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[linux-lvm] Parallel IO on striped logic volume?



I have a performance problem about parallel IO on striped logic volume.

I build a striped LV on 4 scsi disks. If I do sequential IO on one disk
with buffer size 4k, the bandwidth is 9MB/s. Then I do sequential IO on
LV with buffer size 16k. Theoretically I can get almost 4*9=36MB/s because
LVM stripe 4k IO on every disk but I only get 18MB/s. I don't know where
I lost so much performance or it is the overhead of LVM? I do believe LVM 
do parallel IO because if I increase buffer size to 1024k, I can get 
51MB/s which is close to 4*14 MB/s and 14MB/s is the maximum
bandwidth of a single disk.

My question is why I can't see nearly linear scaling of the bandwidth 
when the buffer size is small? Does striping LVM do real parallel IO
similar to software RAID0?
 

--xiaoxiang




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