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From: Zdenek Kabelac <>
To: LVM general discussion and development <>,
	james harvey <>
Subject: Re: [linux-lvm] Is "man lvcreate" for "-Z|--zero" incorrectly worded?
Date: Thu, 10 Jan 2019 10:31:17 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

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.


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.


      reply	other threads:[~2019-01-10  9:31 UTC|newest]

Thread overview: 2+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-10  3:05 [linux-lvm] Is "man lvcreate" for "-Z|--zero" incorrectly worded? james harvey
2019-01-10  9:31 ` Zdenek Kabelac [this message]

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