Re: Writes to LV not updating the mapped PV's disk cache?

John Stoffel <john@stoffel.org> · Fri, 2 Dec 2016 22:52:16 -0500

I suspect you either need to flush the device at the end, or use direction from within your dd test run to force the data to disk.

It would help if you would show the actual commands you run so we can look them over as well.  

Sent from my Amiga 1000

On Dec 1, 2016, at 11:29 AM, Kipper, Matthew <Matthew.Kipper@gd-ms.ca> wrote:

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

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