Обсуждение: Big UPDATE breaking replication
Hello, We are using the 9.1 built-in streaming replication. Recently our slave nodes fell behind because of an UPDATE statement. It took about 3 minutes to execute, but it affected half a million records, hence the replication broke with the "requested WAL segment ... has already been removed" series of error messages. The WAL settings we have are: max_wal_senders = 6 wal_keep_segments = 60 max_standby_archive_delay = 300s I guess increasing the wal_keep_segments value would prevent it from happening in the future, but increase it with how much? What value would be high enough? Also we noticed some strange error message appearing shortly before and after this same statement: "LOG: out of file descriptors: Too many open files; release and retry". Could it be related somehow and what does it mean exactly? Here's an excerpt from the master DB log: May 30 12:23:09 DB1 postgres[28201]: [13-1] user=www,db=xxx LOG: out of file descriptors: Too many open files; release and retry May 30 12:23:09 DB1 postgres[28201]: [13-2] user=www,db=xxx CONTEXT: writing block 0 of relation base/2819385/2820788 May 30 12:23:09 DB1 postgres[28201]: [13-3] user=www,db=xxx STATEMENT: UPDATE May 30 12:23:09 DB1 postgres[28201]: [13-4] ^I message May 30 12:23:09 DB1 postgres[28201]: [13-5] ^I SET May 30 12:23:09 DB1 postgres[28201]: [13-6] ^I sender_has_deleted=TRUE, May 30 12:23:09 DB1 postgres[28201]: [13-7] ^I receiver_has_deleted=TRUE May 30 12:23:09 DB1 postgres[28201]: [13-8] ^I WHERE from_profile_sid=870 ... May 30 12:39:47 DB1 postgres[9053]: [2-1] user=postgres,db=[unknown] FATAL: requested WAL segment 00000001000002DE000000BD has already been removed Regards, -- Kouber Saparev
Kouber Saparev wrote: > We are using the 9.1 built-in streaming replication. > > Recently our slave nodes fell behind because of an UPDATE statement. It > took about 3 minutes to execute, but it affected half a million records, > hence the replication broke with the "requested WAL segment ... has > already been removed" series of error messages. > > The WAL settings we have are: > > max_wal_senders = 6 > wal_keep_segments = 60 > max_standby_archive_delay = 300s > > > I guess increasing the wal_keep_segments value would prevent it from > happening in the future, but increase it with how much? What value would > be high enough? That depends on how much WAL your UPDATEs generate. In other words, there is no safe value that will guarantee that you still have all the WAL files you need. To be on the safe side, you should additionally configure log shipping as described in http://www.postgresql.org/docs/current/static/warm-standby.html That way your standby will always be able to catch up. > Also we noticed some strange error message appearing shortly before and > after this same statement: "LOG: out of file descriptors: Too many open > files; release and retry". > > Could it be related somehow and what does it mean exactly? It means that PostgreSQL tries to open more files than the operating system allows. Try to increase that parameter. I don't know if there is a connection to the standby falling behind, but that can always happen if there is enough activity on the primary. Yours, Laurenz Albe
On 06/04/2013 04:53 AM, Kouber Saparev wrote: > Hello, > > We are using the 9.1 built-in streaming replication. > > Recently our slave nodes fell behind because of an UPDATE statement. It > took about 3 minutes to execute, but it affected half a million records, > hence the replication broke with the "requested WAL segment ... has > already been removed" series of error messages. > > The WAL settings we have are: > > max_wal_senders = 6 > wal_keep_segments = 60 > max_standby_archive_delay = 300s > > > I guess increasing the wal_keep_segments value would prevent it from > happening in the future, but increase it with how much? What value would > be high enough? You can use WAL shipping to protect against this or set wal_keep_segments higher. I set my main server to a tad over 1,000 and know I can do a full restore on the master without coming close to breaking replication. My xlog dir is 17G. A bit of a waste, perhaps, but I've noted no ill effects and it's still only a sliver of the total drive capacity. > > Also we noticed some strange error message appearing shortly before and > after this same statement: "LOG: out of file descriptors: Too many open > files; release and retry". > > Could it be related somehow and what does it mean exactly? What is your setup (Linux? Mac? Windows? VM in the cloud? How many simultaneous connections?...) You will find a lot of info in old messages on the subject but, annotating the docs: If the kernel is enforcing a safe per-process limit, you don't need to worry about this setting. But on some platforms (notably, most BSD systems - looking at you Mac), the kernel will allow individual processes to open many more files than the system can actually support if many processes all try to open that many files.... An old email from Tom Lane notes that you need to make sure your kernel can support approximately max_connections * (max_files_per_process + max_connections) open file handles plus any requirements imposed by other processes on the system and comments that Mac treats each semaphore as an open file. My interpretation is that if your kernel enforces things properly you don't need to worry. If it doesn't, reduce your max_connections and/or max_files_per_process as needed. Cheers, Steve
On 06/04/2013 06:47 PM, Steve Crawford wrote: > On 06/04/2013 04:53 AM, Kouber Saparev wrote: >> Hello, >> >> We are using the 9.1 built-in streaming replication. >> >> Recently our slave nodes fell behind because of an UPDATE statement. It >> took about 3 minutes to execute, but it affected half a million records, >> hence the replication broke with the "requested WAL segment ... has >> already been removed" series of error messages. >> >> The WAL settings we have are: >> >> max_wal_senders = 6 >> wal_keep_segments = 60 >> max_standby_archive_delay = 300s >> >> >> I guess increasing the wal_keep_segments value would prevent it from >> happening in the future, but increase it with how much? What value would >> be high enough? > > You can use WAL shipping to protect against this or set > wal_keep_segments higher. I set my main server to a tad over 1,000 and > know I can do a full restore on the master without coming close to > breaking replication. My xlog dir is 17G. A bit of a waste, perhaps, > but I've noted no ill effects and it's still only a sliver of the > total drive capacity. Well, the streaming replication involves the WAL shipping already. Do you mean to archive the WALs somewhere and then scp them with a cron job? I doubt it would be fast enough neither. > > >> >> Also we noticed some strange error message appearing shortly before and >> after this same statement: "LOG: out of file descriptors: Too many open >> files; release and retry". >> >> Could it be related somehow and what does it mean exactly? > > What is your setup (Linux? Mac? Windows? VM in the cloud? How many > simultaneous connections?...) You will find a lot of info in old > messages on the subject but, annotating the docs: If the kernel is > enforcing a safe per-process limit, you don't need to worry about this > setting. But on some platforms (notably, most BSD systems - looking at > you Mac), the kernel will allow individual processes to open many more > files than the system can actually support if many processes all try > to open that many files.... > > An old email from Tom Lane notes that you need to make sure your > kernel can support approximately > max_connections * (max_files_per_process + max_connections) open file > handles plus any requirements imposed by other processes on the system > and comments that Mac treats each semaphore as an open file. > > My interpretation is that if your kernel enforces things properly you > don't need to worry. If it doesn't, reduce your max_connections and/or > max_files_per_process as needed. We are running virtual machines with Ubuntu Server 12.04.1 LTS. The error looks very confusing given: root@DB1:~# cat /proc/sys/fs/file-max 1621645 root@DB1:~# lsof | wc -l 7013 It is not present normally in our logs, that's why I am suspecting that it has some correlation with the WAL issue above. Could it be that the master was not able to open properly the wal sender for this transaction? -- Kouber Saparev
On Thu, Jun 6, 2013 at 5:19 AM, Kouber Saparev <kouber@saparev.com> wrote:
Do you mean to archive the WALs somewhere and then scp them with a cron
job? I doubt it would be fast enough neither.
I haven't been paying attention to this thread, but this comment caught my eye. I had a setup at a previous job where the primary and DR datacenters were at opposite ends of the county making the network latency high enough that WALs piled up on the master. What we did was to archive the WALs locally (to the master) in a dedicated volume, then rsync'd them in batches of 50 or so. It made a big difference for us, as we were generating around 100 WAL segments per minute IIRC. I think the latency was around 46ms and during bursts we would fall behind quite a bit.