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

Lars Ellenberg lars.ellenberg at linbit.com
Thu Jan 12 17:00:53 UTC 2017

On Wed, Jan 11, 2017 at 06:23:08PM +0100, knebb at knebb.de wrote:
> 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?

In this case, it happens on first mount.
Also, it is not an "EIO", but an "EOPNOTSUP".

What really happens is that the file system code calls
which will try discard, if discard is available and discard zeroes data,
or, if discard (with discard zeroes data) is not available or returns
failure, tries write-same with ZERO_PAGE,
or, if write-same is not available or returns failure,
tries __blkdev_issue_zeroout() (which uses "normal" writes).

At least in "current upstream", probably very similar in your
almost-3.10.something kernel.

DRBD sits in between, sees the failure return of write-same,
and handles it by detaching.

> 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....

Yes. And it is, as far as I can tell.

> > 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

double check that, please.
all my centos7 / RHEL 7 (and other distributions with sufficiently new
kernel) have that.

there are both the read-only /sys/block/*/queue/write_same_max_bytes
and the write-able /sys/devices/*/*/*/host*/target*/*/scsi_disk/*/max_write_same_blocks

> "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.

show us lsblk -t and lsblk -D from the box that detaches.
(the "7" one)

It may also be that a discard failed, in which case it could be
devicemapper pretending discard was supported, and the backend failing
that discard request. Or some combination there.

Your original logs show
> Jan  7 10:58:44 backuppc kernel: EXT4-fs (dm-2): mounted filesystem with ordered data mode. Opts: (null)
> Jan  7 10:58:48 backuppc kernel: block drbd1: local WRITE IO error sector 5296+3960 on sdc

The "+..." part is the length (number of sectors) of the request.
We don't allow "normal" requests of that size, so this is either a
discard or write-same.

> Jan  7 10:58:48 backuppc kernel: block drbd1: disk( UpToDate -> Failed )

> Jan  7 10:58:48 backuppc kernel: block drbd1: IO ERROR: neither local nor remote data, sector 29096+3968

> Jan  7 10:58:48 backuppc kernel: dm-2: WRITE SAME failed. Manually zeroing.

And here we see that at least some WRITE SAME was issued, and returned failure.
and device mapper, which in your case sits above DRBD,
and consumes that error, has its own fallback code for failed write-same.
Which can no longer be services, because DRBD already detached.

So yes,
I'm pretty sure that I did not pull my "best guess" out of thin air only


: Lars Ellenberg
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