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[linux-lvm] Does LVM-LV take system cache?



Does LVM-LV take system cache?

In my test, it does not take system cache on a RHEL5 box, is it a bug or a feature?

 

Test1, run dd with raw block device, it seems system cache can boost up reread operation.

 

[ djt_18_69 ~]# dd if=/dev/sdc bs=1M of=/dev/null count=500

524288000 bytes (524 MB) copied, 1.88267 seconds, 278 MB/s

[ djt_18_69 ~]# dd if=/dev/sdc bs=1M of=/dev/null count=500

524288000 bytes (524 MB) copied, 0.13445 seconds, 3.9 GB/s

 

Test2, run dd with lv, it seems system cache CAN NOT boost up reread operation.

 

[ djt_18_69 ~]# dd if=/dev/vgext/lvtest5 bs=1M of=/dev/null count=500

524288000 bytes (524 MB) copied, 2.17848 seconds, 241 MB/s

[ djt_18_69 ~]# dd if=/dev/vgext/lvtest5 bs=1M of=/dev/null count=500

524288000 bytes (524 MB) copied, 2.17451 seconds, 241 MB/s

 

Both 2.6.18-164.15.1.el5 and 2.6.18-194.el5 do the same behavior.

 

 

  Regards

  Kirby Zhou

 


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