Re: [PATCHES] Doc update for pg_start_backup

On Jun 29, 2007, at 4:25 AM, Heikki Linnakangas wrote:

> Tom Lane wrote:
>> Heikki Linnakangas <heikki@enterprisedb.com> writes:
>>> Added a note to the docs that pg_start_backup can take a long  
>>> time to finish now that we spread out checkpoints:
>> I was starting to wordsmith this, and then wondered whether it's not
>> just a stupid idea for pg_start_backup to act that way.  The reason
>> you're doing it is to take a base backup, right?  What are you going
>> to take the base backup with?  I do not offhand know of any backup
>> tools that don't suck major amounts of I/O bandwidth.
>
> scp over a network? It's still going to consume a fair amount of I/ 
> O, but the network could very well be the bottleneck.
>
>> That being
>> the case, you're simply not going to schedule the operation during
>> full-load periods.  And that leads to the conclusion that
>> pg_start_backup should just use CHECKPOINT_IMMEDIATE and not slow
>> you down.
>
> That's probably true in most cases. But on a system that doesn't  
> have quite periods, you're still going to have to take the backup.
> To be honest, I've never worked as a DBA and never had to deal with  
> taking backups of a production system, so my gut feelings on this  
> could be totally wrong.

I'll share my two cents having had to back up many terabytes of  
oracle, postgres and mysql every day...

The comments that taking a backup causes a lot of absolutely  
unavoidable I/O is right on the mark.

If you have a large enough database where this matters the technique  
usually looks as follows.

(1) sanity
(2) postgres_start_backup
(3) snap
(4) postgres_stop_backup
(5) backup

Now, the backup will always have to read the data, if it is full it  
reads every block.  If it is incremental, it reads the blocks that  
changed.  You will frequently be in the position of performing a full  
backup.  The bandwidth for doing the read will inevitably happen in  
one or more of the above steps.  I strongly prefer that load to  
happen in (5) and for steps (2,3,4) to happen as quickly as  
possible.  Right now on our largest (slowest) production box which is  
postgres and over a terabyte, steps 2-4 take about 30-60 seconds.   
Step 5 takes *cough* about 18 hours *cough*.

The snap in many of our cases is an logical software enabled snapshot  
(either Veritas, LVM or ZFS).  However, you can use many enterprise  
storage to take a hard snapshot and expose that as a LUN to mount  
elsewhere on attached to the same SAN.  Many confuse this for being  
"free".  Regardless of how the snap is taken you have to pay for it..  
either at snap time, at read time or at release time.  Nothing's free.

// Theo Schlossnagle
// Principal@OmniTI: http://omniti.com
// Esoteric Curio: http://www.lethargy.org/~jesus/