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

[Hank Tsai <hanktsai@qnap.com>]

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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:
https://drbd.linbit.com/users-guide/s-lvm-drbd-as-pv.html

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:
https://books.google.com/books?id=d1ZwAgAAQBAJ&pg=PT192&lpg=PT192&dq=drbd+lvm+
"write_cache_state"
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
the
top of /dev/sda1, and create PV using /dev/drbd1, then LVM won't display
/dev/sda1 as physical volumes.
http://www.linux-iscsi.org/Doc/RTS%20OS%20Admin%20Manual.pdf

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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