Streaming replication, and walsender during recovery
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From: Heikki Linnakangas on 28 Jan 2010 02:47 Fujii Masao wrote: > OK. Here is the patch which supports a walsender process during recovery; > > * Change walsender so as to send the WAL written by the walreceiver > if it has been started during recovery. > * Kill the walsenders started during recovery at the end of recovery > because replication cannot survive the change of timeline ID. I think there's a race condition at the end of recovery. When the shutdown checkpoint is written, with new TLI, doesn't a cascading walsender try to send that to the standby as soon as it's flushed to disk? But it won't find it in the WAL segment with the old TLI that it's reading. Also, when segments are restored from the archive, using restore_command, the cascading walsender won't find them because they're not written in pg_xlog like normal WAL segments. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
From: Fujii Masao on 28 Jan 2010 05:22 On Thu, Jan 28, 2010 at 4:47 PM, Heikki Linnakangas <heikki.linnakangas(a)enterprisedb.com> wrote: > I think there's a race condition at the end of recovery. When the > shutdown checkpoint is written, with new TLI, doesn't a cascading > walsender try to send that to the standby as soon as it's flushed to > disk? But it won't find it in the WAL segment with the old TLI that it's > reading. Right. But I don't think that such a shutdown checkpoint record is worth being sent by a cascading walsender. I think that such a walsender has only to exit without regard to the WAL segment with the new TLI. > Also, when segments are restored from the archive, using > restore_command, the cascading walsender won't find them because they're > not written in pg_xlog like normal WAL segments. Yeah, I need to adjust my approach to the recent 'xlog-refactor' change. The archived file needs to be restored without a name change, and remain in pg_xlog until the bgwriter will have recycled it. But that change would make the xlog.c even more complicated. Should we postpone the 'cascading walsender' feature into v9.1, and, in v9.0, just forbid walsender to be started during recovery? Regards, -- Fujii Masao NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
From: Heikki Linnakangas on 28 Jan 2010 05:43 Fujii Masao wrote: > On Thu, Jan 28, 2010 at 4:47 PM, Heikki Linnakangas > <heikki.linnakangas(a)enterprisedb.com> wrote: >> I think there's a race condition at the end of recovery. When the >> shutdown checkpoint is written, with new TLI, doesn't a cascading >> walsender try to send that to the standby as soon as it's flushed to >> disk? But it won't find it in the WAL segment with the old TLI that it's >> reading. > > Right. But I don't think that such a shutdown checkpoint record is worth > being sent by a cascading walsender. I think that such a walsender has > only to exit without regard to the WAL segment with the new TLI. > >> Also, when segments are restored from the archive, using >> restore_command, the cascading walsender won't find them because they're >> not written in pg_xlog like normal WAL segments. > > Yeah, I need to adjust my approach to the recent 'xlog-refactor' change. > The archived file needs to be restored without a name change, and remain > in pg_xlog until the bgwriter will have recycled it. I guess you could just say that it's working as designed, and WAL files restored from archive can't be streamed. Presumably the cascaded slave can find them in the archive too. But it is pretty weird, doesn't feel right. This reminds me of something I've been pondering anyway. Currently, restore_command copies the restored WAL segment as pg_xlog/RECOVERYXLOG instead of the usual 00000... filename. That avoids overwriting any pre-existing WAL segments in pg_xlog, which may still contain useful data. Using the same filename over and over also means that we don't need to worry about deleting old log files during archive recovery. The downside in standby mode is that once standby has restored segment X from archive, and it's restarted, it must find X in the archive again or it won't be able to start up. The archive better be a directory on the same host. Streaming Replication, however, took another approach. It does overwrite any existing files in pg_xlog, we do need to worry about deleting old files, and if the master goes down, we can always find files we've already streamed in pg_xlog, so the standby can recover even if the master can't be contacted anymore. That's a bit inconsistent, and causes the problem that a cascading walsender won't find the files restored from archive. How about restoring/streaming files to a new directory, say pg_xlog/restored/, with the real filenames? At least in standby_mode, probably best to keep the current behavior in PITR. That would feel more clean, you could easily tell apart files originating from the server itself and those copied from the master. > But that change would make the xlog.c even more complicated. Should we > postpone the 'cascading walsender' feature into v9.1, and, in v9.0, just > forbid walsender to be started during recovery? That's certainly the simplest solution... -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
From: Fujii Masao on 28 Jan 2010 06:40 On Thu, Jan 28, 2010 at 7:43 PM, Heikki Linnakangas <heikki.linnakangas(a)enterprisedb.com> wrote: > How about restoring/streaming files to a new directory, say > pg_xlog/restored/, with the real filenames? At least in standby_mode, > probably best to keep the current behavior in PITR. That would feel more > clean, you could easily tell apart files originating from the server > itself and those copied from the master. When the WAL file with the same name exists in the archive, pg_xlog and pg_xlog/restore/ which directory should we recover it from? I'm not sure that we can always make a right decision about that. How about just making a restore_command copy the WAL files as the normal one (e.g., 0000...) instead of a pg_xlog/RECOVERYXLOG? Though we need to worry about deleting them, we can easily leave the task to the bgwriter. Regards, -- Fujii Masao NIPPON TELEGRAPH AND TELEPHONE CORPORATION NTT Open Source Software Center -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
From: Tom Lane on 28 Jan 2010 10:48
Fujii Masao <masao.fujii(a)gmail.com> writes: > How about just making a restore_command copy the WAL files as the > normal one (e.g., 0000...) instead of a pg_xlog/RECOVERYXLOG? > Though we need to worry about deleting them, we can easily leave > the task to the bgwriter. The reason for doing it that way was to limit disk space usage during a long restore. I'm not convinced we can leave the task to the bgwriter --- it shouldn't be deleting anything at that point. regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers(a)postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers |