Re: [LSF/MM/BPF BoF] BoF for Zoned Storage

Luis Chamberlain
2022-03-04T22:10:13Z

On Fri, Mar 04, 2022 at 11:10:22AM +1100, Dave Chinner wrote:
> On Wed, Mar 02, 2022 at 04:56:54PM -0800, Luis Chamberlain wrote:
> > Thinking proactively about LSFMM, regarding just Zone storage..
> >
> > I'd like to propose a BoF for Zoned Storage. The point of it is
> > to address the existing point points we have and take advantage of
> > having folks in the room we can likely settle on things faster which
> > otherwise would take years.
> >
> > I'll throw at least one topic out:
> >
> > * Raw access for zone append for microbenchmarks:
> > - are we really happy with the status quo?
> > - if not what outlets do we have?
> >
> > I think the nvme passthrogh stuff deserves it's own shared
> > discussion though and should not make it part of the BoF.
>
> Reading through the discussion on this thread, perhaps this session
> should be used to educate application developers about how to use
> ZoneFS so they never need to manage low level details of zone
> storage such as enumerating zones, controlling write pointers
> safely for concurrent IO, performing zone resets, etc.

I'm not even sure users are really aware that given cap can be different
than zone size and btrfs uses zone size to compute size, the size is a
flat out lie.

modprobe null_blk nr_devices=0
mkdir /sys/kernel/config/nullb/nullb0
echo 0 > /sys/kernel/config/nullb/nullb0/completion_nsec
echo 0 > /sys/kernel/config/nullb/nullb0/irqmode
echo 2 > /sys/kernel/config/nullb/nullb0/queue_mode
echo 1024 > /sys/kernel/config/nullb/nullb0/hw_queue_depth
echo 1 > /sys/kernel/config/nullb/nullb0/memory_backed
echo 1 > /sys/kernel/config/nullb/nullb0/zoned

echo 128 > /sys/kernel/config/nullb/nullb0/zone_size
# 6 zones are implied, we are saying 768 for the full storage size..
# but...
echo 768 > /sys/kernel/config/nullb/nullb0/size

# If we force capacity to be way less than the zone sizes, btrfs still
# uses the zone size to do its data / metadata size computation...
echo 32 > /sys/kernel/config/nullb/nullb0/zone_capacity

# No conventional zones
echo 0 > /sys/kernel/config/nullb/nullb0/zone_nr_conv

echo 1 > /sys/kernel/config/nullb/nullb0/power
echo mq-deadline > /sys/block/nullb0/queue/scheduler

# mkfs.btrfs -f -d single -m single /dev/nullb0
Label: (null)
UUID: e725782a-d2d3-4c02-97fd-0501de117323
Node size: 16384
Sector size: 4096
Filesystem size: 768.00MiB
Block group profiles:
Data: single 128.00MiB
Metadata: single 128.00MiB
System: single 128.00MiB
SSD detected: yes
Zoned device: yes
Zone size: 128.00MiB
Incompat features: extref, skinny-metadata, no-holes, zoned
Runtime features: free-space-tree
Checksum: crc32c
Number of devices: 1
Devices:
ID SIZE PATH
1 768.00MiB /dev/nullb0

# mount /dev/nullb0 /mnt
# btrfs fi show
Label: none uuid: e725782a-d2d3-4c02-97fd-0501de117323
Total devices 1 FS bytes used 144.00KiB
devid 1 size 768.00MiB used 384.00MiB path
/dev/nullb0

# btrfs fi df /mnt
Data, single: total=128.00MiB, used=0.00B
System, single: total=128.00MiB, used=16.00KiB
Metadata, single: total=128.00MiB, used=128.00KiB
GlobalReserve, single: total=3.50MiB, used=0.00B

Since btrfs already has "real size" problems this existing
design takes this a bit further without a fix either. I suspect
quite a bit of puzzled users will be unhappy that even though
ZNS claims to kill overprovisioning we're now somehow lying
about size. I'm not even sure this might be good for the
filesystem / metadata.

Luis