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From: Kai Krakow <kai@kaishome.de>
To: Santiago Castillo Oli <scastillo@aragon.es>
Cc: Coly Li <colyli@suse.de>, linux-bcache@vger.kernel.org
Subject: Re: Low hit ratio and cache usage
Date: Fri, 4 Jun 2021 17:56:44 +0200	[thread overview]
Message-ID: <CAC2ZOYuUbrRgHjFqERMQ8N2NUuDnySbcpvtaCzHGWGTnnZU7_w@mail.gmail.com> (raw)
In-Reply-To: <62f20c57-d502-c362-da84-61a47c891e6d@aragon.es>

Hello!

Am Fr., 4. Juni 2021 um 14:36 Uhr schrieb Santiago Castillo Oli
<scastillo@aragon.es>:
>
> Hi Coli!
>
>
> El 04/06/2021 a las 14:05, Coly Li escribió:
> > What is the kernel version and where do you have the kernel ?  And what
> > is the workload on your machine ?
>
> I'm using debian 10 with default debian kernel (4.19.0-16-amd64) in host
> and guests.
>
> For virtualization I'm using KVM.
>
>
> There is a host, where bcache is running. The filesystem over bcache
> device is ext4. In that filesystem there is only 9 qcow2 files user by
> three VM guests. Two VM are running small nextcloud instances, another
> one is running transmission (bittorrent) for feeding debian and other
> distro iso files (30 files - 60 GiB approx.)

Besides Coly recommending to use a newer kernel, I think there may be
some misunderstanding of how bcache works:

* bcache is mostly about reducing latency so it skips sequential
access, you should measure block access latency instead of throughput
or fill rate

* thus, it probably fills your cache very slowly if a lot of patterns
are sequential

* ext4 has a write journal which turns many random write patterns into
sequential write patterns, YMMV if you disable ordered data mode or
journalling

* qcow2 is copy-on-write: new blocks are appended, resulting in
possible write amplification in bcache, it also creates sequential
write patterns

* kvm/qemu probably use directio / uncached IO by default which may
bypass bcache or caching completely, you should try a different IO
mode in kvm (e.g. unsafe cached)


Regards,
Kai

  parent reply	other threads:[~2021-06-04 15:58 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-04 11:07 Low hit ratio and cache usage Santiago Castillo Oli
2021-06-04 12:05 ` Coly Li
2021-06-04 12:35   ` Santiago Castillo Oli
2021-06-04 12:59     ` Coly Li
2021-06-04 15:56     ` Kai Krakow [this message]
     [not found] ` <f25c7f91-433e-d699-c1f6-7e828023167f@orange.fr>
2021-06-04 12:12   ` Santiago Castillo Oli

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