[SRU xenial-aws][PATCH 0/2] LP: #1801305: xen-blkfront: restore request mode

[Daniel Axtens]

In current Ubuntu kernels, PV blkfront drivers have blk-mq enabled by
default and cannot use the old I/O scheduler.

[Impact]
blk-mq is not as fast as the old request-based scheduler for some
workloads on HDD disks.

[Fix]
Amazon Linux has a commit which reintroduces the request-based
mode. It disables blk-mq by default but allows it to be switched back
on with a kernel parameter.

For X this needs a small patch from upstream for error handling.

For B/C this patchset is bigger as it includes the suspend/resume
patches already in X, and a new fixup. These are desirable as the
request mode patch assumes their presence.

[Regression Potential]
Could potentially break xen based disks on AWS.

For B/C, the patches also add some code to the xen core around suspend
and resume, this code is much smaller and also mirrors code already in
Xenial.

[Tests]
Tested by AWS for Xenial, and their kernel engineers vetted the Xenial
patches. I tested the Bionic and Cosmic patchsets with fio, the system
appears stable and the IOPS promised for EBS Provisioned IOPS disks
were met in my testing. I did an apt update/upgrade and everything
worked (no hash-sum mismatches).

Christoph Hellwig (1):
  xen-blkfront: don't use req->errors

Munehisa Kamata (1):
  xen-blkfront: resurrect request-based mode

 drivers/block/xen-blkfront.c | 333 +++++++++++++++++++++++++++++------
 1 file changed, 276 insertions(+), 57 deletions(-)

-- 
2.17.1