MySQL伺服器預設安裝之後調節效能的方法

My favorite question during Interview for people to work as MySQL DBAs or be involved with MySQL Performance in some way is to ask them what should be tuned in MySQL Server straight after installation, assuming it was installed with default settings.

在面試MySQL DBA或者那些打算做MySQL效能最佳化的人時，我最喜歡問題是：MySQL伺服器按照預設設定安裝完之後，應該做哪些方面的調節呢？

I'm surprised how many people fail to provide any reasonable answer to this question, and how many servers are where in wild which are running with default settings.

令我很驚訝的是，有多少人對這個問題無法給出合理的答案，又有多少伺服器都運行在預設的設定下。

Even though you can tune quite a lot of variables in MySQL Servers only few of them are really important for most common workload. After you get these settings right other changes will most commonly offer only incremental performance improvements.

儘管你可以調節很多MySQL伺服器上的變數，但是在大多數通常的工作負載下，只有少數幾個才真正重要。如果你把這些變數設定正確了，那麼修改其他變數最多隻能對系統效能改善有一定提升。

key_buffer_size - Very important if you use MyISAM tables. Set up to 30-40% of available memory if you use MyISAM tables exclusively. Right size depends on amount of indexes, data size and workload - remember MyISAM uses OS cache to cache the data so you need to leave memory for it as well, and data can be much larger than indexes in many cases. Check however if all of key_buffer is used over time - it is not rare to see key_buffer being set to 4G while combined size of .MYI files is just 1GB. This would be just a waste. If you use few MyISAM tables you'll want to keep it lower but still at least 16-32Mb so it is large enough to accommodate indexes for temporary tables which are created on disk.

key_buffer_size - 這對MyISAM表來說非常重要。如果只是使用MyISAM表，可以把它設定為可用記憶體的 30-40%。合理的值取決於索引大小、資料量以及負載 -- 記住，MyISAM表會使用作業系統的緩衝來快取資料，因此需要留出部分記憶體給它們，很多情況下資料比索引大多了。儘管如此，需要總是檢查是否所有的 key_buffer 都被利用了 -- .MYI 檔案只有 1GB，而 key_buffer 卻設定為 4GB 的情況是非常少的。這麼做太浪費了。如果你很少使用MyISAM表，那麼也保留低於 16-32MB 的 key_buffer_size 以適應給予磁碟的暫存資料表索引所需。

innodb_buffer_pool_size This is very important variable to tune if you're using Innodb tables. Innodb tables are much more sensitive to buffer size compared to MyISAM. MyISAM may work kind of OK with default key_buffer_size even with large data set but it will crawl with default innodb_buffer_pool_size. Also Innodb buffer pool caches both data and index pages so you do not need to leave space for OS cache so values up to 70-80% of memory often make sense for Innodb only installations. Same rules as for key_buffer apply - if you have small data set and it is not going to grow dramatically do not oversize innodb_buffer_pool_size you might find better use for memory available.

innodb_buffer_pool_size - 這對Innodb表來說非常重要。Innodb相比MyISAM表對緩衝更為敏感。MyISAM可以在預設的 key_buffer_size 設定下啟動並執行可以，然而Innodb在預設的 innodb_buffer_pool_size 設定下卻跟蝸牛似的。由於Innodb把資料和索引都緩衝起來，無需留給作業系統太多的記憶體，因此如果只需要用Innodb的話則可以設定它高達 70-80% 的可用記憶體。一些應用於 key_buffer 的規則有 -- 如果你的資料量不大，並且不會暴增，那麼無需把 innodb_buffer_pool_size 設定的太大了。

innodb_additional_pool_size This one does not really affect performance too much, at least on OS with decent memory allocators. Still you might want to have it 20MB (sometimes larger) so you can see how much memory Innodb allocates for misc needs.

innodb_additional_pool_size - 這個選項對效能影響並不太多，至少在有差不多足夠記憶體可分配的作業系統上是這樣。不過如果你仍然想設定為 20MB(或者更大)，因此就需要看一下Innodb其他需要分配的記憶體有多少。

innodb_log_file_size Very important for write intensive workloads especially for large data sets. Larger sizes offer better performance but increase recovery times so be careful. I normally use values 64M-512M depending on server size.

innodb_log_file_size 在高寫入負載尤其是大資料集的情況下很重要。這個值越大則效能相對越高，但是要注意到可能會增加恢復。我經常設定為 64-512MB，跟據伺服器大小而異。

innodb_log_buffer_size Default for this one is kind of OK for many workloads with medium write load and shorter transactions. If you have update activity spikes however or work with blobs a lot you might want to increase it. Do not set it too high however as it would be waste of memory - it is flushed every 1 sec anyway so you do not need space for more than 1 sec worth of updates. 8MB-16MB are typically enough. Smaller installations should use smaller values.

innodb_log_buffer_size 預設的設定在中等強度寫入負載以及較短事務的情況下，伺服器效能還可以。如果存在更新操作峰值或者負載較大，就應該考慮加大它的值了。如果它的值設定太高了，可能會浪費記憶體 -- 它每秒都會重新整理一次，因此無需設定超過1秒所需的記憶體空間。通常 8-16MB 就足夠了。越小的系統它的值越小。

innodb_flush_logs_at_trx_commit Crying about Innodb being 100 times slower than MyISAM ? You probably forgot to adjust this value. Default value of 1 will mean each update transaction commit (or each statement outside of transaction) will need to flush log to the disk which is rather expensive, especially if you do not have Battery backed up cache. Many applications, especially those moved from MyISAM tables are OK with value 2 which means do not flush log to the disk but only flush it to OS cache. The log is still flushed to the disk each second so you normally would not loose more than 1-2 sec worth of updates. Value 0 is a bit faster but is a bit less secure as you can lose transactions even in case MySQL Server crashes. Value 2 only cause data loss with full OS crash.

innodb_flush_logs_at_trx_commit 是否為Innodb比MyISAM慢1000倍而頭大？看來也許你忘了修改這個參數了。預設值是 1，這意味著每次提交的更新事務（或者每個事務之外的語句）都會重新整理到磁碟中，而這相當耗費資源，尤其是沒有電池備用緩衝時。很多應用程式，尤其是從 MyISAM轉變過來的那些，把它的值設定為 2 就可以了，也就是不把日誌重新整理到磁碟上，而只重新整理到作業系統的緩衝上。日誌仍然會每秒重新整理到磁碟中去，因此通常不會丟失每秒1-2次更新的消耗。如果設定為 0 就快很多了，不過也相對不安全了 -- MySQL伺服器崩潰時就會丟失一些事務。設定為 2 指揮丟失重新整理到作業系統緩衝的那部分事務。

table_cache - Opening tables can be expensive. For example MyISAM tables mark MYI header to mark table as currently in use. You do not want this to happen so frequently and it is typically best to size your cache so it is large enough to keep most of your tables open. It uses some OS resources and some memory but for modern hardware it is typically not the problem. 1024 is good value for applications with couple hundreds tables (remember each connection needs its own entry) if you have many connections or many tables increase it larger. I've seen values over 100.000 used.

table_cache -- 開啟一個表的開銷可能很大。例如MyISAM把MYI檔案頭標誌該表正在使用中。你肯定不希望這種操作太頻繁，所以通常要加大緩衝數量，使得足以最大限度地緩衝開啟的表。它需要用到作業系統的資源以及記憶體，對當前的硬體設定來說當然不是什麼問題了。如果你有200多個表的話，那麼設定為 1024 也許比較合適（每個線程都需要開啟表），如果串連數比較大那麼就加大它的值。我曾經見過設定為 100,000 的情況。

thread_cache Thread creation/destructions can be expensive, which happen at each connect/disconnect. I normally set this value to at least 16. If application has large jumps in amount of concurrent connections and I see fast growth of
Threads_Created variable I boost it higher. The goal is not to have threads created in normal operation.

thread_cache -- 線程的建立和銷毀的開銷可能很大，因為每個線程的串連/斷開都需要。我通常至少設定為 16。如果應用程式中有大量的跳躍並發串連並且 Threads_Created 的值也比較大，那麼我就會加大它的值。它的目的是在通常的操作中無需建立新線程。

query_cache If your application is read intensive and you do not have application level caches this can be great help. Do not set it too large as it may slow things down as its maintenance may get expensive. Values from 32M to 512M normally make sense. Check it however after a while and see if it is well used. For certain workloads cache hit ratio is lower than would justify having it enabled.

query_cache -- 如果你的應用程式有大量讀，而且沒有應用程式層級的緩衝，那麼這很有用。不要把它設定太大了，因為想要維護它也需要不少開銷，這會導致MySQL變慢。通常設定為 32-512Mb。設定完之後最好是跟蹤一段時間，查看是否運行良好。在一定的負載壓力下，如果快取命中率太低了，就啟用它。

Note: as you can see all of these are global variables. These variables depend on hardware and mix of storage engines, while per session variables are typically workload specific. If you have simple queries there is no reason to increase sort_buffer_size even if you have 64GB of memory to waste. Furthermore doing so may decrease performance.
I normally leave per session variable tuning to second step after I can analyze workload.

注意：就像你看到的上面這些全域表量，它們都是依據硬體設定以及不同的儲存引擎而不同，但是會話變數通常是根據不同的負載來設定的。如果你只有一些簡單的查詢，那麼就無需增加 sort_buffer_size 的值了，儘管你有 64GB 的記憶體。搞不好也許會降低效能。
我通常在分析系統負載後才來設定會話變數。

P.S Note MySQL distribution contains bunch of sample my.cnf files which may be great templates to use. Typically they would already be much better than defaults if you chose correct one.

P.S，MySQL的發行版已經包含了各種 my.cnf 範例檔案了，可以作為配置模板使用。通常這比你使用預設設定好的多了。