tree 3f2957cc99f6d572ed176f48007a0b62c9af7db4
parent d6f9469a03d832dcd17041ed67774ffb5f3e73b3
author Coly Li <colyli@suse.de> 1590144479 +0800
committer Coly Li <colyli@suse.de> 1591172613 +0800

bcache: export bcache zone information for zoned backing device

When using a zoned device e.g. SMR hard drive as the backing device,
if bcache can export the zoned device information then it is possible
to help the upper layer code to accelerate hot READ I/O requests.

This patch adds the report_zones method for the bcache device which has
zoned device as backing device. Now such bcache device can be treated as
a zoned device, by configured to writethrough, writearound mode or none
mode, zonefs can be formated on top of such bcache device.

Here is a simple performance data for read requests via zonefs on top of
bcache. The cache device of bcache is a 4TB NVMe SSD, the backing device
is a 14TB host managed SMR hard drive. The formatted zonefs has 52155
zone files, 523 of them are for conventional zones (1 zone is reserved
for bcache super block and not reported), 51632 of them are for
sequential zones.

First run to read first 4KB from all the zone files with 50 processes,
it takes 5 minutes 55 seconds. Second run takes 12 seconds only because
all the read requests hit on cache device.

29 times faster is as expected for an ideal case when all READ I/Os hit
on NVMe cache device.

Besides providing report_zones method of the bcache gendisk structure,
this patch also initializes the following zoned device attribution for
the bcache device,
- number of zones: the total number of zones minus reserved zone(s) for
  bcache super block.
- zone size: same size as reported from the underlying zoned device
- zone model: same model as reported from the underlying zoned device
Currently blk_revalidate_disk_zones() does not accept non-mq drivers, so
all the above attribution are initialized manaully in bcache code.

This patch just provides the report_zones method only. Handling all zone
management commands will be addressed in following patches.

Signed-off-by: Coly Li <colyli@suse.de>
Cc: Damien Le Moal <damien.lemoal@wdc.com>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Johannes Thumshirn <johannes.thumshirn@wdc.com>