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From: Gionatan Danti <>
To: LVM general discussion and development <>
Cc: "Roy Sigurd Karlsbakk" <>, Håkon <>
Subject: Re: [linux-lvm] Looking ahead - tiering with LVM?
Date: Wed, 09 Sep 2020 21:44:22 +0200	[thread overview]
Message-ID: <> (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 

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.


Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. -
email: -
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

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