Loop devices not supported concurrent access ?
From: Gregory Auzanneau
Date: Sat Jan 08 2011 - 20:17:11 EST
Hello All,
During some tests, I've remark an interesting issue with loop devices.
loop devices queues concurrent access by multi-threaded software.
This issue can easily be highlighted by this tool :
http://box.houkouonchi.jp/seeker_baryluk.c
This tool permit to test how many iop/s your system can handle (don't
forget to use deadline scheduler).
First of all, we will define a loop device on a raw device (raw device
permit to avoid filesystem interaction) :
losetup /dev/loop1 /dev/sda5
Now we will test the number our raw device and our loop device can
handle with only one thread :
./seeker_baryluk /dev/sda5 1 => Results: 196 seeks/second, 5.081 ms
random access time
./seeker_baryluk /dev/loop1 1 => Results: 194 seeks/second, 5.131 ms
random access time
Results : raw device and loop device have approximately the same
performance with one thread.
Here is the problem : we will now test with 32 threads :
./seeker_baryluk /dev/sda5 32 => Results: 631 seeks/second, 1.585 ms
random access time
./seeker_baryluk /dev/loop1 32 => Results: 194 seeks/second, 5.148 ms
random access time
As you can see, loop device deals request one by one even with parrallel
request.
This involve that we lost a lot of performance performed by NCQ/TCQ
and/or disk balancing.
The same thing also appears in Xen when mapping a disk with a "file".
Is this problem solvable ?
/Thank you all for the good work with linux, keep up with it ! :)
Greg
/
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