Database Recovery Procedures

[Network Administrator]

Looks like for the first time in 6 years, I'm experienced some database table
corruption. This was due to the space filling up on a server (you don't want to
know how that happened).

I have 3 tables corrupt and the others are fine (which I dumped to be safe). I
have a backup which I could use but then I realized that maybe there might be
some "surgery" I could perform to get the table "repaired". Note that the
normal recovery that the database does on its own did not work in this case.

I looked through the documentation (Admin 7.3.2) and I thought there was a
disaster recovery section but there is only "recovery" discussed as part of
backup/restore. If this information is out there somewhere else if someone
could provide a link that would be a great help as well.

My question/statement is that I think this is something that is important to
have. At least in regards to different strategies one could try to surgically
recover data BEFORE use the broad sword method of going to a backup. One of the
successful "sell" points I use to my clients is how resilient Linux/Unix
filesystems are. As well as Pg on Linux. In the case here, though I don't have
FS corruption so I'd like to know what should and could I do in this case.

Suggestions?

Oh and here is the output of a "select *" on one of the corrupt tables...

(saved as draft email here on 9/12/03)

...Ok, I was going to paste that in the email but now the database isn't coming
up at all. Here is the start up message

~~~
DEBUG: FindExec: found "/usr/local/pgsql/bin/postgres" using argv[0]
DEBUG: invoking IpcMemoryCreate (size=1466368)
DEBUG: FindExec: found "/usr/local/pgsql/bin/postmaster" using argv[0]
LOG: database system shutdown was interrupted at 2003-09-16 15:11:36 EDT
LOG: checkpoint record is at 5/2D497FC0
LOG: redo record is at 5/2D497FC0; undo record is at 0/0; shutdown TRUE
LOG: next transaction id: 5287090; next oid: 26471
LOG: database system was not properly shut down; automatic recovery in progress
LOG: ReadRecord: unexpected pageaddr 5/27498000 in log file 5, segment 45,
offset 4816896
LOG: redo is not required
PANIC: XLogWrite: write request 5/2D498000 is past end of log 5/2D498000
DEBUG: reaping dead processes
LOG: startup process (pid 17031) was terminated by signal 6
LOG: aborting startup due to startup process failure
DEBUG: proc_exit(1)
DEBUG: shmem_exit(1)
DEBUG: exit(1)
~~~

Thanks in advance to all

--
Keith C. Perry
Director of Networks & Applications
VCSN, Inc.
http://vcsn.com
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[Tom Lane]

Network Administrator <ne******@vcsn. com> writes:

PANIC: XLogWrite: write request 5/2D498000 is past end of log 5/2D498000

I'll bet you are running 7.3.3. You need to update to 7.3.4 to escape
this startup bug.

regards, tom lane

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[Network Administrator]

Ahhh, we it just so happens that I have 7.3.4 waiting to be built. I was trying
to wait up 7.4 was released but other than this startup bug does 7.3.4 do a
better job of recovery?

*pause*

Ok, wait- at this point I have to go to 7.3.4 because I would not be able to go
to 7.4 'cause I need the dump/restore process that we've been talking about
first... so, I'll do that and when/if I have a problem, I'll repost :)

Quoting Tom Lane <tg*@sss.pgh.pa .us>:

Network Administrator <ne******@vcsn. com> writes:

PANIC: XLogWrite: write request 5/2D498000 is past end of log 5/2D498000

I'll bet you are running 7.3.3. You need to update to 7.3.4 to escape
this startup bug.

regards, tom lane

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--
Keith C. Perry
Director of Networks & Applications
VCSN, Inc.
http://vcsn.com

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[Network Administrator]

Ok, upgrade done and the system is up but when try hit database through the app
(browser front end just does selects) on the debug screen (level 3) I get this...

DEBUG: child process (pid 21248) was terminated by signal 6
LOG: server process (pid 21248) was terminated by signal 6
LOG: terminating any other active server processes
LOG: all server processes terminated; reinitializing shared memory and semaphores
DEBUG: shmem_exit(0)
DEBUG: invoking IpcMemoryCreate (size=1466368)
LOG: database system was interrupted at 2003-09-17 10:52:16 EDT
LOG: checkpoint record is at 5/2D498110
LOG: redo record is at 5/2D498110; undo record is at 0/0; shutdown TRUE
LOG: next transaction id: 5287090; next oid: 26471
LOG: database system was not properly shut down; automatic recovery in progress
LOG: ReadRecord: record with zero length at 5/2D498150
LOG: redo is not required
LOG: database system is ready
DEBUG: proc_exit(0)
DEBUG: shmem_exit(0)
DEBUG: exit(0)

...if I using the psql client, and issue a "select * from <corrupt table name>
limit 5" if get this...
PANIC: read of clog file 5, offset 16384 failed: Success
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: Failed.
!#

...I've never seen the prompt got to "!#"
So back to my original question. What are the recover procedures (if any) that
should be tried before I grab my PGDATA path from tape?
Quoting Tom Lane <tg*@sss.pgh.pa .us>:

Network Administrator <ne******@vcsn. com> writes:

PANIC: XLogWrite: write request 5/2D498000 is past end of log 5/2D498000

I'll bet you are running 7.3.3. You need to update to 7.3.4 to escape
this startup bug.

regards, tom lane

--
Keith C. Perry
Director of Networks & Applications
VCSN, Inc.
http://vcsn.com

_______________ _______________ ______
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[Tom Lane]

Network Administrator <ne******@vcsn. com> writes:

..if I using the psql client, and issue a "select * from <corrupt table name>
limit 5" if get this... PANIC: read of clog file 5, offset 16384 failed: Success
Hm, not good :-(. What files actually exist in $PGDATA/pg_clog/ (names
and sizes)?
So back to my original question. What are the recover procedures (if
any) that should be tried before I grab my PGDATA path from tape?

You may be up against having to do that, but some investigation first
seems called for.

regards, tom lane

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[Network Administrator]

Quoting Tom Lane <tg*@sss.pgh.pa .us>:

Network Administrator <ne******@vcsn. com> writes:

..if I using the psql client, and issue a "select * from <corrupt table name>

limit 5" if get this...

PANIC: read of clog file 5, offset 16384 failed: Success

Hm, not good :-(. What files actually exist in $PGDATA/pg_clog/ (names
and sizes)?

Here you go...

-rw------- 1 postgres users 262144 Jul 20 15:53 0000
-rw------- 1 postgres users 262144 Jul 31 12:57 0001
-rw------- 1 postgres users 262144 Aug 12 17:32 0002
-rw------- 1 postgres users 262144 Aug 26 00:15 0003
-rw------- 1 postgres users 262144 Sep 9 23:44 0004
-rw------- 1 postgres users 16384 Sep 10 21:21 0005

So back to my original question. What are the recover procedures (if
any) that should be tried before I grab my PGDATA path from tape?

You may be up against having to do that, but some investigation first
seems called for.

Yep, its ready to go. When and if nothing surgical can be done.
regards, tom lane

--
Keith C. Perry
Director of Networks & Applications
VCSN, Inc.
http://vcsn.com

_______________ _______________ ______
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[Tom Lane]

Network Administrator <ne******@vcsn. com> writes:

PANIC: read of clog file 5, offset 16384 failed: Success

Hm, not good :-(. What files actually exist in $PGDATA/pg_clog/ (names
and sizes)?

-rw------- 1 postgres users 262144 Jul 20 15:53 0000
-rw------- 1 postgres users 262144 Jul 31 12:57 0001
-rw------- 1 postgres users 262144 Aug 12 17:32 0002
-rw------- 1 postgres users 262144 Aug 26 00:15 0003
-rw------- 1 postgres users 262144 Sep 9 23:44 0004
-rw------- 1 postgres users 16384 Sep 10 21:21 0005

Okay, it's trying to read off the end of the clog, no doubt looking for
a transaction number just slightly larger than what's known to clog.
This probably indicates more serious problems (because WAL replay really
should have prevented such an inconsistency), but you can get past the
immediate panic relatively easily: just append an 8k page of zeroes to
clog. Assuming your system has /dev/zero, something like this should
do it:

dd bs=8k count=1 < /dev/zero >> $PGDATA/pg_clog/0005

(do this with care of course, and you should probably shut down the
postmaster first). You might possibly have to add more than one page,
if you then get similar PANICs with larger offsets, but try one page
for starters.

If this does suppress the failure messages, you are still not really out
of the woods; you should do what you can to check for data consistency.
A paranoid person would probably take a complete pg_dump and try to diff
it against the last known good dump. At the very least, I'd treat the
table involved in the problem with great suspicion.

regards, tom lane

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[Network Administrator]

Quoting Tom Lane <tg*@sss.pgh.pa .us>:

Network Administrator <ne******@vcsn. com> writes:

PANIC: read of clog file 5, offset 16384 failed: Success Hm, not good :-(. What files actually exist in $PGDATA/pg_clog/ (names
and sizes)?

-rw------- 1 postgres users 262144 Jul 20 15:53 0000
-rw------- 1 postgres users 262144 Jul 31 12:57 0001
-rw------- 1 postgres users 262144 Aug 12 17:32 0002
-rw------- 1 postgres users 262144 Aug 26 00:15 0003
-rw------- 1 postgres users 262144 Sep 9 23:44 0004
-rw------- 1 postgres users 16384 Sep 10 21:21 0005

Okay, it's trying to read off the end of the clog, no doubt looking for
a transaction number just slightly larger than what's known to clog.
This probably indicates more serious problems (because WAL replay really
should have prevented such an inconsistency), but you can get past the
immediate panic relatively easily: just append an 8k page of zeroes to
clog. Assuming your system has /dev/zero, something like this should
do it:

dd bs=8k count=1 < /dev/zero >> $PGDATA/pg_clog/0005

(do this with care of course, and you should probably shut down the
postmaster first). You might possibly have to add more than one page,
if you then get similar PANICs with larger offsets, but try one page
for starters.

Well whatdoyaknow! That did it- EVERYTHING is there! I only needed the one
page. I'm going to have to read up on pg_clog (WAL) so that I understand what
it does a little better.

It makes total sense too because obvious if there is no more space to write too
anything on disk get "frozen" where as application is just going to keep moving
along. At least for a time.
If this does suppress the failure messages, you are still not really out
of the woods; you should do what you can to check for data consistency.
A paranoid person would probably take a complete pg_dump and try to diff
it against the last known good dump. At the very least, I'd treat the
table involved in the problem with great suspicion.

regards, tom lane

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Well, I fortunately was on the side of the road and not in the woods in this
case. I'm assuming the next thing to do is run a vacuum analyse and they a dump
all. I'll see how it things perform over the next 48 hours or so.

Now that we've done that, how should should this surgery be documented? I would
think a "tip" like this should be somewhere in the docs (not just the archive).
I'd be more than will to write up this case but I'm trying to establish a long
goal here- if I may be so bold as to suggest one.

Thanks again.

--
Keith C. Perry
Director of Networks & Applications
VCSN, Inc.
http://vcsn.com

_______________ _______________ ______
This email account is being host by:
VCSN, Inc : http://vcsn.com

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