From: Zdenek Kabelac <zkabelac@redhat.com>
To: Eric Toombs <ewtoombs@uwaterloo.ca>,
LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] faster snapshot creation?
Date: Wed, 26 Feb 2020 11:01:26 +0100 [thread overview]
Message-ID: <a4708de8-93f9-e663-3184-099a8e40ac3c@redhat.com> (raw)
In-Reply-To: <bfd3824a-f360-b4ab-9a08-6a51c8c37a94@uwaterloo.ca>
Dne 25. 02. 20 v 22:35 Eric Toombs napsal(a):
> I tried thin pools and got the same snapshot creation time, about .45s.
>
As it has been said - lvm2 is NOT only sending couple ioctl() to create snapshot.
Every lvm2 command essentially does a full system validation with respect to
currently configured set of metadata - this exec-time by faaaaaar exceeds
the actual snapshot creation time.
(Also snapshot monitoring is pretty 'time expensive' operation on its own)
> I also tried creating the entire vg in memory with losetup and that
> didn't change the creation time either.
lvm2 was never optimized towards 'hyper fast' manipulation with a single
device - after all you are impacting your system resources in a major way -
thus couple milliseconds here and there for majority of users doesn't really
matter.
What is the most important in terms of speed is the 'minimal' delay which can
be observed on origin being in use (aka 'suspend-resume' time-frame of origin
takes short amount of time to block actual usage of the origin device - user
should not see big latency...)
If you are aspiring of creating tens of snapshot per second - you probably
have very unusual workflow/requirement and mostly likely lvm2 is not the right
tool for such task ATM.
There used to exist 'slight' optimization where lvm2 executed as 'lvm shell'
(so you basically stream lvm command though a pipe to such lvmshell) was
'sharing' some cached content between runs of individual commands - but this
was mostly lost and most likely would not squeeze a lot the time you refer.
So my advice - if you really need to use and create old snaps in very fast way
is to developed your very own tool working in you restricted environment where
you might probably not care about meta/data consistencies, deal with speed of
udev and gazillion other issues - you can run your small 'ioctl()' stream much
more efficiently.
On the other hand it's well beyond my imagination what is good for 'very fast'
creation of a snapshot when the overall performance of the system is then
degraded by 50% or more... but user can choice...
Zdenek
next prev parent reply other threads:[~2020-02-26 10:01 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-02-22 11:58 Eric Toombs
2020-02-25 9:31 ` Gionatan Danti
2020-02-25 9:38 ` Zdenek Kabelac
2020-02-25 21:35 ` Eric Toombs
2020-02-26 10:01 ` Zdenek Kabelac [this message]
2020-02-26 14:03 ` Douglas Paul
2020-02-26 0:38 ` Stuart D. Gathman
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