Re: [rdiff-backup-users] Filechange during backup

[Neil]

no. you have misunderstood lvm snapshots

LVM snapshots are far better than a dd (for this purpose at least) they hold a 
copy of the any changed data before it was changed

from the lvcreate man page-
Snapshots provide a 'frozen image' of the contents of the origin while the 
origin can still be updated. They enable consistent backups and online 
recovery of removed/overwritten data/files. The snapshot does not need the 
same amount of storage the origin has. In a typical scenario, 15-20% might be 
enough.
---

if you want to imagine it in your head it is as from the time the snapshot 
logical volume is created any data that would have been overwritten on the 
origin logical volume is instead kept (on the snapshot logical volume). when 
you read from the snapshot volume if that data has changed you get the copy 
from the snapshot logical volume if the data has not changed it comes from 
the origin logical volume

creating the snapshot is a fast operation, subsequent disk operations on the 
origin have a ?slight? overhead whilst the snapshot exists so delete the 
snapshot once the backup is done (and verified ;)

Any clearer? Have an experiment with them

be aware that just because it is a consistent snapshot of what was on the disk 
at a point in time doesn't mean it will be useful. For example using a 
database you need to get the database to write to disk all the info it needs 
to be a valid usable snapshot. The easy generic way around this is to shut 
the database down; take a snapshot; start the database up again; backup the 
snapshot;remove the snapshot. This time this saves over shutdown database; 
backup; startup database can be huge. or course you may not want to shut your 
database down then you getting into database specific territory

On Saturday 08 January 2005 15:44, Rainer Zocholl wrote:
> address@hidden(Neil)  07.01.05 10:55
>
> >one good workaround if you can is to use and lvm snapshot
> >you take a snapshot and back that up then remove the snapshot
> >
> >http://www.tldp.org/HOWTO/LVM-HOWTO/snapshotintro.html
>
> Wouldn't that double the space requirements?
>
> The LVM-snapshot documentation is always talking about
> "a copy of the data". I did not find any hints
> how the snapshot feature is really working/implemented.
>
> That sounds to me as the "snapshot" is just a "dd",
> which needs a long time to process and only ensures
> "stability" while reading the copy.
> Is that true?
> Where is it decscripted?
>
> What happens if you have 2 files, say a database and an index.
> Making the snapshot may need 10 minutes.
> What if the index is already stored in the first minute but
> in the 5th minute a change in the database is done which causes
> an rebuilt of the index? In 7th minute the database will
> be copied, and, oops, the index file already stored will not fit
> anymore!
>
>
> I know that there are snapshot file systems.
> But the don't copy all the data but only the directory structures
> meta intos, like a journaling filesystem.
> So they are very fast and aware of changes during
> the very short time the snapshot needs. Too they only need
> to store the changed data fractions. (hm, where did i read
> that recently? ;-))
>