On 18.12.2012 00:35, Simon Riggs wrote:
On 17 December 2012 17:39, Heikki Linnakangas<hlinnakan...@vmware.com> wrote:
(This is different from the other issue related to timeline switches I just
posted about. There's no timeline switch involved in this one.)
If you do "pg_basebackup -x" against a standby server, in some circumstances
the backup fails to restore with error like this:
C 2012-12-17 19:09:44.042 EET 7832 LOG: database system was not properly
shut down; automatic recovery in progress
C 2012-12-17 19:09:44.091 EET 7832 LOG: record with zero length at
0/1764F48
C 2012-12-17 19:09:44.091 EET 7832 LOG: redo is not required
C 2012-12-17 19:09:44.091 EET 7832 FATAL: WAL ends before end of online
backup
C 2012-12-17 19:09:44.091 EET 7832 HINT: All WAL generated while online
backup was taken must be available at recovery.
C 2012-12-17 19:09:44.092 EET 7831 LOG: startup process (PID 7832) exited
with exit code 1
C 2012-12-17 19:09:44.092 EET 7831 LOG: aborting startup due to startup
process failure
I spotted this bug while reading the code, and it took me quite a while to
actually construct a test case to reproduce the bug, so let me begin by
discussing the code where the bug is. You get the above error, "WAL ends
before end of online backup", when you reach the end of WAL before reaching
the backupEndPoint stored in the control file, which originally comes from
the backup_label file. backupEndPoint is only used in a base backup taken
from a standby, in a base backup taken from the master, the end-of-backup
WAL record is used instead to mark the end of backup. In the xlog redo loop,
after replaying each record, we check if we've just reached backupEndPoint,
and clear it from the control file if we have. Now the problem is, if there
are no WAL records after the checkpoint redo point, we never even enter the
redo loop, so backupEndPoint is not cleared even though it's reached
immediately after reading the initial checkpoint record.
To deal with the similar situation wrt. reaching consistency for hot standby
purposes, we call CheckRecoveryConsistency() before the redo loop. The
straightforward fix is to copy-paste the check for backupEndPoint to just
before the redo loop, next to the CheckRecoveryConsistency() call. Even
better, I think we should move the backupEndPoint check into
CheckRecoveryConsistency(). It's already responsible for keeping track of
whether minRecoveryPoint has been reached, so it seems like a good idea to
do this check there as well.
Attached is a patch for that (for 9.2), as well as a script I used to
reproduce the bug. The script is a bit messy, and requires tweaking the
paths at the top. Anyone spot a problem with this?
Yep. The problem is one of design, not merely a coding error.
The design assumes that the master is up, connected and doing work.
Which is perfectly reasonable, since this is the reason we are doing a
backup from the standby. But obviously not always true, so what we're
saying is that the selection of backupEndPoint is incorrect in the
first place. Putting code in to cope with that poor choice at recovery
seems wrong - we should record the correct location.
Hmm, pg_controlinfo says:
pg_control version number: 922
Catalog version number: 201204301
Database system identifier: 5823207860608399803
Database cluster state: in crash recovery
pg_control last modified: ti 18. joulukuuta 2012 11.00.58
Latest checkpoint location: 0/1764EE8
Prior checkpoint location: 0/1764EE8
Latest checkpoint's REDO location: 0/1764EE8
Latest checkpoint's TimeLineID: 1
...
Minimum recovery ending location: 0/1764F48
Backup start location: 0/1764EE8
Backup end location: 0/1764F48
...
That seems correct to me. The backup is considered valid after reaching
0/1764F48, which is where the checkpoint record ends. minRecoveryPoint
is set to the same. What do you think it should be set to?
In passing, I see another problem as well. We assume that the backup
is still valid if we promote the standby to a master during the
backup, but that isn't documented and so isn't full analysed to see
that is true.
No, that is taken care of in do_pg_stop_backup:
/*
* Parse the BACKUP FROM line. If we are taking an online backup from
the
* standby, we confirm that the standby has not been promoted during the
* backup.
*/
ptr = strstr(remaining, "BACKUP FROM:");
if (!ptr || sscanf(ptr, "BACKUP FROM: %19s\n", backupfrom) != 1)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("invalid data in file \"%s\"",
BACKUP_LABEL_FILE)));
if (strcmp(backupfrom, "standby") == 0 && !backup_started_in_recovery)
ereport(ERROR,
(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
errmsg("the standby was promoted during online
backup"),
errhint("This means that the backup being taken is
corrupt "
"and should not be used. "
"Try taking another online
backup.")));
- Heikki
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