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From: "John Stoffel" <john@stoffel.org>
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, 9 Sep 2020 14:47:37 -0400	[thread overview]
Message-ID: <24409.9033.527504.36789@quad.stoffel.home> (raw)
In-Reply-To: <3503b4f5b55345beb24de4b156ee75c7@assyoma.it>

>>>>> "Gionatan" == Gionatan Danti <g.danti@assyoma.it> writes:

Gionatan> Il 2020-09-09 17:01 Roy Sigurd Karlsbakk ha scritto:
>> First, filelevel is usually useless. Say you have 50 VMs with Windows
>> server something. A lot of them are bound to have a ton of equal
>> storage in the same areas, but the file size and content will vary
>> over time. With blocklevel tiering, that could work better.

Gionatan> It really depends on the use case. I applied it to a
Gionatan> fileserver, so working at file level was the right
Gionatan> choice. For VMs (or big files) it is useless, I agree.

This assumes you're tiering whole files, not at the per-block level
though, right? 

>> This is all known.

Gionatan> But the only reason to want tiering vs cache is the
Gionatan> additional space the former provides. If this additional
Gionatan> space is so small (compared to the combined, total volume
Gionatan> space), tiering's advantage shrinks to (almost) nothing.

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.

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.  

  reply	other threads:[~2020-09-09 18:54 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 [this message]
2020-09-09 19:10         ` Zdenek Kabelac
2020-09-09 19:21           ` John Stoffel
2020-09-09 19:44         ` Gionatan Danti
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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