From d46b1f904e21b3dfcfd520aa3b1081bee6df86c2 Mon Sep 17 00:00:00 2001 From: Tom Lane Date: Tue, 17 Feb 2004 07:36:47 +0000 Subject: [PATCH] Minor editorializing on cost-based vacuum description. --- doc/src/sgml/runtime.sgml | 69 ++++++++++++++++++++++----------------- 1 file changed, 39 insertions(+), 30 deletions(-) diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml index 1f2e547f86..950bae2512 100644 --- a/doc/src/sgml/runtime.sgml +++ b/doc/src/sgml/runtime.sgml @@ -1,5 +1,5 @@ @@ -995,24 +995,24 @@ SET ENABLE_SEQSCAN TO OFF; Cost-Based Vacuum Delay - During the execution of VACUUM, - VACUUM FULL and ANALYZE, - the system mantains an internal counter that keeps track of the - cost of the various I/O operations that are performed. When the - accumulated cost reaches a limit - (specified by vacuum_cost_limit), the backend performing - the operation will sleep for a while (specified by + During the execution of VACUUM + and ANALYZE commands, + the system maintains an internal counter that keeps track of the + estimated cost of the various I/O operations that are performed. + When the accumulated cost reaches a limit + (specified by vacuum_cost_limit), the process + performing the operation will sleep for a while (specified by vacuum_cost_naptime). Then it will reset the counter and continue execution. - The intent of this feature is to allow administrators the reduce + The intent of this feature is to allow administrators to reduce the I/O impact of these commands on concurrent database activity. There are some situations in which it is not very - important that maintainence commands like + important that maintenance commands like VACUUM and ANALYZE finish - quickly; however, it is usually very important these these + quickly; however, it is usually very important that these commands do not significantly interfere with the ability of the system to perform other database operations. Cost-based vacuum delay provides a way for administrators to achieve this. @@ -1020,7 +1020,7 @@ SET ENABLE_SEQSCAN TO OFF; This feature is disabled by default. To enable it, set the - vacuum_cost_naptime variable to a reasonable + vacuum_cost_naptime variable to a nonzero value. @@ -1029,7 +1029,7 @@ SET ENABLE_SEQSCAN TO OFF; vacuum_cost_page_hit (integer) - The cost for vacuuming a buffer found in the shared buffer + The estimated cost for vacuuming a buffer found in the shared buffer cache. It represents the cost to lock the buffer pool, lookup the shared hash table and scan the content of the page. The default value is 1. @@ -1041,7 +1041,7 @@ SET ENABLE_SEQSCAN TO OFF; vacuum_cost_page_miss (integer) - The cost for vacuuming a buffer that has to be read from + The estimated cost for vacuuming a buffer that has to be read from disk. This represents the effort to lock the buffer pool, lookup the shared hash table, read the desired block in from the disk and scan its content. The default value is 10. @@ -1053,7 +1053,7 @@ SET ENABLE_SEQSCAN TO OFF; vacuum_cost_page_dirty (integer) - The extra cost added when vacuum modifies a block that was + The estimated cost charged when vacuum modifies a block that was previously clean. It represents the extra I/O required to flush the dirty block out to disk again. The default value is 20. @@ -1065,7 +1065,7 @@ SET ENABLE_SEQSCAN TO OFF; vacuum_cost_limit (integer) - The accumulated cost that will cause the backend to briefly + The accumulated cost that will cause the vacuuming process to briefly nap. The default value is 200. @@ -1075,25 +1075,34 @@ SET ENABLE_SEQSCAN TO OFF; vacuum_cost_naptime (integer) - The length of time in milliseconds that a backend will nap - when the cost limit has been exceeded. There are certain bulk - operations that hold critical locks and should therefore - complete as quickly as possible. Because of that it is - possible that the cost actually accumulates far higher than - this limit. To compensate for this, the final naptime is - calculated as vacuum_cost_naptime * - accumulated_balance / - vacuum_cost_limit with a maximum of - vacuum_cost_naptime * 4. - - - + The length of time, in milliseconds, that the process will nap + when the cost limit has been exceeded. The default value is 0, which disables the cost-based vacuum - delay feature. + delay feature. Positive values enable cost-based vacuuming. + Note however that on many systems, the effective resolution + of sleep delays is 10 milliseconds; setting + vacuum_cost_naptime to a value that is + not a multiple of 10 may have the same results as setting it + to the next higher multiple of 10. + + + + There are certain bulk operations that hold critical locks and should + therefore complete as quickly as possible. Cost-based vacuum + delays do not occur during such operations. Therefore it is + possible that the cost accumulates far higher than the specified + limit. To avoid uselessly long delays in such cases, the actual + naptime is calculated as vacuum_cost_naptime * + accumulated_balance / + vacuum_cost_limit with a maximum of + vacuum_cost_naptime * 4. + + +