Re: Online Backup and WAL archives

"Morus Walter" <morus(dot)walter(at)tanto(dot)de> ha scritto nel messaggio
news:16895(dot)32035(dot)914311(dot)612690(at)tanto-xipolis(dot)de(dot)(dot)(dot)
> Hi,
>
> I'm currently considering to use postgresql 8.0 online backups.
>
> The documentation says
> ' To make use of this backup, you will need to keep around all the
> WAL segment files generated at or after the starting time of the backup. '
>
> Now I'm wondering how much of these WAL segment files do I really need
> in order to recover the databases to a consistent state.

It depends from the date of your "last" base backup. You must keep around
all WAL segments since.
The only way to keep WAL sets to a minimum is to base-backup frequently, but
how much is the size of your DB ? How many transactions your users produce ?

>
> Let's say I cannot write the WAL segments to tape dynamically when they
> are archived. Then a complete disk failure would mean, that I loose WAL
> segments also.

The command to archive fully used segments is there just for this. We would
use it to (keeping simple) rcp segments on another system.

> Therefore I would like to save a minimum number of WAL segments at or
> after the online backup that allows recovery.
> Is that possible? How would I decide how much of the WAL I need?

see answer #1

>
> Of course I could recover to the state of database at about the time of
the
> backup only, but it would be a consistent state at least.
>
> Or should I use the previous online backup plus all WAL segements up to
> the current backup?

Last base backup + WAL segments from that point on = Your Database

>
> I expect the online backup to faster on recovery than an SQL dump, since
> the latter would imply recreation of indexes during recovery.
> Therefore I would prefer to use online backups.

Replaying transactions is surely faster than a complete restore.

Regards
Claudio Duffini
>