diff --git a/doc/src/sgml/runtime.sgml b/doc/src/sgml/runtime.sgml index 93b5687373..1f2e547f86 100644 --- a/doc/src/sgml/runtime.sgml +++ b/doc/src/sgml/runtime.sgml @@ -1,5 +1,5 @@ @@ -992,83 +992,107 @@ SET ENABLE_SEQSCAN TO OFF; - Cost Based Vacuum Delay + 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 + vacuum_cost_naptime). Then it will reset the + counter and continue execution. + + + + The intent of this feature is to allow administrators the 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 + VACUUM and ANALYZE finish + quickly; however, it is usually very important these 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. + + + + This feature is disabled by default. To enable it, set the + vacuum_cost_naptime variable to a reasonable + value. + - - vacuum_cost_page_hit (integer) - - - During a default VACUUM (not - FULL) the system maintains and internal counter - accumulating the cost of various operations performed. When the - accumulated cost reaches a limit, the backend performing the - VACUUM will sleep for a while, reset the - accumulator and continue. The intention is to lower the IO impact - of VACUUM. - + + vacuum_cost_page_hit (integer) + + + The 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. + + + - - The variable vacuum_cost_page_hit is the cost - for vacuuming a buffer found inside the shared buffer cache. - It represents the cost to lock the buffer pool, lookup the - shared hash table and to actually scan the block content. - - - + + vacuum_cost_page_miss (integer) + + + The 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. + + + - - vacuum_cost_page_miss (integer) - - - The cost for vacuuming a buffer that has to be read from disk. - This represents the effort to lock the buffer pool, lookup the - cache directory, reading the block from disk and scanning the - content. - - - + + vacuum_cost_page_dirty (integer) + + + The extra cost added 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. + + + - - vacuum_cost_page_dirty (integer) - - - This extra cost is added when vacuum modifies a block that was - clean before. It represents the extra IO required to flush the - dirty block out to disk again. - - - + + vacuum_cost_limit (integer) + + + The accumulated cost that will cause the backend to briefly + nap. The default value is 200. + + + - - vacuum_cost_limit (integer) - - - This is the cost limit that must be reached or exceeded before - the VACUUM will nap. - - - - - - vacuum_cost_naptime (integer) - - - The time im milliseconds the VACUUM will - nap when the cost limit has been reached or exceeded. - There are certain bulk operations that hold critical - locks and should therefore perform - 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. - - - + + 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 default value is 0, which disables the cost-based vacuum + delay feature. + + +