linux-lvm.redhat.com archive mirror
 help / color / mirror / Atom feed
From: Gionatan Danti <g.danti@assyoma.it>
To: LVM general discussion and development <linux-lvm@redhat.com>
Cc: "Roy Sigurd Karlsbakk" <roy@karlsbakk.net>, Håkon <hawken@thehawken.org>
Subject: Re: [linux-lvm] Looking ahead - tiering with LVM?
Date: Wed, 09 Sep 2020 21:44:22 +0200	[thread overview]
Message-ID: <7d62cb86425416d5a3db115afdbd996c@assyoma.it> (raw)
In-Reply-To: <24409.9033.527504.36789@quad.stoffel.home>

Il 2020-09-09 20:47 John Stoffel ha scritto:
> This assumes you're tiering whole files, not at the per-block level
> though, right?

The tiered approach I developed and maintained in the past, yes. For any 
LVM-based tiering, we are speaking about block-level tiering (as LVM 
itself has no "files" concept).

> Do you have numbers?  I'm using DM_CACHE on my home NAS server box,
> and it *does* seem to help, but only in certain cases.   I've got a
> 750gb home directory LV with an 80gb lv_cache writethrough cache
> setup.  So it's not great on write heavy loads, but it's good in read
> heavy ones, such as kernel compiles where it does make a difference.

Numbers for available space for tiering vs cache can vary based on your 
setup. However, storage tiers generally are at least 5-10X apart from 
each other (ie: 1 TB SSD for 10 TB HDD). Hence my gut fealing that 
tiering is not drastically better then  lvm cache. But hey - I reserve 
the right to be totally wrong ;)

> So it's not only the caching being per-file or per-block, but how the
> actual cache is done?  writeback is faster, but less reliable if you
> crash.  Writethrough is slower, but much more reliable.

writeback cache surely is more prone to failure vs writethoug cache. The 
golden rule is that writeback cache should use a mirrored device (with 
device-level powerloss protected writeback cache if sync write speed is 
important).

But this is somewhat ortogonal to the original question: block-level 
tiering itself increases the chances of data loss (ie: losing the SSD 
component will ruin the entire filesystem), so you should used mirrored 
(or parity) device for tiering also.

Regards.

-- 
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8

  parent reply	other threads:[~2020-09-09 19:44 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-02 18:38 Roy Sigurd Karlsbakk
2020-09-05 11:47 ` Gionatan Danti
2020-09-09 15:01   ` Roy Sigurd Karlsbakk
2020-09-09 18:16     ` Gionatan Danti
2020-09-09 18:47       ` John Stoffel
2020-09-09 19:10         ` Zdenek Kabelac
2020-09-09 19:21           ` John Stoffel
2020-09-09 19:44         ` Gionatan Danti [this message]
2020-09-09 19:53           ` John Stoffel
2020-09-09 20:20             ` Gionatan Danti
2020-09-09 19:41       ` Roy Sigurd Karlsbakk
2020-09-09 19:49         ` 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:
  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=7d62cb86425416d5a3db115afdbd996c@assyoma.it \
    --to=g.danti@assyoma.it \
    --cc=hawken@thehawken.org \
    --cc=linux-lvm@redhat.com \
    --cc=roy@karlsbakk.net \
    --subject='Re: [linux-lvm] Looking ahead - tiering with LVM?' \
    /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).