Re: A few questions to real pgsql gurus

Dmitry Tkach wrote:
> Tom Lane wrote:
<snip>
>> 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...

If it's of any help, recent versions of Solaris 8 and higher come with
this built in too.

Regards and best wishes,

Justin Clift

> 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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