Re: During promotion, new master tries to archive same segment twice
Adrian Klaver wrote: On 08/16/2018 01:48 AM, Phil Endecott wrote: Adrian Klaver wrote: On 08/15/2018 01:25 PM, Phil Endecott wrote: Dear Experts, The above is not clear to me. My best guess: It's not part of the error for the archive command; it's just the next thing in the log file. Y is (1) trying write WAL segment E8 to the Y-to-Z archive and failing because it is already there and (2) trying to read WAL segment E9 from the X-to-Y archive and failing because it doesn't exist (and never will). (1) is the problem, (2) is expected. Hmm, still not sure why they are both showing up in the same LOG: entry as the entry was for the failure of the archive_command. It's not. It's just the next thing in the log file. It seems that stderr output from scp appears raw in the log file. What is archive_mode set to on Y? archive_mode = always Regards, Phil.
Re: During promotion, new master tries to archive same segment twice
On 08/16/2018 01:48 AM, Phil Endecott wrote: Adrian Klaver wrote: On 08/15/2018 01:25 PM, Phil Endecott wrote: Dear Experts, The above is not clear to me. My best guess: It's not part of the error for the archive command; it's just the next thing in the log file. Y is (1) trying write WAL segment E8 to the Y-to-Z archive and failing because it is already there and (2) trying to read WAL segment E9 from the X-to-Y archive and failing because it doesn't exist (and never will). (1) is the problem, (2) is expected. Hmm, still not sure why they are both showing up in the same LOG: entry as the entry was for the failure of the archive_command. What is archive_mode set to on Y? -- Adrian Klaver adrian.kla...@aklaver.com
Re: During promotion, new master tries to archive same segment twice
Greetings, * Phil Endecott (spam_from_pgsql_li...@chezphil.org) wrote: > David Steele wrote: > >pgBackRest has done this for years and it saves a *lot* of headaches. > > The system to which I am sending the WAL files is a rsync.net > account. I use it because of its reliability, but methods for > transferring files are limited largely to things like scp and > rsync. You might consider using a well-tested PG backup tool locally and then simply rsync that backup over to rsync.net. Certainly with pgbackrest, we intend and expect people to use more routine "just copy all the files" backup methods for backing up the repository. Basically, you could pgbackrest to /some/local/path and then rsync from there over to rsync.net. Having an rsync or scp 'storage' option (similar to the s3 one) for pgbackrest might be interesting.. We discussed having the ability to start a backup from the PG server at one point but in that discussion we were thinking pgbackrest would also be installed on the backup server. This would be different from that in that the remote side would only need to support rsync or scp. You'd have to accept that if the the backup server dies then you lose data though, since I don't believe there's a way to ask for an fsync() through rsync or scp, which makes it a much less compelling feature, unless rsync.net guarantees writes somehow..? Thanks! Stephen signature.asc Description: PGP signature
Re: During promotion, new master tries to archive same segment twice
On 8/16/18 4:37 AM, Phil Endecott wrote: > David Steele wrote: >> On 8/15/18 4:25 PM, Phil Endecott wrote: >>> - Should my archive_command detect the case where it is asked to >>> write the same file again with the same contents, and report success >>> in that case? >> >> Yes. > >> There are a number of cases where the same WAL >> segment can be pushed more than once, especially after failures where >> Postgres is not sure that the command completed. The archive command >> should handle this gracefully. > > Hmm, OK. Here's what the current docs say: > > Section 25.3.1: > > "The archive command should generally be designed to refuse to > overwrite any pre-existing archive file. This is an important > safety feature to preserve the integrity of your archive in case > of administrator error (such as sending the output of two > different servers to the same archive directory). > > It is advisable to test your proposed archive command to ensure > that it indeed does not overwrite an existing file, and that it > returns nonzero status in this case." > > And section 26.2.9: > > "When continuous WAL archiving is used in a standby, there > are two different scenarios: the WAL archive can be shared > between the primary and the standby, or the standby can > have its own WAL archive. When the standby has its own WAL > archive, set archive_mode to always, and the standby will call > the archive command for every WAL segment it receives, whether > it's by restoring from the archive or by streaming replication. > The shared archive can be handled similarly, but the > archive_command must test if the file being archived exists > already, and if the existing file has identical contents. > This requires more care in the archive_command, as it must be > careful to not overwrite an existing file with different contents, > but return success if the exactly same file is archived twice. > And all that must be done free of race conditions, if two > servers attempt to archive the same file at the same time." > > So you're saying that that's wrong, and that I must always > handle the case when the same WAL segment is written twice. Seems like an omission in section 25.3.1 rather than a problem in 26.2.9. Duplicate WAL is possible in *all* cases. A trivial example is that Postgres calls archive_command and it succeeds but an error happens (e.g. network) right before Postgres is notified. It will wait a bit and try the same WAL segment again. > I'll file a bug against the documentation. OK. >> pgBackRest has done this for years and it saves a *lot* of headaches. > > The system to which I am sending the WAL files is a rsync.net > account. I use it because of its reliability, but methods for > transferring files are limited largely to things like scp and > rsync. Rsync and scp are not good tools to use for backup because there is no guarantee of durability, i.e. the file is not synced to disk before success is returned. rsync.net may have durability guarantees but you should verify that with them. Even so, crafting a safe archive_command using these tools is going to be very tricky. Regards, -- -David da...@pgmasters.net
Re: During promotion, new master tries to archive same segment twice
Adrian Klaver wrote: On 08/15/2018 01:25 PM, Phil Endecott wrote: Dear Experts, Here is my latest issue with replication: I have 3 systems, X, Y and Z. Initially X is replicated to Y and Y is replicated to Z; in each case the replication involves log-shipping using archive_command and restore_command (via a 4th system called "backup") and then streaming. The plan is to shut down X and to promote Y to be the new master. I shut down X, and as expected Y complains that it con no longer connect to it: 2018-08-15 15:10:58.785 UTC [617] LOG: replication terminated by primary server 2018-08-15 15:10:58.785 UTC [617] DETAIL: End of WAL reached on timeline 1 at 7/E998. 2018-08-15 15:10:58.785 UTC [617] FATAL: could not send end-of-streaming message to primary: no COPY in progress scp: backup/postgresql/archivedir/0001000700E9: No such file or directory 2018-08-15 15:10:59.928 UTC [354] LOG: invalid resource manager ID 22 at 7/E998 2018-08-15 15:11:00.092 UTC [5856] FATAL: could not connect to the primary server: FATAL: the database system is shutting down Note that in the middle of that, after streaming has failed it tries to fetch the next WAL segment, E9, from the backup; this fails because this segment doesn't exist. Then I pg_ctl promote on Y: 2018-08-15 15:12:11.888 UTC [354] LOG: received promote request 2018-08-15 15:12:11.888 UTC [354] LOG: redo done at 7/E928 2018-08-15 15:12:11.888 UTC [354] LOG: last completed transaction was at log time 2018-08-15 14:45:05.961153+00 scp: backup/postgresql/archivedir/0001000700E9: No such file or directory scp: backup/postgresql/archivedir/0002.history: No such file or directory 2018-08-15 15:12:13.316 UTC [354] LOG: selected new timeline ID: 2 2018-08-15 15:12:13.368 UTC [354] FATAL: could not open file "recovery.conf": Permission denied So in the process of exiting recovery mode it tries to rename recovery.conf to recovery.done, but it fails to do so because my recovery.conf was owned by root, not user postgres. Oooops. Perhaps it would be appropriate to check for that when it initially reads recovery.conf? Anyway, I fix the permissions and restart it. It starts in recovery mode and cannot connect to the master, as expected. 2018-08-15 15:16:58.061 UTC [6036] LOG: restored log file "0001000700E8" from archive 2018-08-15 15:16:58.108 UTC [6036] LOG: redo starts at 7/E8678300 2018-08-15 15:16:58.731 UTC [6087] LOG: archive command failed with exit code 1 2018-08-15 15:16:58.731 UTC [6087] DETAIL: The failed archive command was: ssh backup test ! -f backup/postgresql/Y/0001000700E8 && scp pg_xlog/0001000700E8 backup:backup/postgresql/Y/0001000700E8 scp: backup/postgresql/archivedir/0001000700E9: No such file or directory The above is not clear to me. My best guess: 1) The ssh backup ... is archiving WAL's from Y to Y specific archive on 'backup'? Yes, there are two archive directories on "backup", one called "archivedir" which is for the X-to-Y replication and a second called "Y" which is for the Y-to-Z replication. 2) The scp: backup ... is the restore_command fetching archived WAL's that originated on X and where sent to backup/postgresql/archivedir/ ? Yes. 3) If 2) is true why is it showing up in the error for the archive command? It's not part of the error for the archive command; it's just the next thing in the log file. Y is (1) trying write WAL segment E8 to the Y-to-Z archive and failing because it is already there and (2) trying to read WAL segment E9 from the X-to-Y archive and failing because it doesn't exist (and never will). (1) is the problem, (2) is expected. Can you show us your actual archive and restore commands? On Y: restore_command = 'scp backup:backup/postgresql/archivedir/%f %p' archive_command = 'ssh backup test ! -f backup/postgresql/Y/%f && scp %p backup:backup/postgresql/Y/%f' 2018-08-15 15:16:58.823 UTC [6036] LOG: consistent recovery state reached at 7/E998 2018-08-15 15:16:58.823 UTC [6036] LOG: invalid resource manager ID 22 at 7/E998 2018-08-15 15:16:58.824 UTC [6035] LOG: database system is ready to accept read only connections 2018-08-15 15:16:58.855 UTC [6094] FATAL: could not connect to the primary server: could not connect to server: Connection refused I can now connect to the db and do read/write operations, but it continues to try and fail to re-write segment E8 to its archive. Eventually I manually delete E8 from the archive; it then succeeds at writing it with the same content as before and seems to continue to function normally. Which archive? The Y-to-Z archive, backup:backup/postgresql/Y/...E8. Questions: - Can all of this be blamed on the failure of the first attempt to promote due to the wrong permissions on recovery.conf? - Should my archive_command detect the case where it is asked to write the same file again with the same
Re: During promotion, new master tries to archive same segment twice
David Steele wrote: On 8/15/18 4:25 PM, Phil Endecott wrote: - Should my archive_command detect the case where it is asked to write the same file again with the same contents, and report success in that case? Yes. There are a number of cases where the same WAL segment can be pushed more than once, especially after failures where Postgres is not sure that the command completed. The archive command should handle this gracefully. Hmm, OK. Here's what the current docs say: Section 25.3.1: "The archive command should generally be designed to refuse to overwrite any pre-existing archive file. This is an important safety feature to preserve the integrity of your archive in case of administrator error (such as sending the output of two different servers to the same archive directory). It is advisable to test your proposed archive command to ensure that it indeed does not overwrite an existing file, and that it returns nonzero status in this case." And section 26.2.9: "When continuous WAL archiving is used in a standby, there are two different scenarios: the WAL archive can be shared between the primary and the standby, or the standby can have its own WAL archive. When the standby has its own WAL archive, set archive_mode to always, and the standby will call the archive command for every WAL segment it receives, whether it's by restoring from the archive or by streaming replication. The shared archive can be handled similarly, but the archive_command must test if the file being archived exists already, and if the existing file has identical contents. This requires more care in the archive_command, as it must be careful to not overwrite an existing file with different contents, but return success if the exactly same file is archived twice. And all that must be done free of race conditions, if two servers attempt to archive the same file at the same time." So you're saying that that's wrong, and that I must always handle the case when the same WAL segment is written twice. I'll file a bug against the documentation. pgBackRest has done this for years and it saves a *lot* of headaches. The system to which I am sending the WAL files is a rsync.net account. I use it because of its reliability, but methods for transferring files are limited largely to things like scp and rsync. Thanks, Phil.
Re: During promotion, new master tries to archive same segment twice
On 08/15/2018 01:25 PM, Phil Endecott wrote: Dear Experts, Here is my latest issue with replication: I have 3 systems, X, Y and Z. Initially X is replicated to Y and Y is replicated to Z; in each case the replication involves log-shipping using archive_command and restore_command (via a 4th system called "backup") and then streaming. The plan is to shut down X and to promote Y to be the new master. I shut down X, and as expected Y complains that it con no longer connect to it: 2018-08-15 15:10:58.785 UTC [617] LOG: replication terminated by primary server 2018-08-15 15:10:58.785 UTC [617] DETAIL: End of WAL reached on timeline 1 at 7/E998. 2018-08-15 15:10:58.785 UTC [617] FATAL: could not send end-of-streaming message to primary: no COPY in progress scp: backup/postgresql/archivedir/0001000700E9: No such file or directory 2018-08-15 15:10:59.928 UTC [354] LOG: invalid resource manager ID 22 at 7/E998 2018-08-15 15:11:00.092 UTC [5856] FATAL: could not connect to the primary server: FATAL: the database system is shutting down Note that in the middle of that, after streaming has failed it tries to fetch the next WAL segment, E9, from the backup; this fails because this segment doesn't exist. Then I pg_ctl promote on Y: 2018-08-15 15:12:11.888 UTC [354] LOG: received promote request 2018-08-15 15:12:11.888 UTC [354] LOG: redo done at 7/E928 2018-08-15 15:12:11.888 UTC [354] LOG: last completed transaction was at log time 2018-08-15 14:45:05.961153+00 scp: backup/postgresql/archivedir/0001000700E9: No such file or directory scp: backup/postgresql/archivedir/0002.history: No such file or directory 2018-08-15 15:12:13.316 UTC [354] LOG: selected new timeline ID: 2 2018-08-15 15:12:13.368 UTC [354] FATAL: could not open file "recovery.conf": Permission denied So in the process of exiting recovery mode it tries to rename recovery.conf to recovery.done, but it fails to do so because my recovery.conf was owned by root, not user postgres. Oooops. Perhaps it would be appropriate to check for that when it initially reads recovery.conf? Anyway, I fix the permissions and restart it. It starts in recovery mode and cannot connect to the master, as expected. 2018-08-15 15:16:58.061 UTC [6036] LOG: restored log file "0001000700E8" from archive 2018-08-15 15:16:58.108 UTC [6036] LOG: redo starts at 7/E8678300 2018-08-15 15:16:58.731 UTC [6087] LOG: archive command failed with exit code 1 2018-08-15 15:16:58.731 UTC [6087] DETAIL: The failed archive command was: ssh backup test ! -f backup/postgresql/Y/0001000700E8 && scp pg_xlog/0001000700E8 backup:backup/postgresql/Y/0001000700E8 scp: backup/postgresql/archivedir/0001000700E9: No such file or directory The above is not clear to me. My best guess: 1) The ssh backup ... is archiving WAL's from Y to Y specific archive on 'backup'? 2) The scp: backup ... is the restore_command fetching archived WAL's that originated on X and where sent to backup/postgresql/archivedir/ ? 3) If 2) is true why is it showing up in the error for the archive command? Can you show us your actual archive and restore commands? More below. 2018-08-15 15:16:58.823 UTC [6036] LOG: consistent recovery state reached at 7/E998 2018-08-15 15:16:58.823 UTC [6036] LOG: invalid resource manager ID 22 at 7/E998 2018-08-15 15:16:58.824 UTC [6035] LOG: database system is ready to accept read only connections 2018-08-15 15:16:58.855 UTC [6094] FATAL: could not connect to the primary server: could not connect to server: Connection refused I can now connect to the db and do read/write operations, but it continues to try and fail to re-write segment E8 to its archive. Eventually I manually delete E8 from the archive; it then succeeds at writing it with the same content as before and seems to continue to function normally. Which archive? Questions: - Can all of this be blamed on the failure of the first attempt to promote due to the wrong permissions on recovery.conf? - Should my archive_command detect the case where it is asked to write the same file again with the same contents, and report success in that case? - Is this a bug? Thanks, Phil. -- Adrian Klaver adrian.kla...@aklaver.com
Re: During promotion, new master tries to archive same segment twice
Hi Phil, On 8/15/18 4:25 PM, Phil Endecott wrote: > > Questions: > > - Can all of this be blamed on the failure of the first attempt > to promote due to the wrong permissions on recovery.conf? Yes, it looks that way. > - Should my archive_command detect the case where it is asked to > write the same file again with the same contents, and report > success in that case? Yes. pgBackRest has done this for years and it saves a *lot* of headaches. > - Is this a bug? I don't think so. There are a number of cases where the same WAL segment can be pushed more than once, especially after failures where Postgres is not sure that the command completed. The archive command should handle this gracefully. Regards, -- -David da...@pgmasters.net
