+++ /dev/null
-From ccshag@cclabs.missouri.edu Wed Jul 10 04:27:16 1996
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-Date: Wed, 10 Jul 1996 03:26:41 -0500 (CDT)
-From: "Paul 'Shag' Walmsley" <ccshag@cclabs.missouri.edu>
-Sender: ccshag@cclabs.missouri.edu
-To: postgres95-hackers@oozoo.vnet.net
-cc: pixel@tiger.coe.missouri.edu, michael.siebenborn@ae3.Hypo.DE,
- scrappy@ki.net
-Subject: Workaround for database corruption problems [long]
-Message-ID: <Pine.SGI.3.93.960710023021.673B-100000@realtime.cc.missouri.edu>
-X-Disclaimer: The opinions of this writer do not necessarily represent those of the University of Missouri-Columbia.
-MIME-Version: 1.0
-Content-Type: TEXT/PLAIN; charset=US-ASCII
-Status: RO
-X-Status:
-
-
-I've been poking through the guts of POSTGRES95 in pursuit of the
-previously-mentioned "database corruption" problem, and I think that I
-have some preliminary answers and a workaround for people to play around
-with.
-
-Specifically, the problem that I'm referring to is manifested by
-pg_log, pg_time, or pg_variable either growing to huge sizes (usually
-128MB) or showing evidence of internal corruption. Symptoms that appear
-to the user include receiving messages like the following in psql or
-monitor:
- "WARN:cannot open segment 1 of relation pg_time"
- "WARN:cannot write block 16384 of pg_log"
-.. which are followed up in a 'postmaster -d 3' debug log by a
- "NOTICE:AbortTransaction and not in in-progress state"
-
-It's been my experience that this problem is difficult to reproduce on
-demand; I've tried several different ways and haven't succeeded. (If
-anyone knows how to reliably reproduce this, please E-mail me.) I was,
-however, able to obtain a database in a corrupted state from one of the
-fellows on campus here running POSTGRES95; and following is something of a
-post-mortem.
-
-----------
-The problem with the database is that after any query, POSTGRES95 attempts
-to generate a huge pg_log file and fails (running out of disk space) with
-a "WARN:cannot write block 16384 of pg_log" message. The contents of the
-database and its internal structure were reasonably intact - I discovered
-that I was able to execute a single SQL query per-backend, and could
-therefore back the contents of the database up with a COPY command. (I
-wasn't able to use pg_dump; it failed on its BEGIN TRANSACTION step.)
-Inspection of data/pg_log, data/pg_variable, and data/pg_time revealed the
-following;
-
-realtime /usr/postgres95 > od -x data/pg_log.old | head
-0000000 0060 2000 2000 2000 f680 0000 ed00 0000
-0000020 e300 0000 dc00 0000 ce00 0000 bc00 0000
-0000040 b140 0000 a740 0000 9dc0 0000 9180 0000
-0000060 8580 0000 7c80 0000 6fc0 0000 6480 0000
-0000100 58c0 0000 4fc0 0000 4640 0000 3a80 0000
-0000120 30c0 0000 23c0 0000 1900 0000 0c40 0000
-0000140 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0000200 8800 0000 0000 0000 0800 0000 0000 0000
-0000220 0800 0000 0000 0000 0800 0000 0000 0000
-
-
-Now, already something is obviously wrong: pg_log shouldn't have any
-values other than 0, 1, 2, 4, 8, a, c, or e for each nybble (see the XID_
-... definitions in src/backend/access/transam.h:35; two of these can be
-stored per nybble.)
-
-
-realtime /usr/postgres95 > od -x data/pg_variable.old | head
-0000000 0058 2000 2000 2740 f240 0000 e400 0000
-0000020 d780 0000 cac0 0000 b840 0000 a900 0000
-0000040 9f80 0000 9280 0000 8580 0000 78c0 0000
-0000060 6c80 0000 6200 0000 5800 0000 4f80 0000
-0000100 44c0 0000 39c0 0000 2d80 0000 2600 0000
-0000120 1c00 0000 0e00 0000 0000 0000 0000 0000
-0000140 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0000700 0000 01bc 0000 009c 0014 ffff ffff 0000
-0000720 0004 5f0b 0000 0000 0000 0ba3 0000 0c23
-
-
-This is obviously broken too; the only bytes that should be non-zero in
-pg_variable are the first 16 (see src/backend/access/transam.h:99).
-
-
-realtime /usr/postgres95 > od -x data/pg_time.old | head -10
-0000000 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0004000 31c6 09ae 0000 0000 31c6 09b3 31a9 79a6
-0004020 31a9 79a6 31a9 79a8 31a9 79a9 31a9 79a9
-0004040 31a9 79a9 31a9 79aa 31a9 79aa 31a9 79aa
-0004060 31a9 79aa 31a9 79aa 31a9 79ab 31a9 79ab
-0004100 31a9 79ab 31a9 79ac 31a9 79ac 31a9 79ac
-0004120 31a9 79ac 31a9 79ad 31a9 79ad 31a9 79ae
-0004140 31a9 79ae 31a9 79af 31a9 79af 31a9 79af
-0004160 31a9 79b0 31a9 79b0 31a9 79b4 31a9 79b4
-realtime /usr/postgres95 > od -x data/pg_time.old +32b | head -10
-0032000 6665 6520 666f 7220 31c5 cb61 696e 6720
-0032020 6465 7665 6c6f 7065 7273 2062 7579 2061
-0032040 206c 6f77 2d69 6e63 6f6d 6520 6170 6172
-0032060 746d 656e 7420 636f 6d70 6c65 7869 6e20
-0032100 7468 6520 7472 6561 7375 7265 7227 7320
-0032120 7761 7264 2066 726f 6d20 7468 6520 6665
-0032140 6465 7261 6c20 676f 7665 726e 6d65 6e74
-0032160 2066 6f72 2024 313b 2064 6576 656c 6f70
-0032200 6572 7361 6c73 6f20 31c5 cb7c 31be 91a2
-0032220 31be 91a2 31be 91a2 31be 91a2 31be 91a3
-
-The first part of pg_time looked okay -- filled with 4-byte time values
-with the occasional "00 00 00 00" here and there -- but inspection into
-pg_time's guts revealed some preposterous numbers.
-
-For your comparing pleasure, I include copies of what these files
-_should_ look like -- more or less -- from a newly initdb'd database
-directory:
-
-realtime /usr/postgres95 > od -x data-new-initdb/pg_log
-0000000 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0000200 8aaa aaaa aaaa aaa8 0000 0000 0000 0000
-0000220 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0020000
-realtime /usr/postgres95 > od -x data-new-initdb/pg_variable
-0000000 0000 0000 0000 0222 0000 021e 0000 5120
-0000020 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0020000
-realtime /usr/postgres95 > od -x data-new-initdb/pg_time
-0000000 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0004000 31e3 4fb3 0000 0000 31e3 4fb4 31e3 4fb4
-0004020 31e3 4fb4 31e3 4fb5 31e3 4fb5 31e3 4fb5
-0004040 31e3 4fb6 31e3 4fb6 31e3 4fb6 31e3 4fb6
-0004060 31e3 4fb7 31e3 4fb7 31e3 4fb7 31e3 4fb7
-0004100 31e3 4fb8 31e3 4fb8 31e3 4fb8 31e3 4fb9
-0004120 31e3 4fb9 31e3 4fb9 31e3 4fb9 31e3 4fba
-0004140 31e3 4fba 31e3 4fba 31e3 4fbb 31e3 4fbb
-0004160 31e3 4fbb 31e3 4fbb 31e3 4fbb 0000 0000
-0004200 0000 0000 0000 0000 0000 0000 0000 0000
-*
-0020000
-
-
-What's more, if I view pg_log, pg_variable, and pg_time with a text
-editor, I discover that the corrupted areas contain fragments of tuples
-from the database!
-----------
-
-
-Since I have been unable to reproduce this problem on demand (and
-therefore have a complete picture of all of the queries executed on the
-database), I've only been able to speculate on the original cause of the
-problem. One explanation would be some kind of buffer manager bug; or
-perhaps the backend is running low on file descriptors and bollixing
-existing ones? I intend to continue tracking this down as I have the
-time, but in the interim, I've created a quick Perl script that is
-designed to restore some sense of rationality to the
-pg_{log,time,variable} files. It's a horribly barbaric way to do this --
-at the very least, it almost certainly breaks time travel, and will
-almost certainly result in duplicate oid problems -- but it restored my
-pathological database here to a sufficiently working state such that I
-could run pg_dump on it and do various other queries with it. The script
-is included at the bottom of this message.
-
-Anyway, fellow posthackers, assuming that no one raises any red flags
-about this, I'll post it on postgres95@oozoo.vnet.net as a temporary
-stop-gap workaround to allow people to recover their databases when this
-sort of problem occurs.
-
-I wish I had more time to do this kind of thing.
-
-
-- Paul "Shag" Walmsley <ccshag@cclabs.missouri.edu>
- "Knowing is not enough." -- Hal Hartley, "Surviving Desire"
-
-
+++ /dev/null
-#!/bin/perl
-#
-# zap_ltv - attempt to restore a POSTGRES95 database afflicted with
-# pg_log, pg_time, or pg_variable corruption to
-# minimal functionality
-#
-# Paul Walmsley <ccshag@cclabs.missouri.edu>
-#
-# Legalese:
-#
-# In no event shall Paul Walmsley be liable to any party for direct,
-# indirect, special, incidental, or consequential damages, including
-# lost profits, arising from the use of this software, even if Paul Walmsley
-# has been advised of the possibility of such damage. Paul Walmsley
-# specifically disclaims any warranties, including, but not limited to,
-# the implied warranties of merchantability and fitness for a particular
-# purpose. The software provided hereunder is on an "as is" basis,
-# and Paul Walmsley has no obligations to provide maintenance, support,
-# updates, enhancements, or modifications.
-#
-# Thanks, Berkeley ;-)
-
-print "This program should only be run if POSTGRES95 is not currently\n";
-print "running on this system. It should also not be run unless you \n";
-print "are having seemingly unrecoverable problems with your POSTGRES95\n";
-print "database related to pg_log, pg_time, or pg_variable corruption.\n\n";
-print "This program replaces the existing pg_log, pg_time, and pg_variable\n";
-print "files with \"clean\" copies of those files. This will almost \n";
-print "certainly result in duplicate IDs when any INSERTs are attempted,\n";
-print "and probably has other side-effects as well. Back up your databases\n"
-print "and re-initdb from scratch after using this!\n\n";
-print "This program will attempt to make a backup of your pg_time,\n";
-print "pg_log, and pg_variable files (to pg_time.backup, pg_log.backup,\n";
-print "and pg_variable.backup, respectively).\n\n";
-print "This program bears no guarantees nor any warranties whatsoever.\n";
-print "View the source for details.\n\n";
-print "Press ENTER to zap your pg_log, pg_time, and pg_variable files:";
-
-$trash=<STDIN>;
-
-$pg_log_data=pack('xxxx@8192',0);
-$pg_time_data=pack('xxxx@8192',0);
-# next_tid, last_tid, and oid are pulled from a fresh initdb
-$pg_variable_data=pack('xxxxxxCCxxCCxxCC@8192',2,34,2,30,81,32);
-
-if (length($pg_log_data)!=8192) {
- die "pg_log_data must be exactly 8192 bytes long";
-}
-if (length($pg_time_data)!=8192) {
- die "pg_time_data must be exactly 8192 bytes long";
-}
-if (length($pg_variable_data)!=8192) {
- die "pg_variable_data must be exactly 8192 bytes long";
-}
-
-
-if (! -f 'pg_database') {
- die "This program must be run from your POSTGRES95 data directory.";
-}
-
-system('cp pg_log pg_log.backup');
-open(PG_LOG,'>pg_log');
-binmode(PG_LOG);
-$written=syswrite(PG_LOG,$pg_log_data,8192);
-close(PG_LOG);
-
-if ($written!=8192) {
- die "pg_log write failed: $!";
-}
-
-$written=0;
-system('cp pg_time pg_time.backup');
-open(PG_TIME,'>pg_time');
-binmode(PG_TIME);
-$written=syswrite(PG_TIME,$pg_time_data,8192);
-close(PG_TIME);
-
-if ($written!=8192) {
- die "pg_time write failed: $!";
-}
-
-$written=0;
-system('cp pg_variable pg_variable.backup');
-open(PG_VARIABLE,'>pg_variable');
-binmode(PG_VARIABLE);
-$written=syswrite(PG_VARIABLE,$pg_variable_data,8192);
-close(PG_VARIABLE);
-
-if ($written!=8192) {
- die "pg_variable write failed: $!";
-}
-
-print "Done.\n";
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