archive mirror
 help / color / mirror / Atom feed
From: Roger Heflin <>
To: LVM general discussion and development <>
Cc: Malin Bruland <>
Subject: Re: [linux-lvm] lvconvert --uncache takes hours
Date: Wed, 1 Mar 2023 18:51:08 -0600	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Wed, Mar 1, 2023 at 4:50 PM 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?
> 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.

linux-lvm mailing list
read the LVM HOW-TO at

  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

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).