[dm-devel] [linux-lvm] Mounted, but idle LVM volume causes constant disk writes

[Tomas Vanderka]

On 06/04/2013 07:46 AM, Bostjan Skufca wrote:
> Hello,
> 
> If LVM2 volume is mounted (ext4 fs), why is there constant write activity to the devices below (sw raid in this case) despite not using the mounted volume at all?
> 
> I am seing this on multiple hosts. On this particular one I am seeing around 20 writes per second on average, but they occur in batches of around 50-90 writeIOps on every
> 3-4 seconds. If I mount the same filesystem (ext4) on sw raid1 without LVM in between, I see the expected 0 write IOps when not using the filesystem.
> 

Just out of curiosity I looked at my systems and I don't see any such IO, even for mounted FS.

You can try running following commands to log writes and where are they comming from (show the result via dmesg)

echo 1 > /proc/sys/vm/block_dump; sleep 10; echo 0 > /proc/sys/vm/block_dump;

By doing so I noticed dm-thin writing to my otherwise unused dm thin pool every second. After looking at the code in dm-thin.c, it looks like the kernel is periodically
commiting metadata every second even if there was no IO to the pool in the time period. I think process_deferred_bios function should probably check if anything changed
before commiting metadata unconditionally when 1s timer is up. Looks like dm-cache-target.c is doing it right.

t.