[linux-lvm] Is "man lvcreate" for "-Z|--zero" incorrectly worded?

[linux-lvm] Is "man lvcreate" for "-Z|--zero" incorrectly worded?

[james harvey]

On lvm2 2.02.183, man lvcreate includes:

-Z|--zero y|n
   Controls zeroing of the first 4KiB of data in the new LV.  Default
is y.  Snapshot COW volumes are always zeroed.  LV is not zeroed if
the read only flag is set.  Warning: trying to mount an unzeroed LV
can cause the system to hang.

This says only the "first 4KiB" is zeroed.

Yet, when running "lvcreate --chunksize <something large>", it says:

   WARNING: Pool zeroing and ###.00 MiB large chunk size slows down
thin provisioning.
   WARNING: Consider disabling zeroing (-Zn) or using smaller chunk
size (<512.00 KiB).

Which seems to indicate it's going to zero the entire newly allocated
chunk size, rather than only the first 4KiB.

The lvcreate warning goes more along with the purpose of zeroing, to
prevent reading data one shouldn't have access to.

Re: [linux-lvm] Is "man lvcreate" for "-Z|--zero" incorrectly worded?

[Zdenek Kabelac]

Dne 10. 01. 19 v 4:05 james harvey napsal(a):
> On lvm2 2.02.183, man lvcreate includes:
> 
> -Z|--zero y|n
>     Controls zeroing of the first 4KiB of data in the new LV.  Default
> is y.  Snapshot COW volumes are always zeroed.  LV is not zeroed if
> the read only flag is set.  Warning: trying to mount an unzeroed LV
> can cause the system to hang.
> 
> This says only the "first 4KiB" is zeroed.
> 
> Yet, when running "lvcreate --chunksize <something large>", it says:
> 
>     WARNING: Pool zeroing and ###.00 MiB large chunk size slows down
> thin provisioning.
>     WARNING: Consider disabling zeroing (-Zn) or using smaller chunk
> size (<512.00 KiB).
> 
> Which seems to indicate it's going to zero the entire newly allocated
> chunk size, rather than only the first 4KiB.
> 

Hi

With thin-pool and thin-volumes -   -Z  has slightly modified meaning.
User controls thin-pool zeroing - it's possible the information got
somewhat lost over the 'man page' rework where it's now fully generated
from the source code.

So with thin-pool --zero means whether zeroing is enabled or skipped for 
provisioned chunks.

With zeroing is skipped  (-Zn) -  there is still ALWAYS 'cleared' initial 
device header - otherwise such device would become unpredictable for the 
system - as unprovisioned chunks always return 'zero' - so i.e. udev device 
scanning may guess it's empty device and later upon write to first chunk - we 
must make sure that at least initial disk header will be zeroed.

That's why user cannot have control over traditional aka 'zeroing for linears' 
behavior here - and we 'overloaded' --zero setting to give it this meaning.

I hope this better explains it  - I'll try to enhance man page - meanwhile
check  'man lvmthin'  Zeroing.

Zdenek.