[linux-lvm] Snapshot documentation

[linux-lvm] Snapshot documentation

[Jay Weber]

Hi,

Been playing with snapshots and am noticing I can create a snapshot with
less PE's than my source LV that I'm snapshotting, with the small side
effect of:

lvcreate -- WARNING: all snapshots will be disabled if more than 12 MB are
changed

Is the above message implying that if I change more than 12MB of data on
my source LV that the snapshot is then defunct/useless?  Does that include
deletes, etc?

Mainly a pointer to any information or documentation regarding how this
all works would be well appreciated.  I'm noticing LVM has some HP history
involved, so I'll go see if they have relevant data in their HPUX docs.

Maybe this is included in the recent EN-HOWTO I noticed mention of as
well?

Re: [linux-lvm] Snapshot documentation

[Michael Marxmeier]

Jay Weber wrote:
> Been playing with snapshots and am noticing I can create a snapshot with
> less PE's than my source LV that I'm snapshotting, with the small side
> effect of:
> 
> lvcreate -- WARNING: all snapshots will be disabled if more than 12 MB are
> changed

For now snapshots use a COW model at the block level. Each modified 
block gets saved (once) to the snapshot area. If the space dedicated 
to the snapshot area is exhausted the snapshot is discarded (and the
the saved blocks are simply discarded). Otherwise the fs would become
inconsistent.

When/if LVM once has persistent snapshots this might need to change.

> Is the above message implying that if I change more than 12MB of data on
> my source LV that the snapshot is then defunct/useless?  Does that include
> deletes, etc?

Yes. Though deletes should only affect the inode and lists of indirect
blocks (if any) so deleting a 12MB files does not necessarily result 
in 12MB of changes.


Hope this helps
Michael

-- 
Michael Marxmeier           Marxmeier Software AG
E-Mail: mike@msede.com      Besenbruchstrasse 9
Phone : +49 202 2431440     42285 Wuppertal, Germany
Fax   : +49 202 2431420     http://www.msede.com/

Re: [linux-lvm] Snapshot documentation

[Heinz Mauelshagen]

Enabling you to create snapshots with less space than that of the
Original Logical Volume is an option to save space in case you have
data which doesn't change a lot.

The only chance to deal with for eg. a filesystem which changes rapidly
and in the extrem case totally is to create equal sized snapshots.

Heinz

> Hi,
> 
> Been playing with snapshots and am noticing I can create a snapshot with
> less PE's than my source LV that I'm snapshotting, with the small side
> effect of:
> 
> lvcreate -- WARNING: all snapshots will be disabled if more than 12 MB are
> changed
> 
> Is the above message implying that if I change more than 12MB of data on
> my source LV that the snapshot is then defunct/useless?  Does that include
> deletes, etc?
> 
> Mainly a pointer to any information or documentation regarding how this
> all works would be well appreciated.  I'm noticing LVM has some HP history
> involved, so I'll go see if they have relevant data in their HPUX docs.
> 
> Maybe this is included in the recent EN-HOWTO I noticed mention of as
> well?
> 
-- 

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Systemmanagement CS-TS                           T-Nova
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