Обсуждение: Multiple WALs for PITR ?
I wan`t to use Point in Time Recovery I allread read http://developer.postgresql.org/docs/postgres/backup-online.html#BACKUP-ARCHIVING-WAL But I still wonder, if it is possible for PostgreSQL 8.0 to write multiple, redundant WAL-Files like Oracles with its redo-log-groups ? Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ? Thanks Michael Kleiser
Michael Kleiser wrote: > I wan`t to use Point in Time Recovery > I allread read > http://developer.postgresql.org/docs/postgres/backup-online.html#BACKUP-ARCHIVING-WAL > > > But I still wonder, if it is possible for PostgreSQL 8.0 to write > multiple, redundant WAL-Files > like Oracles with its redo-log-groups ? Could you explain what do this feature that PITR implementation don't support ? > Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ? Given that this is Postgresql list could you kindly explain what do this command, and also why do you think Postgresql must support it? Regards Gaetano Mendola
As an Oracle DBA primarily, moving more towards PostGreSQL as time passes, the "alter system switch logfile" command is very often used at the end of a hot backup to force a logfile switch and consequently provide a full archive log set. The command causes the database engine to cease using the current online redo log and have it archived to the archive log destination. This is the only way to be sure that all required archive logs are present in a backup to reconstitute the database at another location. Dick Goulet Senior Oracle DBA Oracle Certified 8i DBA -----Original Message----- From: simon@2ndquadrant.com [mailto:simon@2ndquadrant.com] Sent: Wednesday, October 06, 2004 5:36 PM To: mkl@webde-ag.de Cc: pgsql-admin@postgresql.org Subject: Re: [ADMIN] Multiple WALs for PITR ? Michael Kleiser <mkl@webde-ag.de> wrote on 06.10.2004, 16:29:08: > I wan`t to use Point in Time Recovery > I allread read > http://developer.postgresql.org/docs/postgres/backup-online.html#BACKU > P-ARCHIVING-WAL > Good! > But I still wonder, if it is possible for PostgreSQL 8.0 to write > multiple, redundant WAL-Files like Oracles with its redo-log-groups ? Not currently. That was something I considered, though in the end lacked both implementation time and justification for. If you can explain why you think that might be necessary, I'd be happy to consider it for v8.1. RAID-10 seems a good solution for me... > > Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ? > Not currently. Again, if you can say why we might want that, it can also be added - other than "they have it, so we want it too" :-) This is a likely addition for 8.1, since we will most likely want to archive xlog files every N minutes, whatever the transaction rate, allowing people to specify a maximum period of data loss - but this seemed like an enhancement rather than a definite feature for 8.0 > Thanks > Michael Kleiser > Glad you're interested. Your questions show you understand - good luck. Best Regards Simon Riggs ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
Goulet, Dick wrote: > As an Oracle DBA primarily, moving more towards PostGreSQL as time > passes, the "alter system switch logfile" command is very often used at > the end of a hot backup to force a logfile switch and consequently > provide a full archive log set. The command causes the database engine > to cease using the current online redo log and have it archived to the > archive log destination. This is the only way to be sure that all > required archive logs are present in a backup to reconstitute the > database at another location. > > > Dick Goulet > Senior Oracle DBA > Oracle Certified 8i DBA > -----Original Message----- > From: simon@2ndquadrant.com [mailto:simon@2ndquadrant.com] > Sent: Wednesday, October 06, 2004 5:36 PM > To: mkl@webde-ag.de > Cc: pgsql-admin@postgresql.org > Subject: Re: [ADMIN] Multiple WALs for PITR ? > > > Michael Kleiser <mkl@webde-ag.de> wrote on 06.10.2004, 16:29:08: > > I wan`t to use Point in Time Recovery > > I allread read > > http://developer.postgresql.org/docs/postgres/backup-online.html#BACKU > > P-ARCHIVING-WAL > > > > Good! > > > But I still wonder, if it is possible for PostgreSQL 8.0 to write > > multiple, redundant WAL-Files like Oracles with its redo-log-groups ? > > Not currently. > > That was something I considered, though in the end lacked both > implementation time and justification for. > > If you can explain why you think that might be necessary, I'd be happy > to consider it for v8.1. > > RAID-10 seems a good solution for me... I heared about some RAIDs saying "yes your data is on disk" to the oracle-database, but it wasn't so. So Oracle can't do a complete restore. The solution was to store the redo-log-files (=WALS) to local attached disk, but of course redundant on multiple disks. But other oracle-dba use RAIDs to store the redo-logs with no problems. > > > > > Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ? > > > > Not currently. > > Again, if you can say why we might want that, it can also be added - > other than "they have it, so we want it too" :-) In the online-backup-strategie described in http://developer.postgresql.org/docs/postgres/backup-online.html#BACKUP-ARCHIVING-WAL thre must be a special attention fir active WAL, if one want to backup it also. With "ALTER SYSTEM SWITCH LOGFILE" one could make it to a not active WAL. This would make it easier to backup it, especially if one want to use backups-programms like amanda. If there where multiple redundant WALs, one could compare them as a consistency-check. But this is only possible if nobody writes to the compared files. > > This is a likely addition for 8.1, since we will most likely want to > archive xlog files every N minutes, whatever the transaction rate, > allowing people to specify a maximum period of data loss - but this > seemed like an enhancement rather than a definite feature for 8.0 > Great! Regards Michael Kleiser
Some disk array manufacturers had a problem with their writes in that although the write the data was being stored in volatile memory instead of on disk. The disk buffers were volatile in that if power suddenly died all writes in memory that had not made it to disk were lost. Lately most array manufacturers (at least those I have experience with namely EMC & NetApp) have internal battery backup systems in place so that in the event of a power failure the array will accept no new data, but has sufficient battery power available to flush memory to a disk drive cleanly before crashing. This makes the probability that you end up with a corrupt database a whole lot lower. But there are still some out there, names not to be mentioned, who do not provide this functionality. Therefore buyer beware. Also with the advent of better disk arrays I am not currently aware of any DBA's using Oracle's redo mirroring. I've stopped using it in favor of EMC's disk mirrors or Business Continuance Volumes (BCV's). Now when Oracle writes a redo log file I really get three copies for the cost of one. Works just as well on PostGreSQL WAL & database files. But of course having the ability to force a wal file switch would be a real bennie. Hope it makes it into 8.1. Dick Goulet Senior Oracle DBA Oracle Certified 8i DBA -----Original Message----- From: Michael Kleiser [mailto:mkl@webde-ag.de] Sent: Thursday, October 07, 2004 4:38 AM To: simon@2ndquadrant.com Cc: pgsql-admin@postgresql.org Subject: Re: [ADMIN] Multiple WALs for PITR ? Goulet, Dick wrote: > As an Oracle DBA primarily, moving more towards PostGreSQL as time > passes, the "alter system switch logfile" command is very often used > at the end of a hot backup to force a logfile switch and consequently > provide a full archive log set. The command causes the database > engine to cease using the current online redo log and have it archived > to the archive log destination. This is the only way to be sure that > all required archive logs are present in a backup to reconstitute the > database at another location. > > > Dick Goulet > Senior Oracle DBA > Oracle Certified 8i DBA > -----Original Message----- > From: simon@2ndquadrant.com [mailto:simon@2ndquadrant.com] > Sent: Wednesday, October 06, 2004 5:36 PM > To: mkl@webde-ag.de > Cc: pgsql-admin@postgresql.org > Subject: Re: [ADMIN] Multiple WALs for PITR ? > > > Michael Kleiser <mkl@webde-ag.de> wrote on 06.10.2004, 16:29:08: > > I wan`t to use Point in Time Recovery > I allread read > > http://developer.postgresql.org/docs/postgres/backup-online.html#BACKU > > P-ARCHIVING-WAL > > > > Good! > > > But I still wonder, if it is possible for PostgreSQL 8.0 to write > > multiple, redundant WAL-Files like Oracles with its redo-log-groups ? > > Not currently. > > That was something I considered, though in the end lacked both > implementation time and justification for. > > If you can explain why you think that might be necessary, I'd be happy > to consider it for v8.1. > > RAID-10 seems a good solution for me... I heared about some RAIDs saying "yes your data is on disk" to the oracle-database, but it wasn't so. So Oracle can't do a complete restore. The solution was to store the redo-log-files (=WALS) to local attached disk, but of course redundant on multiple disks. But other oracle-dba use RAIDs to store the redo-logs with no problems. > > > > > Is threre an equivalent to Oracles "ALTER SYSTEM SWITCH LOGFILE" ? > > > > Not currently. > > Again, if you can say why we might want that, it can also be added - > other than "they have it, so we want it too" :-) In the online-backup-strategie described in http://developer.postgresql.org/docs/postgres/backup-online.html#BACKUP- ARCHIVING-WAL thre must be a special attention fir active WAL, if one want to backup it also. With "ALTER SYSTEM SWITCH LOGFILE" one could make it to a not active WAL. This would make it easier to backup it, especially if one want to use backups-programms like amanda. If there where multiple redundant WALs, one could compare them as a consistency-check. But this is only possible if nobody writes to the compared files. > > This is a likely addition for 8.1, since we will most likely want to > archive xlog files every N minutes, whatever the transaction rate, > allowing people to specify a maximum period of data loss - but this > seemed like an enhancement rather than a definite feature for 8.0 > Great! Regards Michael Kleiser ---------------------------(end of broadcast)--------------------------- TIP 9: the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match
On Thu, Oct 07, 2004 at 10:32:04AM -0400, Goulet, Dick scratched on the wall:
> Some disk array manufacturers had a problem with their writes in that
> although the write the data was being stored in volatile memory instead
> of on disk. The disk buffers were volatile in that if power suddenly
> died all writes in memory that had not made it to disk were lost.
> Lately most array manufacturers (at least those I have experience with
> namely EMC & NetApp) have internal battery backup systems in place so
> that in the event of a power failure the array will accept no new data,
> but has sufficient battery power available to flush memory to a disk
> drive cleanly before crashing.
All of the RAID systems I've dealt with work a little differently. In
most cases the internal array battery is only there to keep the
contents of the buffer alive-- there is not enough power to actually
keep a disk spinning and write the data out. You'd need a real UPS
for that. Typically the internal batteries have enough power to
keep the volatile memory contents active for two weeks or more.
Once power is re-applied to the array, the buffers are
automatically flushed by the RAID controller.
They are also, frankly, a pain in the butt. We've had most of our
batteries fail within 18 months or so, and replacements are often
extremely expensive. For most of our applications we've just
disabled the RAID controller write-caches (our application writes
data at a very very slow rate; the main reason we have RAIDs is
protection, size, and *read* speed).
-j
--
Jay A. Kreibich | Comm. Technologies, R&D
jak@uiuc.edu | Campus IT & Edu. Svcs.
<http://www.uiuc.edu/~jak> | University of Illinois at U/C
>Goulet, Dick > As an Oracle DBA primarily, moving more towards PostGreSQL as time > passes, the "alter system switch logfile" command is very often used at > the end of a hot backup to force a logfile switch and consequently > provide a full archive log set. The command causes the database engine > to cease using the current online redo log and have it archived to the > archive log destination. This is the only way to be sure that all > required archive logs are present in a backup to reconstitute the > database at another location. > Yep, know what it does - just wanted to check that people really used it. I always felt I did that as a precaution rather than for good reason. OK - 8.1 it is then, Best Regards Simon Riggs