linux-lvm.redhat.com archive mirror
 help / color / mirror / Atom feed
From: nl6720 <nl6720@gmail.com>
To: LVM general discussion and development <linux-lvm@redhat.com>,
	Zdenek Kabelac <zkabelac@redhat.com>
Subject: Re: [linux-lvm] Why isn't issue_discards enabled by default?
Date: Tue, 22 Sep 2020 12:14:19 +0300	[thread overview]
Message-ID: <2262429.nQIR6vq4BO@walnut> (raw)
In-Reply-To: <02099334-08df-112d-1d02-dd76cc5d8f65@redhat.com>

[-- Attachment #1: Type: text/plain, Size: 2198 bytes --]

On Monday, 21 September 2020 21:51:39 EEST Zdenek Kabelac wrote:
> Dne 21. 09. 20 v 16:14 nl6720 napsal(a):
> > Hi,
> > 
> > I wanted to know why the "issue_discards" setting isn't enabled by
> > default. Are there any dangers in enabling it or if not is there a
> > chance of getting the default changed?
> > 
> > Also it's not entirely clear to me if/how "issue_discards" affects
> > thin pool discard passdown.
> 
> Hi
> 
> Have you checked it's enclosed documentation in within
> /etc/lvm/lvm.conf ?
> 
> issue_discards is PURELY & ONLY related to sending discard to removed
> disk extents/areas after 'lvremove'.
> 
> It is't not in ANY way related to actual discard handling of the LV
> itself. So if you have LV on SSD it is automatically processing
> discards. From the same reason it's unrelated to discard processing
> of thin-pools.
> 
> And finally why we prefer issue_discards to be disabled (=0) by
> default. It's very simple - with lvm2 we try (when we can) to support
> one-command-back restore - so if you do 'lvremove' - you can use
> vgcfgrestore to restore previous metadata and you have your LV back
> with all the data inside.
> 
> When you have issue_discards=1  - the device gets TRIM - so all the
> data are discarded at device level - so when you try to restore your
> previous metadata - well it's nice - but content is gone forever....
> 
> If user can live with this 'risk' and prefers immediate discard -
> perfectly fine - but it should be (IMHO) admin's  decision.
> 
> Regards
> 
> Zdenek


Thanks for your answer, so the reason it's not enabled by default is to 
allow vgcfgrestore to function.

I have read /etc/lvm/lvm.conf and understand that issue_discards affects 
things like lvremove. But I'd like to know, is it only for lvremove or 
also lvreduce and lvconvert (--merge/--uncache)? And what is its 
relation to thin_pool_discards; with issue_discards = 0 and 
thin_pool_discards = passdown (both the defaults) how far down are the 
discards passed?

Lastly, there's no fstrim equivalent for trimming unused space in a PV, 
right? To do that I'd need to lvcreate a LV occupying all free space and 
then use `lvremove --config devices/issue_discards = 1`.

[-- Attachment #2: This is a digitally signed message part. --]
[-- Type: application/pgp-signature, Size: 228 bytes --]

  reply	other threads:[~2020-09-22  9:14 UTC|newest]

Thread overview: 9+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-21 14:14 nl6720
2020-09-21 14:26 ` Mark Mielke
2020-09-22  8:13   ` Zdenek Kabelac
2020-09-21 18:51 ` Zdenek Kabelac
2020-09-22  9:14   ` nl6720 [this message]
2020-09-22  9:20     ` nl6720
2020-09-22 10:15     ` Zdenek Kabelac
2020-09-22 10:38       ` nl6720
2020-09-22 10:59         ` 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=2262429.nQIR6vq4BO@walnut \
    --to=nl6720@gmail.com \
    --cc=linux-lvm@redhat.com \
    --cc=zkabelac@redhat.com \
    --subject='Re: [linux-lvm] Why isn'\''t issue_discards enabled by default?' \
    /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

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).