[linux-lvm] Writes to LV not updating the mapped PV's disk cache?

[emmanuel segura]

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 at 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 at redhat.com
> https://www.redhat.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/



-- 
  .~.
  /V\
 //  \\
/(   )\
^`~'^