Re: A few questions to real pgsql gurus

And what about all the stuff in memory, pending transactions, etc?

2/24/2003 9:33:55 AM, Dmitry Tkach  wrote:

>Tom Lane wrote:
>
>>Dmitry Tkach  writes:
>>  
>>
>>>- The database files sit on a NetApp box (this is a very reliable disk array with parity 
control etc...
>>>plus, it has a 'snapshot' capability, that allows you to take a live snapshot of any directory 
in real time - that creates
>>>a read-only copy of the content of that directory within a second or so).
>>>- The snapshots are taken hourly by a cron job on the whole database (older snapshots get 
removed after a while), and the most recent snapshot
>>>gets backed up to the tape every night.
>>>    
>>>
>>
>>Have you ever actually restored one of those snapshots?  I don't believe
>>this will work, unless you shut down the database while taking the
>>snapshot.  Or unless the snapshot is instantaneous across all files in
>>the $PGDATA directory tree --- but your "within a second or so" doesn't
>>sound real promising.  If there's any time skew between copying
>>different files, you've got problems.
>>
>>	
>>
>Yes, it *is* instanteneous, that's the point!
>The idea is, that it does not really copy anything at all - just freezes 
>all the stuff under that directory exactly the way it is, and then, if 
>somebody (like postmaster) is writing to that directory, it replaces the 
>particular block that's is neing modified, so that the snapshot contiues 
>to look at the old block, and the 'real thing' referes to the new one...
>
>That 'under a second' time is what it needs to setup its bookeeping 
>recors, and it freezes all writes to the directory during that time...
>
>Dima
>
>
>
>
>---------------------------(end of broadcast)---------------------------
>TIP 6: Have you searched our list archives?
>
>http://archives.postgresql.org
>