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 <dmitry(at)openratings(dot)com> wrote:

>Tom Lane wrote:
>
>>Dmitry Tkach <dmitry(at)openratings(dot)com> 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
>
>
>
>
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