This series, formerly titled "scsi-mq support for ZBC disks", implements
support for ZBC disks for system using the scsi-mq I/O path.
The current scsi level support of ZBC disks guarantees write request ordering
using a per-zone write lock which prevents issuing simultaneously multiple
write commands to a zone, doing so avoid reordering of sequential writes to
sequential zones. This method is however ineffective when scsi-mq is used with
zoned block devices. This is due to the different execution model of blk-mq
which passes a request to the scsi layer for dispatching after the request has
been removed from the I/O scheduler queue. That is, when the scsi layer tries
to lock the target zone of the request, the request may already be out of
order and zone write locking fails to prevent that.
Various approaches have been tried to solve this problem directly from the core
code of blk-mq. All of them had the serious disadvantage of cluttering blk-mq
code with zoned block device specific conditions and processing, making
maintenance and testing difficult.
This series adds blk-mq support for zoned block devices at the I/O scheduler
level with simple modifications of the mq-deadline scheduler. Implementation
is done with reusable helpers defined in the zoned block device support file
(blk-zoned.c). These helpers provide per zone write locking control functions
similar to what was implemented directly in the SCSI layer in sd_zbc.c.
The zone write locking mechanism is used by mq-deadline for the exact same
purpose, that is, to limit writes per zone to at most one request to avoid
reordering.
The changes to mq-deadline do not affect its operation with regular disks. The
same scheduling behavior is maintained for these devices. Compared to the SCSI
layer zone locking implementation, this series optimizes avoids locking
conventional zones which result in a use of these zone that is comparable to a
regular disk.
This series also implements changes to the legacy deadline-iosched. Doing so,
the zone locking code at the SCSI layer in sd.c and sd_zbc.c can be removed.
This results in a significant simplification of the sd driver command handling.
Patch 1 to 5 introduce the zone locking helpers in the block layer and modify
the deadline and mq-deadline schedulers.
Patch 6 and 7 remove the SCSI layer zone locking and initialize the device
request queue zone information.
All patches apply without conflicts to the scsi tree branch 4.16/scsi-queue, to
the block tree branch for-linus as well as to the current 4.15-rc4 tree.
Of note is that this series imposes the use of the deadline and mq-deadline
schedulers with zoned block devices. A system can trivialy enforce this using
a udev rule such as:
ACTION=="add|change", KERNEL=="sd[a-z]", ATTRS{queue/zoned}=="host-managed", \
ATTR{queue/scheduler}="deadline"
This rules applies equally for the legacy SCSI path as well as the scsi-mq path
thanks to "mq-deadline" being aliased to "deadline".
Comments are as always very much appreciated.
Changes from v8:
* Rebased on 4.15-rc4
Changes from v7:
* Merged patch 8 into patch 6
* Fixed various typos and commit messages
Changes from v6:
* Implement zone write locking helpers in the block layer
* Also modify legacy path deadline scheduler to remove all zone write locking
code from the scsi layer
Changes from v5:
* Refactor patches to introduce the zone_lock spinlock only when needed
* Addressed Bart's comments (in particular explanations of the zone_lock
spinlock use)
Changes from v4:
* Various fixes and improvements (From Christoph's comments)
* Dropped zones_wlock scheduler tunable attribute
Changes from v3:
* Integrated support directly into mq-deadline instead of creating a new I/O
scheduler.
* Disable setting of default mq scheduler for single queue devices
Changes from v2:
* Introduced blk_zoned structure
* Moved I/O scheduler from drivers/scsi to block
Changes from v1:
* Addressed Bart's comments for the blk-mq patches (declarations files)
* Split (former) patch 4 into multiple patches to facilitate review
* Fixed scsi disk lookup from io scheduler by introducing
scsi_disk_from_queue()
Christoph Hellwig (1):
block: introduce zoned block devices zone write locking
Damien Le Moal (6):
mq-deadline: Introduce dispatch helpers
mq-deadline: Introduce zone locking support
deadline-iosched: Introduce dispatch helpers
deadline-iosched: Introduce zone locking support
sd_zbc: Initialize device request queue zoned data
sd: Remove zone write locking
block/blk-core.c | 1 +
block/blk-zoned.c | 42 +++++++++
block/deadline-iosched.c | 114 ++++++++++++++++++++---
block/mq-deadline.c | 130 ++++++++++++++++++++++++--
drivers/scsi/sd.c | 41 +--------
drivers/scsi/sd.h | 11 ---
drivers/scsi/sd_zbc.c | 235 +++++++++++++++++++++++++++++------------------
include/linux/blkdev.h | 111 ++++++++++++++++++++++
include/scsi/scsi_cmnd.h | 3 +-
9 files changed, 528 insertions(+), 160 deletions(-)
--
2.14.3
