try to use IO direct when you use the dd command, man dd and look oflag option 2016-12-01 17:29 GMT+01:00 Kipper, Matthew <Matthew.Kipper@gd-ms.ca>: > Hi, > > > > I ran into a strange (to me) issue while testing something in LVM – I wanted > to check that zeroing a Logical Volume would really zero the mapped Physical > Extents. My setup was simple – I had a single Volume Group (vg0) containing > a single Physical Volume (/dev/mmcblk0p2) and a few Logical Volumes. Nothing > fancy. > > > > The test itself was also simple: > > 1. Write a test pattern to an LV using ‘dd’ > > 2. Check how the LV is mapped with ‘pvdisplay -m’ > > 3. Read back the mapped PEs from the PVs block device with ‘dd’, > checking that they match the test pattern written to the LV > > > > It seems like Linux’s disk cache doesn’t like this test. If I follow the > steps, the readback from the PV’s block device returns stale data. But if I > empty my cache (‘echo 3 > /proc/sys/vm/drop_caches’) and do the readback > again, I can see the pattern that was written to the LV. > > > > Is this the expected behavior? I don’t know much about Linux subsystems, but > I’d assume that when LVM maps LV -> PV and does a write, Linux would get the > memo and update its disk cache accordingly. The results I’m seeing suggest > that Linux doesn’t update the cache or flag it as dirty, because I can > repeatedly read back stale data prior to emptying my cache. > > > > Here are a few details about my setup: > > · Platform: Zynq 7000 (Dual-Core ARM Cortex-A9) > > · Linux: 4.0.0 (built from Xilinx repo) > > · LVM Version: 2.02.162(2) (2016-07-28) > > · LVM Library Version: 1.02.132 (2016-07-28) > > · LVM Driver Version: 4.30.0 > > · Test Media: eMMC device > > > > I don’t know if I’m misunderstanding how LVM/Linux are supposed to mingle or > if this is an actual issue. > > Matthew Kipper > > > _______________________________________________ > linux-lvm mailing list > linux-lvm@redhat.com > https://www.redhat.com/mailman/listinfo/linux-lvm > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ -- .~. /V\ // \\ /( )\ ^`~'^ _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
