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

Zdenek Kabelac zdenek.kabelac at gmail.com
Thu Feb 3 12:04:44 UTC 2022

Dne 03. 02. 22 v 1:23 Demi Marie Obenour napsal(a):
> 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):

>> 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.

I'd avoid cherry-picking unless you have deep knowledge about all connections 
between patches.
Always use the latest released kernel for your comments whether things are 
slow or fast.

>> 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?

Inefficient design - you should prefer to pass devices directly.
AKA you might have some benefits at creation times, but overall performance of 
VM will be lower during its actual usage... pick your poison...

Especially if the backend is made by NVMe - any layer adds tremendous amount 
of latencies (and in fact DM alone is also noticeable)...



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