raid1 narrow_write_error with 4K disks, sd "bad block number requested" messages

[Nate Dailey <nate.dailey@stratus.com>]

I'm writing about something that appears to be an issue with raid1's 
narrow_write_error, particular to non-512-byte-sector disks. Here's what 
I'm doing:

- 2 disk raid1, 4K disks, each connected to a different SAS HBA
- mount a filesystem on the raid1, run a test that writes to it
- remove one of the SAS HBAs (echo 1 > 
/sys/bus/pci/devices/0000\:45\:00.0/remove)

At this point, writes fail and narrow_write_error breaks them up and 
retries, one sector at a time. But these are 512-byte sectors, and sd 
doesn't like it:

[ 2645.310517] sd 3:0:1:0: [sde] Bad block number requested
[ 2645.310610] sd 3:0:1:0: [sde] Bad block number requested
[ 2645.310690] sd 3:0:1:0: [sde] Bad block number requested
...

There appears to be no real harm done, but there can be a huge number of 
these messages in the log.

I can avoid this by disabling bad block tracking, but it looks like 
maybe the superblock's bblog_shift is intended to address this exact 
issue. However, I don't see a way to change it. Presumably this is 
something mdadm should be setting up? I don't see bblog_shift ever set 
to anything other than 0.

This is on a RHEL 7.1 kernel, version 3.10.0-221.el7. I took a look at 
upstream sd and md changes and nothing jumps out at me that would have 
affected this (but I have not tested to see if the bad block messages do 
or do not happen on an upstream kernel).

I'd appreciate any advice re: how to handle this. Thanks!

Nate Dailey
Stratus Technologies