Re: [linux-lvm] LVM performance vs direct dm-thin

From: "Stuart D. Gathman" <stuart@gathman.org>
To: LVM general discussion and development <linux-lvm@redhat.com>,
	Zdenek Kabelac <zdenek.kabelac@gmail.com>
Subject: Re: [linux-lvm] LVM performance vs direct dm-thin
Date: Sun, 30 Jan 2022 16:39:30 -0500	[thread overview]
Message-ID: <30ba3f0683afb9c3b60d96ac0019ced4ce9bb5b8.camel@gathman.org> (raw)
In-Reply-To: <YfbApm+9ac6iBbbg@itl-email>

On Sun, 2022-01-30 at 11:45 -0500, Demi Marie Obenour wrote:
> On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote:
> > 
> 
> > Since you mentioned ZFS - you might want focus on using 'ZFS-only'
> > solution.
> > Combining  ZFS or Btrfs with lvm2 is always going to be a painful
> > way as
> > those filesystems have their own volume management.
> 
> Absolutely!  That said, I do wonder what your thoughts on using loop
> devices for VM storage are.  I know they are slower than thin
> volumes,
> but they are also much easier to manage, since they are just ordinary
> disk files.  Any filesystem with reflink can provide the needed
> copy-on-write support.

I use loop devices for test cases - especially with simulated IO
errors.  Devs really appreciate having an easy reproducer for
database/filesystem bugs (which often involve handling of IO errors). 
But not for production VMs.

I use LVM as flexible partitions (i.e. only classic LVs, no thin pool).
Classic LVs perform like partitions, literally using the same driver
(device mapper) with a small number of extents, and are if anything
more recoverable than partition tables.  We used to put LVM on bare
drives (like AIX did) - who needs a partition table?  But on Wintel,
you need a partition table for EFI and so that alien operating systems
know there is something already on a disk.

Your VM usage is different from ours - you seem to need to clone and
activate a VM quickly (like a vps provider might need to do).  We
generally have to buy more RAM to add a new VM :-), so performance of
creating a new LV is the least of our worries.

Since we use LVs like partitions - mixing with btrfs is not an issue. 
Just use the LVs like partitions.  I haven't tried ZFS on linux - it
may have LVM like features that could fight with LVM.  ZFS would be my
first choice on a BSD box.

We do not use LVM raid - but either run mdraid underneath, or let btrfs
do it's data duplication thing with LVs on different spindles.

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