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

[Marian Csontos]

On Sun, Jan 30, 2022 at 11:17 PM Demi Marie Obenour <
demi at invisiblethingslab.com> wrote:

> On Sun, Jan 30, 2022 at 04:39:30PM -0500, Stuart D. Gathman wrote:
> > 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.
>

Out of curiosity, is snapshot creation the main culprit to boot a VM in
under 100ms? Does Qubes OS use tweaked linux distributions, to achieve the
desired boot time?

Back to business. Perhaps I missed an answer to this question: Are the
Qubes OS VMs throw away?  Throw away in the sense like many containers are
- it's just a runtime which can be "easily" reconstructed. If so, you can
ignore the safety belts and try to squeeze more performance by sacrificing
(meta)data integrity.

And the answer to that question seems to be both Yes and No. Classical pets
vs cattle.

As I understand it, except of the system VMs, there are at least two kinds
of user domains and these have different requirements:

1. few permanent pet VMs (Work, Personal, Banking, ...), in Qubes OS called
AppVMs,
2. and many transient cattle VMs (e.g. for opening an attachment from
email, or browsing web, or batch processing of received files) called
Disposable VMs.

For AppVMs, there are only "few" of those and these are running most of the
time so start time may be less important than data safety. Certainly
creation time is only once in a while operation so I would say use LVM for
these. And where snapshots are not required, use plain linear LVs, one less
thing which could go wrong. However, AppVMs are created from Template VMs,
so snapshots seem to be part of the system. But data may be on linear LVs
anyway as these are not shared and these are the most important part of the
system. And you can still use old style snapshots for backing up the data
(and by backup I mean snapshot, copy, delete snapshot. Not a long term
snapshot. And definitely not multiple snapshots).

Now I realized there is the third kind of user domains - Template VMs.
Similarly to App VM, there are only few of those, and creating them
requires downloading an image, upgrading system on an existing template, or
even installation of the system, so any LVM overhead is insignificant for
these. Use thin volumes.

For the Disposable VMs it is the creation + startup time which matters. Use
whatever is the fastest method. These are created from template VMs too.
What LVM/DM has to offer here is external origin. So the templates
themselves could be managed by LVM, and Qubes OS could use them as external
origin for Disposable VMs using device mapper directly. These could be held
in a disposable thin pool which can be reinitialized from scratch on host
reboot, after a crash, or on a problem with the pool. As a bonus this would
also address the absence of thin pool shrinking.

I wonder if a pool of ready to be used VMs could solve some of the startup
time issues - keep $POOL_SIZE VMs (all using LVM) ready and just inject the
data to one of the VMs when needed and prepare a new one asynchronously. So
you could have to some extent both the quick start and data safety as a
solution for the hypothetical third kind of domains requiring them - e.g. a
Disposable VM spawn to edit a file from a third party - you want to keep
the state on a reboot or a system crash.
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