[linux-lvm] LVM performance vs direct dm-thin
Demi Marie Obenour
demi at invisiblethingslab.com
Wed Feb 2 02:09:52 UTC 2022
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):
> > > > > > How much slower are operations on an LVM2 thin pool compared to manually
> > > > > > managing a dm-thin target via ioctls? I am mostly concerned about
> > > > > > volume snapshot, creation, and destruction. Data integrity is very
> > > > > > important, so taking shortcuts that risk data loss is out of the
> > > > > > question. However, the application may have some additional information
> > > > > > that LVM2 does not have. For instance, it may know that the volume that
> > > > > > it is snapshotting is not in use, or that a certain volume it is
> > > > > > creating will never be used after power-off.
> > > > > >
> > > >
> > > > > So brave developers may always write their own management tools for their
> > > > > constrained environment requirements that will by significantly faster in
> > > > > terms of how many thins you could create per minute (btw you will need to
> > > > > also consider dropping usage of udev on such system)
> > > >
> > > > What kind of constraints are you referring to? Is it possible and safe
> > > > to have udev running, but told to ignore the thins in question?
> > >
> > > Lvm2 is oriented more towards managing set of different disks,
> > > where user is adding/removing/replacing them. So it's more about
> > > recoverability, good support for manual repair (ascii metadata),
> > > tracking history of changes, backward compatibility, support
> > > of conversion to different volume types (i.e. caching of thins, pvmove...)
> > > Support for no/udev & no/systemd, clusters and nearly every linux distro
> > > available... So there is a lot - and this all adds quite complexity.
> > I am certain it does, and that makes a lot of sense. Thanks for the
> > hard work! Those features are all useful for Qubes OS, too — just not
> > in the VM startup/shutdown path.
> > > So once you scratch all this - and you say you only care about single disc
> > > then you are able to use more efficient metadata formats which you could
> > > even keep permanently in memory during the lifetime - this all adds great
> > > performance.
> > >
> > > But it all depends how you could constrain your environment.
> > >
> > > It's worth to mention there is lvm2 support for 'external' 'thin volume'
> > > creators - so lvm2 only maintains 'thin-pool' data & metadata LV - but thin
> > > volume creation, activation, deactivation of thins is left to external tool.
> > > This has been used by docker for a while - later on they switched to
> > > overlayFs I believe..
> > That indeeds sounds like a good choice for Qubes OS. It would allow the
> > data and metadata LVs to be any volume type that lvm2 supports, and
> > managed using all of lvm2’s features. So one could still put the
> > metadata on a RAID-10 volume while everything else is RAID-6, or set up
> > a dm-cache volume to store the data (please correct me if I am wrong).
> > Qubes OS has already moved to using a separate thin pool for virtual
> > machines, as it prevents dom0 (privileged management VM) from being run
> > out of disk space (by accident or malice). That means that the thin
> > pool use for guests is managed only by Qubes OS, and so the standard
> > lvm2 tools do not need to touch it.
> > Is this a setup that you would recommend, and would be comfortable using
> > in production? As far as metadata is concerned, Qubes OS has its own
> > XML file containing metadata about all qubes, which should suffice for
> > this purpose. To prevent races during updates and ensure automatic
> > crash recovery, is it sufficient to store metadata for both new and old
> > transaction IDs, and pick the correct one based on the device-mapper
> > status line? I have seen lvm2 get in an inconsistent state (transaction
> > ID off by one) that required manual repair before, which is quite
> > unnerving for a desktop OS.
> 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
> > One feature that would be nice is to be able to import an
> > externally-provided mapping of thin pool device numbers to LV names, so
> > that lvm2 could provide a (read-only, and not guaranteed fresh) view of
> > system state for reporting purposes.
> Once you will have evidence it's the lvm2 causing major issue - you could
> consider whether it's worth to step into a separate project.
> > > > > It's worth to mention - the more bullet-proof you will want to make your
> > > > > project - the more closer to the extra processing made by lvm2 you will get.
> > > >
> > > > Why is this? How does lvm2 compare to stratis, for example?
> > >
> > > Stratis is yet another volume manager written in Rust combined with XFS for
> > > easier user experience. That's all I'd probably say about it...
> > That’s fine. I guess my question is why making lvm2 bullet-proof needs
> > so much overhead.
> 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
Qubes OS comes close to this in practice. systemd and udev versions are
known and fixed, and Qubes OS ships its own kernels.
> > > > > However before you will step into these waters - you should probably
> > > > > evaluate whether thin-pool actually meet your needs if you have that high
> > > > > expectation for number of supported volumes - so you will not end up with
> > > > > hyper fast snapshot creation while the actual usage then is not meeting your
> > > > > needs...
> > > >
> > > > What needs are you thinking of specifically? Qubes OS needs block
> > > > devices, so filesystem-backed storage would require the use of loop
> > > > devices unless I use ZFS zvols. Do you have any specific
> > > > recommendations?
> > >
> > > As long as you live in the world without crashes, buggy kernels, apps and
> > > failing hard drives everything looks very simple.
> > Would you mind explaining further? LVM2 RAID and cache volumes should
> > provide most of the benefits that Qubes OS desires, unless I am missing
> > something.
> I'm not familiar with QubesOS - but in many cases in real-life world we
> can't push to our users latest&greatest - so we need to live with bugs and
> add workarounds...
Qubes OS is more than capable of shipping fixes for kernel bugs. Is
that what you are referring to?
> > > 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.
> 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?
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
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