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