Re: HA PostgreSQL

andrew@libertyrms.info (Andrew Sullivan) wrote:
> On Fri, Nov 01, 2002 at 01:31:24PM -0500, Charles H. Woloszynski wrote:
>>
>> I have seen a proposed solution that uses *rsync* on the database files
>> between machines and linux-ha to roll over the network access to the
>> available machine.  My question is pretty simple; can something as
>> *simple* as rsync make a full copy of the database consistently between
>> the machines?  That seems just too easy.
>
> It is too easy.  This has come up more than once recently, and I've
> never seen an argument that using rsync this way is no better than
> using tar on a live data area.  If you want high availability, don't
> do it.

There's one /conceivable/ way for this to work, and that's if you're
filesystem mirroring using something along the lines of Digital's
"AdvFS."

The idea:  You have a duplicate mirror that is being kept up-to-date
at the filesystem level.

You then "break" the mirror, temporarily, and then run rsync on the
duplicate.

Then run [whatever is necessary] to get the sync back up and running.

There is the not-minor problem that this would require perform the
logical equivalent to 'fsync' on the database immediately before
breaking the link to ensure that the state of the DB is 'not
deranged.'

Enumerating the total set of vital non-obvious dependancies is left as
an exercise for the reader...
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