Thread: Data recovery
I am recovering a system from a complete HD crash. I have a backup of the Postgres data directory, e.g. /usr/local/pgsql/data/base. Is there a way or method to have Postgres pick these databases up again? The version of Postgres run in this machine was/is 6.4.2. Thanks in advance for any assistance. Regards - Bob Kruger
Bob Kruger <bkruger@mindspring.com> writes:
> I am recovering a system from a complete HD crash.
> I have a backup of the Postgres data directory, e.g.
> /usr/local/pgsql/data/base.
> Is there a way or method to have Postgres pick these databases up again?
> The version of Postgres run in this machine was/is 6.4.2.
If you have a backup of the whole pgsql/data directory, just reinstall
the same Postgres release, restore the data directory (instead of doing
an initdb), and you should be in fat city.
If you only have the data/base subdirectory, you will need to work
harder; you'll have to regenerate the top-level files. I think if you
get pg_shadow and pg_database right you will be OK. First, install and
initdb to get a basic set of files. You will need to recall the old set
of users (including their userIDs) in order to reconstruct pg_shadow.
After you've done the createusers, issue a createdb for each old
database (subdirectory of base/) so that they have entries in
pg_database. Then, shut down the postmaster, blow away the contents of
the base/ subdirectory and restore it from tape, and restart. I think
it'll work...
In any case it's critical to install the same Postgres version you
were using.
regards, tom lane
Tom Lane wrote:
>
> Bob Kruger <bkruger@mindspring.com> writes:
> > I am recovering a system from a complete HD crash.
> > I have a backup of the Postgres data directory, e.g.
> > /usr/local/pgsql/data/base.
> > Is there a way or method to have Postgres pick these databases up again?
> > The version of Postgres run in this machine was/is 6.4.2.
>
> If you have a backup of the whole pgsql/data directory, just reinstall
> the same Postgres release, restore the data directory (instead of doing
> an initdb), and you should be in fat city.
>
> If you only have the data/base subdirectory, you will need to work
> harder; you'll have to regenerate the top-level files. I think if you
> get pg_shadow and pg_database right you will be OK. First, install and
> initdb to get a basic set of files. You will need to recall the old set
> of users (including their userIDs) in order to reconstruct pg_shadow.
> After you've done the createusers, issue a createdb for each old
> database (subdirectory of base/) so that they have entries in
> pg_database. Then, shut down the postmaster, blow away the contents of
> the base/ subdirectory and restore it from tape, and restart. I think
> it'll work...
>
> In any case it's critical to install the same Postgres version you
> were using.
NO - this cannot work. He surely needs the entire data
directory because the information in the heap's relies on the
bits in data/pg_log. And that info (which XID's are
committed and which not) cannot be reconstructed from the
files - no chance.
Jan
--
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#======================================== jwieck@debis.com (Jan Wieck) #
> > If you only have the data/base subdirectory, you will need to work > > harder; you'll have to regenerate the top-level files. I think if you > > get pg_shadow and pg_database right you will be OK. First, install and > > initdb to get a basic set of files. You will need to recall the old set > > of users (including their userIDs) in order to reconstruct pg_shadow. > > After you've done the createusers, issue a createdb for each old > > database (subdirectory of base/) so that they have entries in > > pg_database. Then, shut down the postmaster, blow away the contents of > > the base/ subdirectory and restore it from tape, and restart. I think > > it'll work... > > > > In any case it's critical to install the same Postgres version you > > were using. > > NO - this cannot work. He surely needs the entire data > directory because the information in the heap's relies on the > bits in data/pg_log. And that info (which XID's are > committed and which not) cannot be reconstructed from the > files - no chance. Very, very hard, but not impossible. If you update a row, and do a select on that row, the select updates the transaction status so the next select doesn't need to look at the pg_log table. What this means is that pg_log could probably be reconstructed from existing data, with just 'unselected' changes not appearing properly. -- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026
>
> > > If you only have the data/base subdirectory, you will need to work
> > > harder; you'll have to regenerate the top-level files. I think if you
> > > get pg_shadow and pg_database right you will be OK. First, install and
> > > initdb to get a basic set of files. You will need to recall the old set
> > > of users (including their userIDs) in order to reconstruct pg_shadow.
> > > After you've done the createusers, issue a createdb for each old
> > > database (subdirectory of base/) so that they have entries in
> > > pg_database. Then, shut down the postmaster, blow away the contents of
> > > the base/ subdirectory and restore it from tape, and restart. I think
> > > it'll work...
> > >
> > > In any case it's critical to install the same Postgres version you
> > > were using.
> >
> > NO - this cannot work. He surely needs the entire data
> > directory because the information in the heap's relies on the
> > bits in data/pg_log. And that info (which XID's are
> > committed and which not) cannot be reconstructed from the
> > files - no chance.
>
> Very, very hard, but not impossible. If you update a row, and do a
> select on that row, the select updates the transaction status so the
> next select doesn't need to look at the pg_log table. What this means
> is that pg_log could probably be reconstructed from existing data, with
> just 'unselected' changes not appearing properly.
So at the end you have some data that you cannot trust. I
don't think that's worth the efford.
Jan
--
#======================================================================#
# It's easier to get forgiveness for being wrong than for being right. #
# Let's break this rule - forgive me. #
#======================================== jwieck@debis.com (Jan Wieck) #
> > Very, very hard, but not impossible. If you update a row, and do a > > select on that row, the select updates the transaction status so the > > next select doesn't need to look at the pg_log table. What this means > > is that pg_log could probably be reconstructed from existing data, with > > just 'unselected' changes not appearing properly. > > So at the end you have some data that you cannot trust. I > don't think that's worth the efford. Yes. True. It is just a point that came up recently when Tom found the first select on a table slow. -- Bruce Momjian | http://www.op.net/~candle maillist@candle.pha.pa.us | (610) 853-3000 + If your life is a hard drive, | 830 Blythe Avenue + Christ can be your backup. | Drexel Hill, Pennsylvania 19026