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Re: slow queries and high % of wait I/O
Posted by: Rick James
Date: January 20, 2009 01:28AM

To read a _random_ block from disk costs on average at least half a rotation. So, depending on rotational speed (SATA: 7200rpm, SCSI up to 15K, etc), we are looking at (in round numbers) 10ms -- 100 reads/sec. That assumes the desired block was not cached (in controller, MySQL cache, etc). And it assumes an isolated read.

Add RAID to the mix, together with multiple requestors, now you can get more. With 6 drives, you might get closer to 600 reads/sec.

Writes are another matter -- with BBWC, writes are virtually instantaneous (until you saturate that).

Alas, the numbers don't quite add up. Perhaps something else is going on that Jawahar has not mentioned.

SELECT * FROM HISTORY WHERE ... -- If it is 200 rows, it could take more than a second.

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Written By
January 19, 2009 05:36PM
January 19, 2009 11:37PM
January 21, 2009 01:22AM
Re: slow queries and high % of wait I/O
January 20, 2009 01:28AM
January 20, 2009 08:41PM

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