Friday, 23 July 2010

Key cache hit ratio

When we talk about key cache performance we usually look at these two status variables.

1. Key_read_requests
The number of requests to read a key block from the cache.

2. Key_reads
The number of physical reads of a key block from disk.

There is a good reason to examine Key_reads, because we know that disks are very slow relative to RAM but the Key_reads aren't always physical disk reads at all. If the requested block of data isn't in the operating system's cache, then a Key_read is a disk read, you will be lucky if it is cached, then it's just a system call. Having said that it is always good to minimize key_reads which may cause randon disk I/O.

The optimum solution is to keep the ratio Key_reads : Key_read_requests should be 1:100 and Key_writes / Key_write_requests should always be less than 1.

Finally I'll like to show you something partially useful you can do with Key_reads:

[xxxxxx ~]$ mysqladmin -uroot -p ext -ri10 | grep Key_reads
Enter password:
| Key_reads | 44605148 |
| Key_reads | 4 |
| Key_reads | 4 |
| Key_reads | 13 |
| Key_reads | 9 |
| Key_reads | 6 |
| Key_reads | 20 |
| Key_reads | 6 |
| Key_reads | 11 |

This server is doing approximately 7 Key_reads every ten seconds but it is hard to say that how many of them are random I/O and how many are just system calls (i.e. read from operrating system's cache)

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