Description

When enabling the blk_mq "vxdmp.dmp_blk_mq_enable" feature, users may encounter unwanted VxFS file system failures and dg disabled events when I/O is in-flight.


blk_mq is a linux feature which DMP started supporting. This will take advantage of modern devices which can process large numbers of parallel I/O.

Linux documentation link:

https://docs.kernel.org/block/blk-mq.html


Workaround
 

Veritas is recommending disabling the blk_mq feature on Linux until the root cause is known.


How to disable blk_mq feature on Linux


# sysctl -w vxdmp.dmp_blk_mq_enable=0 >> /etc/sysctl.conf 

# reboot

To check if the blk_mq feature is enabled, use the below commands to see if a Dynamic Multi-pathing (DMP) device has multiple queues, e.g.

# ls /sys/block/VxDMP*/mq
/sys/block/VxDMP10/mq:
0  1  2  3  4  5
/sys/block/VxDMP11/mq:
0  1  2  3  4  5
/sys/block/VxDMP12/mq:
0  1  2  3  4  5
 

Once the blk-mq feature is disabled and a reboot is taken, then the mq directory itself should not be present for any of the DMP devices:

Example of blk_mq disabled:

# ls /sys/block/VxDMP*/mq
ls: cannot access '/sys/block/VxDMP*/mq': No such file or directory

The blk_mq feature should be disabled across all nodes in the environment.
 

NOTE: Disabling this will not impact the performance of regular hard disks.