From: Demi Marie Obenour <demi@invisiblethingslab.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 17:55:31 -0500 [thread overview]
Message-ID: <Y//X72Iwa6ORU9en@itl-email> (raw)
In-Reply-To: <2023967259.761825.1677710640334.JavaMail.zimbra@karlsbakk.net>
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On Wed, Mar 01, 2023 at 11:44:00PM +0100, Roy Sigurd Karlsbakk 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?
It’s likely normal. HDDs stink at small random writes and RAID-6 makes
this even worse. That said, I *strongly* recommend using three-disk
RAID-1 for the cache, to match the redundancy of the RAID-6. With
write-back caching, a failed cache will result in a corrupt and
unrecoverable filesystem.
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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next prev parent reply other threads:[~2023-03-01 23:02 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 [this message]
2023-03-02 0:51 ` Roger Heflin
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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