The upcoming NVMe ZNS Specification will define a new type of write
command for zoned block devices, zone append.
When when writing to a zoned block device using zone append, the start
sector of the write is pointing at the start LBA of the zone to write to.
Upon completion the block device will respond with the position the data
has been placed in the zone. This from a high level perspective can be
seen like a file system's block allocator, where the user writes to a
file and the file-system takes care of the data placement on the device.
In order to fully exploit the new zone append command in file-systems and
other interfaces above the block layer, we choose to emulate zone append
in SCSI and null_blk. This way we can have a single write path for both
file-systems and other interfaces above the block-layer, like io_uring on
zoned block devices, without having to care too much about the underlying
characteristics of the device itself.
The emulation works by providing a cache of each zone's write pointer, so
zone append issued to the disk can be translated to a write with a
starting LBA of the write pointer. This LBA is used as input zone number
for the write pointer lookup in the zone write pointer offset cache and
the cached offset is then added to the LBA to get the actual position to
write the data. In SCSI we then turn the REQ_OP_ZONE_APPEND request into a
WRITE(16) command. Upon successful completion of the WRITE(16), the cache
will be updated to the new write pointer location and the written sector
will be noted in the request. On error the cache entry will be marked as
invalid and on the next write an update of the write pointer will be
scheduled, before issuing the actual write.
In order to reduce memory consumption, the only cached item is the offset
of the write pointer from the start of the zone, everything else can be
calculated. On an example drive with 52156 zones, the additional memory
consumption of the cache is thus 52156 * 4 = 208624 Bytes or 51 4k Byte
pages. The performance impact is neglectable for a spinning drive.
For null_blk the emulation is way simpler, as null_blk's zoned block
device emulation support already caches the write pointer position, so we
only need to report the position back to the upper layers. Additional
caching is not needed here.
Testing has been conducted by translating RWF_APPEND DIOs into
REQ_OP_ZONE_APPEND commands in the block device's direct I/O function and
injecting errors by bypassing the block layer interface and directly
writing to the disc via the SCSI generic interface.
The whole series is relative to Jens' block-5.6 branch 14afc5936197
("block, bfq: fix overwrite of bfq_group pointer in bfq_find_set_group()").
Damien Le Moal (2):
null_blk: Support REQ_OP_ZONE_APPEND
block: Introduce zone write pointer offset caching
Johannes Thumshirn (8):
block: provide fallbacks for blk_queue_zone_is_seq and
blk_queue_zone_no
block: introduce bio_add_append_page
block: introduce BLK_STS_ZONE_RESOURCE
block: introduce blk_req_zone_write_trylock
block: factor out requeue handling from dispatch code
block: delay un-dispatchable request
scsi: sd_zbc: factor out sanity checks for zoned commands
scsi: sd_zbc: emulate ZONE_APPEND commands
Keith Busch (1):
block: Introduce REQ_OP_ZONE_APPEND
block/bio.c | 41 +++-
block/blk-core.c | 49 +++++
block/blk-map.c | 2 +-
block/blk-mq.c | 54 +++++-
block/blk-settings.c | 16 ++
block/blk-sysfs.c | 15 +-
block/blk-zoned.c | 83 +++++++-
block/blk.h | 4 +-
drivers/block/null_blk_main.c | 9 +-
drivers/block/null_blk_zoned.c | 21 +-
drivers/scsi/scsi_lib.c | 1 +
drivers/scsi/sd.c | 28 ++-
drivers/scsi/sd.h | 35 +++-
drivers/scsi/sd_zbc.c | 344 +++++++++++++++++++++++++++++++--
include/linux/bio.h | 3 +-
include/linux/blk_types.h | 14 ++
include/linux/blkdev.h | 42 +++-
17 files changed, 701 insertions(+), 60 deletions(-)
--
2.24.1
