[linux-lvm] Why it suggest to disable "write_cache_state" when using DRBD as physical volumes

Hank Tsai hanktsai at qnap.com
Wed May 13 10:09:56 UTC 2015

Hi All,

I want to use DRBD devices as LVM physical volumes.
I'm curious about why the the DRBD manual suggest
to disable "write_cache_state" in this scenario:

I can only find a few articles that explain the reason.
For example, this book said that could prevent error while the DRBD devices
are down:
But, as I know, LVM can handle the case if the device listed in device cache
are not available. In LVM2.2.02.105, function lvmcache_device_from_pvid()
invokes lvmcache_label_scan() to refresh the cache if PV is missing.

This document also suggest to disable device cache, but I think that
LVM nowadays can handle the issue it described, such as "duplicate
physical volumes being displayed". For example, if we build /dev/drbd1 on
top of /dev/sda1, and create PV using /dev/drbd1, then LVM won't display
/dev/sda1 as physical volumes.

Also, the device cache feature is automatically disabled if lvmetad is
enabled. So, is it necessary to disable "write_cache_state" while
"use_lvmetad" was enabled?

Any help would be appreciate!

Ming-Hung Tsai
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