Re: LVM performance vs direct dm-thin

Demi Marie Obenour <[email protected]> Wed, 2 Feb 2022 19:23:13 -0500
Newsgroups	gmane.linux.lvm.general
Message-ID	<YfsgcwXnOouviZgc@itl-email>
On Wed, Feb 02, 2022 at 11:04:37AM +0100, Zdenek Kabelac wrote:
> Dne 02. 02. 22 v 3:09 Demi Marie Obenour napsal(a):
> > On Sun, Jan 30, 2022 at 06:43:13PM +0100, Zdenek Kabelac wrote:
> > > Dne 30. 01. 22 v 17:45 Demi Marie Obenour napsal(a):
> > > > On Sun, Jan 30, 2022 at 11:52:52AM +0100, Zdenek Kabelac wrote:
> > > > > Dne 30. 01. 22 v 1:32 Demi Marie Obenour napsal(a):
> > > > > > On Sat, Jan 29, 2022 at 10:32:52PM +0100, Zdenek Kabelac wrote:
> > > > > > > Dne 29. 01. 22 v 21:34 Demi Marie Obenour napsal(a):
> > > My biased advice would be to stay with lvm2. There is lot of work, many
> > > things are not well documented and getting everything running correctly will
> > > take a lot of effort  (Docker in fact did not managed to do it well and was
> > > incapable to provide any recoverability)
> > 
> > What did Docker do wrong?  Would it be possible for a future version of
> > lvm2 to be able to automatically recover from off-by-one thin pool
> > transaction IDs?
> 
> Ensuring all steps in state-machine are always correct is not exactly simple.
> But since I've not heard about off-by-one problem for a long while -  I
> believe we've managed to close all the holes and bugs in double-commit
> system
> and metadata handling by thin-pool and lvm2.... (for recent lvm2 & kernel)

How recent are you talking about?  Are there fixes that can be
cherry-picked?  I somewhat recently triggered this issue on a test
machine, so I would like to know.

> > > It's difficult - if you would be distributing lvm2 with exact kernel version
> > > & udev & systemd with a single linux distro - it reduces huge set of
> > > troubles...
> > 
> > Qubes OS comes close to this in practice.  systemd and udev versions are
> > known and fixed, and Qubes OS ships its own kernels.
> 
> Systemd/udev evolves - so fixed today doesn't really mean same version will
> be there tomorrow.  And unfortunately systemd is known to introduce
> backward incompatible changes from time to time...

Thankfully, in Qubes OS’s dom0, the version of systemd is frozen and
will never change throughout an entire release.

> > > Chain filesystem->block_layer->filesystem->block_layer is something you most
> > > likely do not want to use for any well performing solution...
> > > But it's ok for testing...
> > 
> > How much of this is due to the slow loop driver?  How much of it could
> > be mitigated if btrfs supported an equivalent of zvols?
> 
> Here you are missing the core of problem from kernel POV aka
> how the memory allocation is working and what are the approximation in
> kernel with buffer handling and so on.
> So whoever is using  'loop' devices in production systems in the way
> described above has never really tested any corner case logic....

In Qubes OS the loop device is always passed through to a VM or used as
the base device for an old-style device-mapper snapshot.  It is never
mounted on the host.  Are there known problems with either of these
configurations?

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

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