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

From: Zdenek Kabelac <zdenek.kabelac@gmail.com>
To: Demi Marie Obenour <demi@invisiblethingslab.com>
Cc: LVM general discussion and development <linux-lvm@redhat.com>
Subject: Re: [linux-lvm] LVM performance vs direct dm-thin
Date: Wed, 2 Feb 2022 11:04:37 +0100	[thread overview]
Message-ID: <3adf6eb1-94ac-dd91-e3e6-f0d44cd36b89@gmail.com> (raw)
In-Reply-To: <Yfnn+BcjuukqjfVO@itl-email>

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)

>> 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...

>> 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?
not going to starting discussing this topic ;)

>> 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....

Regards

Zdenek

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