基于开销的清理延迟方面请求帮助

tmdxy

状况为执行VACUUM时WEB程序偶尔会提示连不上数据库,没有发现任何错误日志.另外大家给点优化意见，服务器双xeon，2G ram。
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cron:
*/30 * * * * /usr/local/pgsql/bin/vacuumdb --all --analyze -U postgres
05 03 * * * /usr/local/pgsql/bin/vacuumdb --full --analyze --table 't_request' mysms -U postgres
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sysctl -a |grep kern.ipc.shm
kern.ipc.shmmax: 134217728
kern.ipc.shmmin: 1
kern.ipc.shmmni: 192
kern.ipc.shmseg: 128
kern.ipc.shmall: 32768
kern.ipc.shm_use_phys: 1
kern.ipc.shm_allow_removed: 0
-------------------------------------
postgresql.conf:
max_connections = 500

shared_buffers = 1000
max_prepared_transactions = 5000
work_mem = 10240
max_stack_depth = 5024

vacuum_cost_delay = 10 # 0-1000 milliseconds
vacuum_cost_page_hit = 1 # 0-10000 credits
vacuum_cost_page_miss = 10 # 0-10000 credits
vacuum_cost_page_dirty = 20 # 0-10000 credits
vacuum_cost_limit = 200 # 0-10000 credits

另外日志里大量的：
LOG: transaction ID wrap limit is 1074992275, limited by database "postgres"

不太明白是什么意思。事务ID被数据库postgers限制为1074992275？

刚从mssql转到pgsql，多多指教。