lvmcache in writeback mode gets stuck flushing dirtyblocks

[Lakshmi Narasimhan Sundararajan <lns@portworx.com>]

Hi Nikhil,
Thank you for your email. Much appreciated.

In my environment, Chunksize is fixed at 1M irrespective of the pool size. This may take the number of entries over 1M and result in kernel warning. But the class of systems we are using are huge, and so the memory and cpu bottlenecks does not seem to be a factor in our testing.

I looked up at the bugs. The first one about chunksize > 1M, we should be safe on that given our chunksize is fixed at 1MB.
The other one about migration threshold is interesting, I will have to validate this again.

What would be the unit of migration threshold?  Is it the number of 512 byte sectors? And what exactly is its definition?

And also curiously this does not seem to be exported through lvm cli, need to fetch this only through dmsetup?

Thanks
LN
Sent from Mail for Windows 10

From: Nikhil Kshirsagar
Sent: Wednesday, July 31, 2019 3:04 PM
To: LVM2 development
Subject: Re: [lvm-devel] lvmcache in writeback mode gets stuck flushing dirtyblocks

This used to happen if the chunksize increased as a result of needing to use more than a million chunks to store the size of the cached lv. What is the size of the pool?

Regards,
Nikhil.

On Tue, 30 Jul, 2019, 1:25 PM Lakshmi Narasimhan Sundararajan, <lns@portworx.com> wrote:
Hi Team,
A very good day to all.

I am using lvmcache in writeback mode. When there are dirty blocks still in the lv, and if needs to be destroyed or flushed, then
It seems to me that there are some conditions under which the dirty data flush gets stuck forever.
?
?
As an example:
root at pdc4-sm35:~# lvremove -f pwx0/pool
? 367 blocks must still be flushed.
? 367 blocks must still be flushed.
? 367 blocks must still be flushed.
? 367 blocks must still be flushed.
? 367 blocks must still be flushed.
? 367 blocks must still be flushed.
^C
root at pdc4-sm35:~#
?
I am running these version:
root at pdc4-sm35:~# lvm version
? LVM version:???? 2.02.133(2) (2015-10-30)
? Library version: 1.02.110 (2015-10-30)
? Driver version:? 4.34.0
root at pdc4-sm35:~#
?
?
This issue seems old and reported multiple places. There have been some acknowledgement that this issue is resolved in 2.02.133, but still I see it. Also, I have seen some posts report it in 2.02.170+ as well (here: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=878441) (Package: lvm2 Version: 2.02.173-1 Severity: normal)
?
I filed one here myself, https://github.com/lvmteam/lvm2/issues/22, trying? to understand from you experts where we are on this?
?
I would sincerely appreciate your help in understanding the state of this issue in more detail.
?
Best regards
LN
Sent from Mail for Windows 10
?
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