# MySQL server writing to HDD like stupid but what?

**URL:** <https://forums.percona.com/t/mysql-server-writing-to-hdd-like-stupid-but-what/1791>\
**Category:** Other MySQL® Questions\
**Created:** [April 5, 2012, 9:25pm UTC](https://forums.percona.com/t/mysql-server-writing-to-hdd-like-stupid-but-what/1791 "2012-04-05T21:25:50Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![x-man](https://avatars.discourse-cdn.com/v4/letter/x/46a35a/32.png) [@x-man](https://forums.percona.com/u/x-man)\
**Post date:** [April 5, 2012, 9:25pm UTC](https://forums.percona.com/t/mysql-server-writing-to-hdd-like-stupid-but-what/1791/1 "2012-04-05T21:25:50Z")

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On one of my busy servers I using SSD HD only for mysql DB`s but I can`t understand why MUNIN reporting average 12.74 MBytes (yes BYTES) and up to 80 Mbytes/second for WRITING on this HD and only 58.87 KBytes average and max 3 Mbytes for reading from this HD…I know that most reading come from RAM cache but on main busy website for writing I have only sessions and rest is almost nothing…

And this is what MySQL status said (for last 10 hours):

Received  
Total: 53.9 MiB  
Per hour: 5.3 MiB

Sent  
Total: 1.7 GiB  
Per hour: 0.2 GiB

Like you can see data received to mysql only 53.9 MiB and sent 1.7 GiB, how than this can generate up to 80 Mbytes/second for writing??

And this is what iostat -d -x 5 3 said:  
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util  
sdc 0.00 20.40 0.00 83.80 0.00 1142.40 13.63 0.03 0.40 0.14 1.18

If I good understand this, again I have tooo much writing on this HD…

Again I must say that this HD I using only for mysql DB`s, why and what mysql server can write to HD if that is not data?

Thanks.

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**Author:** ![justin.swanhart](https://sea1.discourse-cdn.com/flex019/user_avatar/forums.percona.com/justin.swanhart/32/16904_2.png) [@justin.swanhart](https://forums.percona.com/u/justin.swanhart)\
**Post date:** [April 6, 2012, 6:18pm UTC](https://forums.percona.com/t/mysql-server-writing-to-hdd-like-stupid-but-what/1791/2 "2012-04-06T18:18:22Z")

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Hi,

You might have queries that are creating temporary tables on disk.

You can try pt-ioprofile ( [http://www.percona.com/doc/percona-toolkit/2.0/pt-ioprofile](http://www.percona.com/doc/percona-toolkit/2.0/pt-ioprofile). html) to see exactly where IO is being generated by the MySQL daemon.

–Justin