[xen-users] block devices and how they work

["David Brown" <dmlb2000@xxxxxxxxx>]

I've been trying to use xen in a pretty specific application and was
wondering why during high I/O in a domU does the dom0 also work just
as hard, it seems the block backend devices are working the hardest on
the dom0.  The setup is physical device shared to the domU not a loop
back. Is there any documentation on specifically how the backend block
device driver works?

=== top output ===
 PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND
6408 root      10  -5     0    0    0 S  2.7  0.0   0:27.59
blkback.2.hda1
6933 root      10  -5     0    0    0 S  2.3  0.0   0:26.12
blkback.3.hda1
7527 root      10  -5     0    0    0 S  2.0  0.0   0:23.58
blkback.4.hda1
8108 root      10  -5     0    0    0 S  2.0  0.0   0:26.06 blkback.5.hda1

In this you can see z4-8 is doing a dd to a file drive that the dom0
is giving it.

=== xm top output ===
     NAME  STATE   CPU(sec) CPU(%)     MEM(k) MEM(%)
 Domain-0 -----r       1135   45.7    1093616   10.4    1179648      11.3
     z4-1 --b---        221    0.3    1048576   10.0    1048576      10.0
     z4-2 -----r        192   10.0    1048576   10.0    1048576      10.0
     z4-3 --b---        178   10.0    1048576   10.0    1048576      10.0
     z4-4 --b---        187    8.0    1048576   10.0    1048576      10.0
     z4-5 --b---        201    8.7    1048576   10.0    1048576      10.0
     z4-8 ------        104   36.6    1048576   10.0    1048576      10.0

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