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Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in dis
From: |
Alberto Garcia |
Subject: |
Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk |
Date: |
Fri, 09 Dec 2016 16:00:14 +0100 |
User-agent: |
Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu) |
On Fri 09 Dec 2016 03:18:23 PM CET, Kevin Wolf wrote:
>> I have been making some tests with exactly that scenario and the
>> results look good: storing the cache in disk gives roughly the same
>> performance as storing it in memory.
>>
>> |---------------------+------------+------+------------+--------|
>> | | Random 4k reads | Sequential 4k reads |
>> | | Throughput | IOPS | Throughput | IOPS |
>> |---------------------+------------+------+------------+--------|
>> | Cache in memory/SSD | 406 KB/s | 99 | 84 MB/s | 21000 |
>> | Default cache (1MB) | 200 KB/s | 60 | 83 MB/s | 21000 |
>> | No cache | 200 KB/s | 49 | 56 MB/s | 14000 |
>> |---------------------+------------+------+------------+--------|
>>
>> I'm including the patch that I used to get these results. This is the
>> simplest approach that I could think of.
>>
>> Opinions, questions?
>
> I suppose you used the fact that the cache is now on disk to increase
> the cache size so that it covers the whole image?
Right, the wording on the table is not clear, but that's what I did. I
also don't think this makes much sense if the cache is not big enough to
cover the whole image.
> If so, are you sure that you aren't just testing that accessing memory
> in the kernel page cache is just as fast as accessing memory in qemu's
> own cache? It seems this would just bypass the cache size limit given
> to qemu by instead leaving things cached in the kernel where the limit
> doesn't apply.
Fair question: what I checked is that the PSS/RSS values match the
expected values (i.e, they don't grow as you read from the disk
image). If the kernel is caching those pages so accessing them after
MADV_DONTNEED does not require going to disk again is a possibility that
I haven't ruled out.
Berto
- [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/09
- [Qemu-devel] [PATCH RFC 1/1] qcow2: Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/09
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, no-reply, 2016/12/09
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Kevin Wolf, 2016/12/09
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk,
Alberto Garcia <=
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Max Reitz, 2016/12/09
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/13
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Max Reitz, 2016/12/13
- Re: [Qemu-devel] [PATCH RFC 0/1] Allow storing the qcow2 L2 cache in disk, Alberto Garcia, 2016/12/13