[linux-lvm] [DRBD-user] LVM on top of DRBD [actually: mkfs.ext4 then mount results in detach on RHEL 7 on VMWare]

knebb at knebb.de knebb at knebb.de
Wed Jan 11 17:23:08 UTC 2017

Hi Lars and all,

>> I have to cross-post to LVM as well to DRBD mailing list as I have no
>> clue where the issue is- if it's not a bug...
>> I can not get working LVM  on top of drbd- I am getting I/O erros
>> followed by "diskless" state.
> For some reason, (some? not only?) VMWare virtual disks tend to pretend
> to support "write same", even if they fail such requests later.
> DRBD treats such failed WRITE-SAME the same way as any other backend
> error, and by default detaches.
Ok, it is beyond my knowledge, but I understand what the "write-same"
command does. But if the underlying physical disk offers the command and
reports an error when used this should apply to mkfs.ext4 on the device/
partition as well, shouldn't it? drbd detacheds when an error is
reported- but why does Linux not report an error without drbd? And why
does this only happen when using LVM in-between? Should be the same when
LVM is not used....

> Older kernels (RHEL 6) and also older drbd (8.3) are not affected, because they
> don't know about write-same.
My primary host is running CentOS7 while the secondary ist older
(CentOS6). I will try to create the ext4 on the secondary and then
switch to primary.

> Or tell the system that the backend does not support write-same:
> Check setting:
> 	grep ^ /sys/block/*/device/scsi_disk/*/max_write_same_blocks
> disable:
> 	echo 0 | tee /sys/block/*/device/scsi_disk/*/max_write_same_blocks
A "find /sys -name "*same*"" does not report any files named
"max_write_same_blocks". On none of the both nodes. So I dcan not
disable nor verify if it's enabled. I assume no as it does not exist. So
this might not be the reason.



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