Re: A few questions to real pgsql gurus

Dennis Gearon wrote:

>And what about all the stuff in memory, pending transactions, etc?
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Well... all of that will be gone, of course...
If you ever need to restore from that snapshot, it will look like the
crash happened a few milliseconds before it actually did :-)

Dima

>2/24/2003 9:33:55 AM, Dmitry Tkach <dmitry(at)openratings(dot)com> wrote:
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>>Tom Lane wrote:
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>>>Dmitry Tkach <dmitry(at)openratings(dot)com> writes:
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>>>>- The database files sit on a NetApp box (this is a very reliable disk array with parity
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>control etc...
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>>>>plus, it has a 'snapshot' capability, that allows you to take a live snapshot of any directory
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>in real time - that creates
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>>>>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
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>removed after a while), and the most recent snapshot
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>>>>gets backed up to the tape every night.
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>>>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.
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>>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...
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>>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...
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>>Dima
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