From: Zdenek Kabelac <zkabelac@redhat.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
Gionatan Danti <g.danti@assyoma.it>
Subject: Re: [linux-lvm] Higher than expected metadata usage?
Date: Tue, 27 Mar 2018 12:39:35 +0200 [thread overview]
Message-ID: <c5d1945b-a58e-4e4a-4f0f-83b32635b250@redhat.com> (raw)
In-Reply-To: <597ba4e4-2028-ed62-6835-86ae9015ea5b@assyoma.it>
Dne 27.3.2018 v 09:44 Gionatan Danti napsal(a):
> What am I missing? Is the "data%" field a measure of how many data chunks are
> allocated, or does it even track "how full" are these data chunks? This would
> benignly explain the observed discrepancy, as a partially-full data chunks can
> be used to store other data without any new metadata allocation.
>
Hi
I've forget to mention there is "thin_ls" tool (comes with
device-mapper-persistent-data package (with thin_check) - for those who want
to know precise amount of allocation and what amount of blocks is owned
exclusively by a single thinLV and what is shared.
It's worth to note - numbers printed by 'lvs' are *JUST* really rough
estimations of data usage for both thin_pool & thin_volumes.
Kernel is not maintaining full data-set - only a needed portion of it - and
since 'detailed' precise evaluation is expensive it's deferred to the tool
thin_ls...
And last but not least comment - when you pointed out 4MB extent usage - it's
relatively huge chunk - and if the 'fstrim' wants to succeed - those 4MB
blocks fitting thin-pool chunks needs to be fully released.
So i.e. if there are some 'sparse' filesystem metadata blocks places - they
may prevent TRIM to successeed - so while your filesystem may have a lot of
free space for its data - the actually amount if physically trimmed space can
be much much smaller.
So beware if the 4MB chunk-size for a thin-pool is good fit here....
The smaller the chunk is - the better change of TRIM there is...
For heavily fragmented XFS even 64K chunks might be a challenge....
Regards
Zdenek
next prev parent reply other threads:[~2018-03-27 10:39 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-03-27 7:44 [linux-lvm] Higher than expected metadata usage? Gionatan Danti
2018-03-27 8:30 ` Zdenek Kabelac
2018-03-27 9:40 ` Gionatan Danti
2018-03-27 10:18 ` Zdenek Kabelac
2018-03-27 10:58 ` Gionatan Danti
2018-03-27 11:06 ` Gionatan Danti
2018-03-27 10:39 ` Zdenek Kabelac [this message]
2018-03-27 11:05 ` Gionatan Danti
2018-03-27 12:52 ` Zdenek Kabelac
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:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=c5d1945b-a58e-4e4a-4f0f-83b32635b250@redhat.com \
--to=zkabelac@redhat.com \
--cc=g.danti@assyoma.it \
--cc=linux-lvm@redhat.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* 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).