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* [linux-lvm] size of lvm metadata
@ 2020-09-16 15:50 Tomas Dalebjörk
  2020-09-17 15:21 ` Heinz Mauelshagen
  0 siblings, 1 reply; 3+ messages in thread
From: Tomas Dalebjörk @ 2020-09-16 15:50 UTC (permalink / raw)
  To: linux-lvm

hi

I am trying to understand how big the lvm metadata is

in the vgcfgbackup file, I can see
extent_size = 8192
dev_size = 204800
pe_start = 2048
pe_count 24

pe_count(24) * extent_size(8192) = 196608 bytes usable space of the total dev_size(204800)
metadata size? = dev_size(204800) - 196608 = 8192

but...
pe_start is 2048?
so what is pe_start here? cant be sectors(512)? bytes? well than ther be not aligned

so where starts the actual data?
and where ends the lvm metadata?

regards Tomas
Sent from my iPhone

^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [linux-lvm] size of lvm metadata
  2020-09-16 15:50 [linux-lvm] size of lvm metadata Tomas Dalebjörk
@ 2020-09-17 15:21 ` Heinz Mauelshagen
  2020-09-17 17:13   ` Tomas Dalebjörk
  0 siblings, 1 reply; 3+ messages in thread
From: Heinz Mauelshagen @ 2020-09-17 15:21 UTC (permalink / raw)
  To: linux-lvm, tomas.dalebjork

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Tomas,

the first PE starts at offset 2048 sectors of size 512 bytes by default, 
i.e. the LVM MDA (metadata area)
is ~1MiB big (because the MDA starts at offset 1 page into the device).  
If you plan for large numbers of LVs or expect very scattered 
allocations which both grow the metadata, you may want to create a
bigger MDA using vgcreate's option --metadatasize (also see 
/etc/lvm/lvm.conf description on
metadata/pvmetadatasize).

On 9/16/20 5:50 PM, Tomas Dalebjörk wrote:
> hi  > > I am trying to understand how big the lvm metadata is > > in the 
vgcfgbackup file, I can see extent_size = 8192 dev_size = > 204800 
pe_start = 2048 pe_count 24 > > pe_count(24) * extent_size(8192) = 
196608 bytes usable space of the > total dev_size(204800) metadata size? 
= dev_size(204800) - 196608 = > 8192 > > but... pe_start is 2048? so 
what is pe_start here? cant be > sectors(512)? bytes? well than ther be 
not aligned > > so where starts the actual data? and where ends the lvm 
metadata?

At offset 2048 sectors (1MiB into the device) / MDA ends at sector 2047.

Mind that lvm2 metadata is text formatted (see /etc/lvm/backup/$VGName 
for one)
and thus varies in size (the MDA is used as a ring buffer for 2 copies 
of the MDA to
support atomic updates).  As pointed out above when refering to 
'vgcreate --metadatasize',
in more elaborate setups you may run out of MDA space.

Heinz

>  > regards Tomas Sent from my iPhone > > > 
_______________________________________________ linux-lvm mailing > list 
linux-lvm@redhat.com > https://www.redhat.com/mailman/listinfo/linux-lvm 
read the LVM HOW-TO > at http://tldp.org/HOWTO/LVM-HOWTO/

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^ permalink raw reply	[flat|nested] 3+ messages in thread

* Re: [linux-lvm] size of lvm metadata
  2020-09-17 15:21 ` Heinz Mauelshagen
@ 2020-09-17 17:13   ` Tomas Dalebjörk
  0 siblings, 0 replies; 3+ messages in thread
From: Tomas Dalebjörk @ 2020-09-17 17:13 UTC (permalink / raw)
  To: Heinz Mauelshagen; +Cc: linux-lvm

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ok

Ithanks


> pe_count(24) * extent_size(8192) = 196608 bytes usable space of the
> > total dev_size(204800) metadata size? = dev_size(204800) - 196608 =
> > 8192


Sent from my iPhone

> On 17 Sep 2020, at 17:22, Heinz Mauelshagen <heinzm@redhat.com> wrote:
> 
>  Tomas,
> 
> the first PE starts at offset 2048 sectors of size 512 bytes by default, i.e. the LVM MDA (metadata area)
> is ~1MiB big (because the MDA starts at offset 1 page into the device).  If you plan for large numbers of LVs or expect very scattered allocations which both grow the metadata, you may want to create a
> bigger MDA using vgcreate's option --metadatasize (also see /etc/lvm/lvm.conf description on
> metadata/pvmetadatasize).
> 
> On 9/16/20 5:50 PM, Tomas Dalebjörk wrote:
> > hi
> > 
> > I am trying to understand how big the lvm metadata is
> > 
> > in the vgcfgbackup file, I can see extent_size = 8192 dev_size =
> > 204800 pe_start = 2048 pe_count 24
> > 
> > pe_count(24) * extent_size(8192) = 196608 bytes usable space of the
> > total dev_size(204800) metadata size? = dev_size(204800) - 196608 =
> > 8192
> > 
> > but... pe_start is 2048? so what is pe_start here? cant be
> > sectors(512)? bytes? well than ther be not aligned
> > 
> > so where starts the actual data? and where ends the lvm metadata?
> At offset 2048 sectors (1MiB into the device) / MDA ends at sector 2047.
> 
> Mind that lvm2 metadata is text formatted (see /etc/lvm/backup/$VGName for one)
> and thus varies in size (the MDA is used as a ring buffer for 2 copies of the MDA to
> support atomic updates).  As pointed out above when refering to 'vgcreate --metadatasize',
> in more elaborate setups you may run out of MDA space.
> 
> Heinz
> 
> > 
> > regards Tomas Sent from my iPhone
> > 
> > 
> > _______________________________________________ linux-lvm mailing
> > list linux-lvm@redhat.com 
> > https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO
> > at http://tldp.org/HOWTO/LVM-HOWTO/
> 

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^ permalink raw reply	[flat|nested] 3+ messages in thread

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2020-09-16 15:50 [linux-lvm] size of lvm metadata Tomas Dalebjörk
2020-09-17 15:21 ` Heinz Mauelshagen
2020-09-17 17:13   ` Tomas Dalebjörk

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