From: Roger Heflin <rogerheflin@gmail.com>
To: LVM general discussion and development <linux-lvm@redhat.com>
Cc: Malin Bruland <malin.bruland@pm.me>
Subject: Re: [linux-lvm] lvconvert --uncache takes hours
Date: Wed, 1 Mar 2023 18:51:08 -0600 [thread overview]
Message-ID: <CAAMCDeftn7gLN8s3EMFer+jJ95dRkdpR4007hDMqNNhjrbGDJA@mail.gmail.com> (raw)
In-Reply-To: <2023967259.761825.1677710640334.JavaMail.zimbra@karlsbakk.net>
On Wed, Mar 1, 2023 at 4:50 PM Roy Sigurd Karlsbakk <roy@karlsbakk.net> wrote:
>
> Hi all
>
> Working with a friend's machine, it has lvmcache turned on with writeback. This has worked well, but now it's uncaching and it takes *hours*. The amount of cache was chosen to 100GB on an SSD not used for much else and the dataset that is being cached, is a RAID-6 set of 10x2TB with XFS on top. The system mainly works with file serving, but also has some VMs that benefit from the caching quite a bit. But then - I wonder - how can it spend hours emptying the cache like this? Most write caching I know of last only seconds or perhaps in really worst case scenarios, minutes. Since this is taking hours, it looks to me something should have been flushed ages ago.
>
> Have I (or we) done something very stupid here or is this really how it's supposed to work?
>
> Vennlig hilsen
>
> roy
A spinning raid6 array is slow on writes (see raid6 write penalty).
Because of that the array can only do about 100 write operattions/sec.
If the disk is doing other work then it only has the extra capacity so
it could destage slower.
A lot depends on how big each chunk is. The lvmcache indicates the
smallest chunksize is 32k.
100G / 32k = 3 million, and at 100seeks/sec that comes to at least an hour.
Lvm bookkeeping has to also be written to the spinning disks I would
think, so 2 hours if the array were idle.
Throw in a 50% baseload on the disks and you get 4 hours.
Hours is reasonable.
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next prev parent reply other threads:[~2023-03-02 0:51 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-01 22:44 [linux-lvm] lvconvert --uncache takes hours Roy Sigurd Karlsbakk
2023-03-01 22:55 ` Demi Marie Obenour
2023-03-02 0:51 ` Roger Heflin [this message]
2023-03-02 8:33 ` Roy Sigurd Karlsbakk
2023-03-02 11:27 ` Roger Heflin
2023-03-02 17:34 ` Gionatan Danti
2023-03-02 18:33 ` Roger Heflin
2023-03-02 20:47 ` Gionatan Danti
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