MySQL server writing to HDD like stupid but what?

On one of my busy servers I using SSD HD only for mysql DBs but I cant 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.

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. html) to see exactly where IO is being generated by the MySQL daemon.

–Justin