MySQL server writing to HDD like stupid but what?

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  • MySQL server writing to HDD like stupid but what?

    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):

    Total: 53.9 MiB
    Per hour: 5.3 MiB

    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?


  • #2

    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.