Re: LVM performance vs direct dm-thin

Demi Marie Obenour <[email protected]> Sun, 30 Jan 2022 17:14:43 -0500
Newsgroups	gmane.linux.lvm.general
Message-ID	<YfcN6KHxLe3/krTX@itl-email>
On Sun, Jan 30, 2022 at 04:39:30PM -0500, Stuart D. Gathman wrote:
> 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.

To put it mildly, yes :).  Ideally we could get VM boot time down to
100ms or lower.

-- 
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab

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