On 07/24/2018 03:07 PM, Mike Snitzer wrote:
On Tue, Jul 24 2018 at 2:00am -0400,
Hannes Reinecke <hare@xxxxxxx> wrote:
On 07/23/2018 06:33 PM, Mike Snitzer wrote:
Hi,
I've opened the following public BZ:
https://bugzilla.redhat.com/show_bug.cgi?id=1607527
Feel free to add comments to that BZ if you have a redhat bugzilla
account.
But otherwise, happy to get as much feedback and discussion going purely
on the relevant lists. I've taken ~1.5 weeks to categorize and isolate
this issue. But I've reached a point where I'm getting diminishing
returns and could _really_ use the collective eyeballs and expertise of
the community. This is by far one of the most nasty cases of corruption
I've seen in a while. Not sure where the ultimate cause of corruption
lies (that the money question) but it _feels_ rooted in NVMe and is
unique to this particular workload I've stumbled onto via customer
escalation and then trying to replicate an rbd device using a more
approachable one (request-based DM multipath in this case).
I might be stating the obvious, but so far we only have considered
request-based multipath as being active for the _entire_ device.
To my knowledge we've never tested that when running on a partition.
True. We only ever support mapping the partitions ontop of
request-based multipath (via dm-linear volumes created by kpartx).
So, have you tested that request-based multipathing works on a
partition _at all_? I'm not sure if partition mapping is done
correctly here; we never remap the start of the request (nor bio,
come to speak of it), so it looks as if we would be doing the wrong
things here.
Have you checked that partition remapping is done correctly?
It clearly doesn't work. Not quite following why but...
After running the test the partition table at the start of the whole
NVMe device is overwritten by XFS. So likely the IO destined to the
dm-cache's "slow" (dm-mpath device on NVMe partition) was issued to the
whole NVMe device:
# pvcreate /dev/nvme1n1
WARNING: xfs signature detected on /dev/nvme1n1 at offset 0. Wipe it? [y/n]
# vgcreate test /dev/nvme1n1
# lvcreate -n slow -L 512G test
WARNING: xfs signature detected on /dev/test/slow at offset 0. Wipe it?
[y/n]: y
Wiping xfs signature on /dev/test/slow.
Logical volume "slow" created.
Isn't this a failing of block core's partitioning? Why should a target
that is given the entire partition of a device need to be concerned with
remapping IO? Shouldn't block core handle that mapping?
Only if the device is marked a 'partitionable', which device-mapper
devices are not.
But I thought you knew that ...
Anyway, yesterday I went so far as to hack together request-based
support for DM linear (because request-based DM cannot stack on
bio-based DM) . With this, request-based linear devices instead of
conventional partitioning, I no longer see the XFS corruption when
running the test:
_Actually_, I would've done it the other way around; after all, where't
the point in running dm-multipath on a partition?
Anything running on the other partitions would suffer from the issues
dm-multipath is designed to handle (temporary path loss etc), so I'm not
quite sure what you are trying to achieve with your testcase.
Can you enlighten me?
Cheers,
Hannes
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