分类： Mysql/postgreSQL
2013-07-03 08:31:31
The shared_buffers configuration parameter determines how much memory is dedicated to PostgreSQL to use for caching data. One reason the defaults are low is because on some platforms (like older Solaris versions and SGI), having large values requires invasive action like recompiling the kernel. Even on a modern Linux system, the stock kernel will likely not allow setting shared_buffers to over 32MB without adjusting kernel settings first.
If you have a system with 1GB or more of RAM, a reasonable starting value for shared_buffers is 1/4 of the memory in your system. If you have less RAM you'll have to account more carefully for how much RAM the OS is taking up; closer to 15% is more typical there. There are some workloads where even larger settings for shared_buffers are effective, but given the way PostgreSQL also relies on the operating system cache, it's unlikely you'll find using more than 40% of RAM to work better than a smaller amount.
Be aware that if your system or PostgreSQL build is 32-bit, it might not be practical to set shared_buffers above 2 ~ 2.5GB. See this blog post for details.
Note that on Windows (and on PostgreSQL versions before 8.1), large values for shared_buffers aren't as effective, and you may find better results keeping it relatively low and using the OS cache more instead. On Windows the useful range is 64MB to 512MB, and for earlier than 8.1 versions the effective upper limit is near shared_buffers=50000 (just under 400MB--older versions before 8.2 don't allow using MB values for their settings, you specify this parameter in 8K blocks)
It's likely you will have to increase the amount of memory your operating system allows you to allocate at once to set the value for shared_buffers this high. On UNIX-like systems, if you set it above what's supported, you'll get a message like this:
IpcMemoryCreate: shmget(key=5432001, size=415776768, 03600) failed: Invalid argument This error usually means that PostgreSQL's request for a shared memory segment exceeded your kernel's SHMMAX parameter. You can either reduce the request size or reconfigure the kernel with larger SHMMAX. To reduce the request size (currently 415776768 bytes), reduce PostgreSQL's shared_buffers parameter (currently 50000) and/or its max_connections parameter (currently 12).
See Managing Kernel Resources for details on how to correct this.
Changing this setting requires restarting the database. Also, this is a hard allocation of memory; the whole thing gets allocated out of virtual memory when the database starts.
effective_cache_size should be set to an estimate of how much memory is available for disk caching by the operating system and within the database itself, after taking into account what's used by the OS itself and other applications. This is a guideline for how much memory you expect to be available in the OS and PostgreSQL buffer caches, not an allocation! This value is used only by the PostgreSQL query planner to figure out whether plans it's considering would be expected to fit in RAM or not. If it's set too low, indexes may not be used for executing queries the way you'd expect. The setting for shared_buffers is not taken into account here--only the effective_cache_size value is, so it should include memory dedicated to the database too.
Setting effective_cache_size to 1/2 of total memory would be a normal conservative setting, and 3/4 of memory is a more aggressive but still reasonable amount. You might find a better estimate by looking at your operating system's statistics. On UNIX-like systems, add the free+cached numbers from free or top to get an estimate. On Windows see the "System Cache" size in the Windows Task Manager's Performance tab. Changing this setting does not require restarting the database (HUP is enough).
PostgreSQL writes new transactions to the database in files called WAL segments that are 16MB in size. Every time checkpoint_segments worth of these files have been written, by default 3, a checkpoint occurs. Checkpoints can be resource intensive, and on a modern system doing one every 48MB will be a serious performance bottleneck. Setting checkpoint_segments to a much larger value improves that. Unless you're running on a very small configuration, you'll almost certainly be better setting this to at least 10, which also allows usefully increasing the completion target.
For more write-heavy systems, values from 32 (checkpoint every 512MB) to 256 (every 4GB) are popular nowadays. Very large settings use a lot more disk and will cause your database to take longer to recover, so make sure you're comfortable with both those things before large increases. Normally the large settings (>64/1GB) are only used for bulk loading. Note that whatever you choose for the segments, you'll still get a checkpoint at least every 5 minutes unless you also increase checkpoint_timeout (which isn't necessary on most systems).
The autovacuum process takes care of several maintenance chores inside your database that you really need. Generally, if you think you need to turn regular vacuuming off because it's taking too much time or resources, that means you're doing it wrong. The answer to almost all vacuuming problems is to vacuum more often, not less, so that each individual vacuum operation has less to clean up.
However, it's acceptable to disable autovacuum for short periods of time, for instance when bulk loading large amounts of data.
There are many things you can log that may or may not be important to you. You should investigate the documentation on all of the options, but here are some tips & tricks to get you started:
The database software collects statistics about each of the tables in your database to decide how to execute queries against it. In earlier versions of PostgreSQL, the default setting of 10 doesn't collect very much information, and if you're not getting good execution query plans particularly on larger (or more varied) tables you should increase default_statistics_target then ANALYZE the database again (or wait for autovacuum to do it for you).
If you do a lot of complex sorts, and have a lot of memory, then increasing the work_mem parameter allows PostgreSQL to do larger in-memory sorts which, unsurprisingly, will be faster than disk-based equivalents.
This size is applied to each and every sort done by each user, and complex queries can use multiple working memory sort buffers. Set it to 50MB, and have 30 users submitting queries, and you are soon using 1.5GB of real memory. Furthermore, if a query involves doing merge sorts of 8 tables, that requires 8 times work_mem. You need to consider what you set max_connections to in order to size this parameter correctly. This is a setting where data warehouse systems, where users are submitting very large queries, can readily make use of many gigabytes of memory.
maintenance_work_mem is used for operations like vacuum. Using extremely large values here doesn't help very much, and because you essentially need to reserve that memory for when vacuum kicks in, takes it away from more useful purposes. Something in the 256MB range has anecdotally been a reasonably large setting here.
PostgreSQL 8.3 and later
In 8.3 you can use log_temp_files to figure out if sorts are using disk instead of fitting in memory. In earlier versions, you might instead just monitor the size of them by looking at how much space is being used in the various $PGDATA/base/
After every transaction, PostgreSQL forces a commit to disk out to its write-ahead log. This can be done a couple of ways, and on some platforms the other options are considerably faster than the conservative default. open_sync is the most common non-default setting switched to, on platforms that support it but default to one of the fsync methods. See Tuning PostgreSQL WAL Synchronization for a lot of background on this topic. Note that open_sync writing is buggy on some platforms (such as Linux), and you should (as always) do plenty of tests under a heavy write load to make sure that you haven't made your system less stable with this change. Reliable Writes contains more information on this topic.
Linux kernels starting with version 2.6.33 will cause earlier versions of PostgreSQL to default to wal_sync_method=open_datasync; before that kernel release the default picked was always fdatasync. This can cause a significant performance decrease when combined with small writes and/or small values for wal_buffers.
Increasing wal_buffers from its tiny default of a small number of kilobytes is helpful for write-heavy systems. Benchmarking generally suggests that just increasing to 1MB is enough for some large systems, and given the amount of RAM in modern servers allocating a full WAL segment (16MB, the useful upper-limit here) is reasonable. Changing wal_buffers requires a database restart.
This setting is used for managing 2 phase commit. If you do not use two phase commit (and if you don't know what it is, you don't use it), then you can set this value to 0. That will save a little bit of shared memory. For database systems with a large number (at least hundreds) of concurrent connections, be aware that this setting also affects the number of available lock-slots in pg_locks, so you may want to leave it at the default setting. There is a formula for how much memory gets allocated in the docs and in the default postgresql.conf.
Changing max_prepared_transactions requires a server restart.
PostgreSQL can only safely use a write cache if it has a battery backup. See WAL reliability for an essential introduction to this topic. No, really; go read that right now, it's vital to understand that if you want your database to work right.
You may be limited to approximately 100 transaction commits per second per client in situations where you don't have such a durable write cache (and perhaps only 500/second even with lots of clients).
This setting suggests to the optimizer how long it will take your disks to seek to a random disk page, as a multiple of how long a sequential read (with a cost of 1.0) takes. If you have particularly fast disks, as commonly found with RAID arrays of SCSI disks, it may be appropriate to lower random_page_cost, which will encourage the query optimizer to use random access index scans. Some feel that 4.0 is always too large on current hardware; it's not unusual for administrators to standardize on always setting this between 2.0 and 3.0 instead. In some cases that behavior is a holdover from earlier PostgreSQL versions where having random_page_cost too high was more likely to screw up plan optimization than it is now (and setting at or below 2.0 was regularlly necessary). Since these cost estimates are just that--estimates--it shouldn't hurt to try lower values.
But this not where you should start to search for plan problems. Note that random_page_cost is pretty far down this list (at the end in fact). If you are getting bad plans, this shouldn't be the first thing you look at, even though lowering this value may be effective. Instead, you should start by making sure autovacuum is working properly, that you are collecting enough statistics, and that you have correctly sized the memory parameters for your server--all the things gone over above. After you've done all those much more important things, if you're still getting bad plans then you should see if lowering random_page_cost is still useful.
