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

[Zdenek Kabelac]

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.

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..

> 
>> 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...

>> 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.
And every development costs quite some time & money.

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.

Regards

Zdenek