io read - merged

["Tolga Evren" <tolgae paro com tr>]

Hi ,

 

i have a question regarding to the iostat output:

 

Device:    rrqm/s wrqm/s   r/s   w/s  rsec/s  wsec/s    rkB/s    wkB/s
avgrq-sz avgqu-sz   await  svctm  %util

sdp        86432.30   0.00 3915.20  2.00 124267.20    2.00 62133.60
1.00    31.72    32.86    8.39   0.26 100.00

 

According to this output:

 

rrqm/s : 86432.3 , r/s:3915.2 , rkB/s : 62133.60 .

 

 rrqm/s

                     The  number of read requests merged per sec-

                     ond that were issued to the device.

 

The operating system merges multiple io requests into one read call ,
since these io requests are small from the basic io unit.  

 

I have 86432.3 merge per second , with 3915.2 read per second. This
shows that , for 1 read operation , my system does 22 merge
.(86432.3/3915.2)   . On the other hand , 62133.60 / 3915.2 = 15kb ,
each read operation issues 15Kb. from the underlying disk subsystem. 

 

15kb/22 merge = 0,6 kb more or less 512b. So each io request  in fact is
512byte , but  since the minimum io that os can request is 15kb, they
are merged till the 15kb. size is reached.

 

Are these comments meaningful?

 

Kind Regards,

tolga