogievskiy geschrieben:
> Hi all!
> 
> I know, I have several series waiting for a resend, but I had to switch
> to another task spawned from our customer's bug.
> 
> Original problem: we use O_DIRECT for all vm images in our product, it's
> the policy. The only exclusion is backup target qcow2 image for
> compressed backup, because compressed backup is extremely slow with
> O_DIRECT (due to unaligned writes). Customer complains that backup
> produces a lot of pagecache.
> 
> So we can either implement some internal cache or use fadvise somehow.
> Backup has several async workes, which writes simultaneously, so in both
> ways we have to track host cluster filling (before dropping the cache
> corresponding to the cluster).  So, if we have to track anyway, let's
> try to implement the cache.
> 
> Idea is simple: cache small unaligned write and flush the cluster when
> filled.

I haven't had the time to properly look at the patches, but is there
anything in it that is actually specific to compressed writes?

I'm asking because you may remember that a few years ago I talked at KVM
Forum about how a data cache could be used for small unaligned (to
cluster sizes) writes to reduce COW cost (mostly for sequential access
where the other part of the cluster would be filled soon enough).

So if we're introducing some kind of data cache, wouldn't it be nice to
use it even in the more general case instead of just restricting it to
compression?

Kevin



  parent reply	other threads:[~2021-02-10 12:37 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-29 16:50 [PATCH 0/7] qcow2: compressed write cache Vladimir Sementsov-Ogievskiy
2021-01-29 16:50 ` [PATCH 1/7] qemu/queue: add some useful QLIST_ and QTAILQ_ macros Vladimir Sementsov-Ogievskiy
2021-02-01  8:29   ` Markus Armbruster
2021-02-01  8:34     ` Vladimir Sementsov-Ogievskiy
2021-02-10 17:07   ` Max Reitz
2021-01-29 16:50 ` [PATCH 2/7] block/qcow2: introduce cache for compressed writes Vladimir Sementsov-Ogievskiy
2021-02-10 17:07   ` Max Reitz
2021-02-11 12:49     ` Vladimir Sementsov-Ogievskiy
2021-02-18 15:04       ` Max Reitz
2021-01-29 16:50 ` [PATCH 3/7] block/qcow2: use compressed write cache Vladimir Sementsov-Ogievskiy
2021-02-10 17:11   ` Max Reitz
2021-02-11 12:53     ` Vladimir Sementsov-Ogievskiy
2021-02-18 16:02       ` Max Reitz
2021-01-29 16:50 ` [PATCH 4/7] simplebench: bench_one(): add slow_limit argument Vladimir Sementsov-Ogievskiy
2021-01-29 16:50 ` [PATCH 5/7] simplebench: bench_one(): support count=1 Vladimir Sementsov-Ogievskiy
2021-01-29 16:50 ` [PATCH 6/7] simplebench/bench-backup: add --compressed option Vladimir Sementsov-Ogievskiy
2021-01-29 16:50 ` [PATCH 7/7] simplebench/bench-backup: add target-cache argument Vladimir Sementsov-Ogievskiy
2021-01-29 17:30 ` [PATCH 0/7] qcow2: compressed write cache no-reply
2021-02-01  8:24   ` Vladimir Sementsov-Ogievskiy
2021-02-09 13:25 ` Max Reitz
2021-02-09 14:10   ` Vladimir Sementsov-Ogievskiy
2021-02-09 14:47     ` Max Reitz
2021-02-09 16:39       ` Vladimir Sementsov-Ogievskiy
2021-02-09 18:36         ` Vladimir Sementsov-Ogievskiy
2021-02-09 18:41           ` Denis V. Lunev
2021-02-09 18:51             ` Vladimir Sementsov-Ogievskiy
2021-02-10 10:00               ` Max Reitz
2021-02-10 10:10                 ` Vladimir Sementsov-Ogievskiy
2021-02-09 16:52       ` Denis V. Lunev
2021-02-10 10:00         ` Max Reitz
2021-02-10 12:35 ` Kevin Wolf [this message]
2021-02-10 14:35   ` Vladimir Sementsov-Ogievskiy

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