[Xen-users] Double amount of READ DISK traffic on dom0 than from domU?

[Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>]

Dear list,
did anybody of you see a similar behaviour? I have a test server with a 
single XEN VM running. In it, there's a PostgreSQL DB running a CLUSTER 
command, which basically copies the table in a new order. The domU VM 
said it's 35MB/s read and 35MB/s write, but the dom0 said it's about 
TWICE the amount on *read* only (79MB/s read, 35MB/s write). The command 
with which I watched was
iostat -kx 5 555
on both domU and dom0. The values were most of the time nearly exaclty 
domU-reads * 2 = dom0-reads. Writes were the same numbers both in domU 
and dom0. Of course the values vary a bit because the iostat command 
doesn't run exactly at the same time in domU and dom0, but I watched a 
long time and it's about twice as much reads on dom0. I would understand 
the other way round, that could be because of caching in dom0, but this 
results are strange. Anybody got an explanation?

Real Server dom0:
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
sda               0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
sdb               0,00     1,80 10254,40  231,80 79640,00 40258,40    22,87     
1,16    0,11   0,04  46,96

XEN VM domU:
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
xvda              0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
xvdb              0,00     0,00  839,92    0,00 35794,01     0,00    85,23     
4,90    5,83   1,18  99,00
xvdc              0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
xvdd              0,00     0,00    0,00  821,36     0,00 35823,55    87,23    
51,17   61,97   1,21  99,48

Later I tested again:

Real Server dom0:
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
sda               0,00     0,20    0,00    0,40     0,00     2,40    12,00     
0,00    0,00   0,00   0,00
sdb               0,20     0,00 3246,31   68,66 24988,42  7826,75    19,80     
0,38    0,11   0,07  24,43

XEN VM domU:
Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz 
avgqu-sz   await  svctm  %util
xvda              0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
xvdb              0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
xvdc              0,00     0,00    0,00    0,00     0,00     0,00     0,00     
0,00    0,00   0,00   0,00
xvdd              0,00     0,40  277,84  365,27 11681,44 15674,65    85,07    
26,68   41,49   0,83  53,49


-- 
// Michael Monnerie, Ing.BSc    -----      http://it-management.at
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