Thread: debugging handle exhaustion and 15 min/ 5mil row delete
Hello, We've been a satified user of PostgreSQL for several years, and use it to power a national pet adoption website: http://www.adoptapet.com/ Recently we've had a regularly-timed middle-of-the-night problem where database handles are exhausted for a very brief period. In tracking it down, I have found that the event seems to correspond to a time when a cron script is deleting from a large logging table, but I'm not certain if this is the cause or a correlation. We are deleting about 5 million rows from a time-based logging table that is replicated by Slony. We are currently using a single delete statement, which takes about 15 minutes to run. There is no RI on the table, but the use of Slony means that a trigger call and action is made for every row deleted, which causes a corresponding insertion in another table so the deletion can be replicated to the slave. My questions: - Could this kind of activity lead to an upward spiral in database handle usage? - Would it be advisable to use several small DELETE statements instead, to delete rows in batches of 1,000. We could use the recipe for this that was posted earlier to this list: delete from table where pk in (select pk from table where delete_condition limit X); Partitions seems attractive here, but aren't easy to use Slony. Perhaps once we migrate to PostgreSQL 9.0 and the hot standby feature we can consider that. Thanks for your help! Mark . . . . . . . . . . . . . . . . . . . . . . . . . . . Mark Stosberg Principal Developer mark@summersault.com Summersault, LLC 765-939-9301 ext 202 database driven websites . . . . . http://www.summersault.com/ . . . . . . . .
On Fri, May 07, 2010 at 09:37:42AM -0400, Mark Stosberg wrote: > > Hello, > > We've been a satified user of PostgreSQL for several years, and use it > to power a national pet adoption website: http://www.adoptapet.com/ > > Recently we've had a regularly-timed middle-of-the-night problem where > database handles are exhausted for a very brief period. > > In tracking it down, I have found that the event seems to correspond to > a time when a cron script is deleting from a large logging table, but > I'm not certain if this is the cause or a correlation. > > We are deleting about 5 million rows from a time-based logging table > that is replicated by Slony. We are currently using a single delete > statement, which takes about 15 minutes to run. There is no RI on the > table, but the use of Slony means that a trigger call and action is made > for every row deleted, which causes a corresponding insertion in another > table so the deletion can be replicated to the slave. > > My questions: > > - Could this kind of activity lead to an upward spiral in database > handle usage? Yes. > > - Would it be advisable to use several small DELETE statements instead, > to delete rows in batches of 1,000. We could use the recipe for this > that was posted earlier to this list: Yes, that is the method we use in several cases to avoid this behavior. Deletion is a more intensive process in PostgreSQL, so batching it will keep from dragging down other queries which results in your out-of-handles error. Regards, Ken
El 07/05/2010 15:37, Mark Stosberg escribió: > Hello, > > We've been a satified user of PostgreSQL for several years, and use it > to power a national pet adoption website: http://www.adoptapet.com/ > > Recently we've had a regularly-timed middle-of-the-night problem where > database handles are exhausted for a very brief period. > > In tracking it down, I have found that the event seems to correspond to > a time when a cron script is deleting from a large logging table, but > I'm not certain if this is the cause or a correlation. > > We are deleting about 5 million rows from a time-based logging table > that is replicated by Slony. We are currently using a single delete > statement, which takes about 15 minutes to run. There is no RI on the > table, but the use of Slony means that a trigger call and action is made > for every row deleted, which causes a corresponding insertion in another > table so the deletion can be replicated to the slave. > > My questions: > > - Could this kind of activity lead to an upward spiral in database > handle usage? > > - Would it be advisable to use several small DELETE statements instead, > to delete rows in batches of 1,000. We could use the recipe for this > that was posted earlier to this list: > > delete from table where pk in > (select pk from table where delete_condition limit X); > > Partitions seems attractive here, but aren't easy to use Slony. Perhaps > once we migrate to PostgreSQL 9.0 and the hot standby feature we can > consider that. > > Thanks for your help! > > Mark > > . . . . . . . . . . . . . . . . . . . . . . . . . . . > Mark Stosberg Principal Developer > mark@summersault.com Summersault, LLC > 765-939-9301 ext 202 database driven websites > . . . . . http://www.summersault.com/ . . . . . . . . > > > > You can use TRUNCATE instead DELETE. TRUNCATE is more efficient and faster that DELETE. Now, we need more information about your system to give you a certain solution: Are you using a RAID controller for you data? Do you have separated the xlog directory from the data directory? Which is your Operating System? Which is you architecture? Regards
> You can use TRUNCATE instead DELETE. TRUNCATE is more efficient and
> faster that DELETE.
Thanks for the suggestion. However, TRUNCATE is not compatible with
Slony, and we also have some rows which remain in table.
> Now, we need more information about your system to give you a certain
> solution:
> Are you using a RAID controller for you data?
Yes.
> Do you have separated the xlog directory from the data directory?
No.
> Which is your Operating System?
FreeBSD.
> Which is you architecture?
i386.
Thanks for the feedback. I'm going to try batching the deletes for now,
which is approach was worked well for some of our other long-running
deletes.
Mark
El 07/05/2010 16:10, Mark Stosberg escribió: > >> You can use TRUNCATE instead DELETE. TRUNCATE is more efficient and >> faster that DELETE. >> > Thanks for the suggestion. However, TRUNCATE is not compatible with > Slony, and we also have some rows which remain in table. > > >> Now, we need more information about your system to give you a certain >> solution: >> Are you using a RAID controller for you data? >> > Yes. > > >> Do you have separated the xlog directory from the data directory? >> > No. > > >> Which is your Operating System? >> > FreeBSD. > > >> Which is you architecture? >> > i386. > > Thanks for the feedback. I'm going to try batching the deletes for now, > which is approach was worked well for some of our other long-running > deletes. > > Mark > > Have you valorated to use a 64 bits version of FreeBSD for that? The 64 bits OS can help you very much on large databases because yo can use actually all available RAM that you have on the server. Many experts on this list recommende to separate the xlog directory on a RAID 1 configuration and the data directory on RAID 10 to obtain a better performance. The filesystems are very diverse, but I ´ve seen that ZFS is very useful on these cases. Which version of Slony-I are you using? Regards