btrfs-progs: docs: update Zoned mode
Copy remaining text from wiki. Signed-off-by: David Sterba <dsterba@suse.com>
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David Sterba
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@ -42,7 +42,7 @@ This is inherently incompatible with some features:
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Initial support lacks some features but they're planned:
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* only single profile is supported
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* only single (data, metadata) and DUP (metadata) profile is supported
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* fstrim - due to dependency on free space cache v1
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Super block
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@ -64,3 +64,122 @@ devices is expected to be large, tens of terabytes. Maximum zone size supported
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is 8GiB, which would mean that e.g. offset 0-16GiB would be reserved just for
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the super block on a hypothetical device of that zone size. This is wasteful
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but required to guarantee crash safety.
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Devices
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^^^^^^^
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Real hardware
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"""""""""""""
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The WD Ultrastar series 600 advertises HM-SMR, i.e. the host-managed zoned
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mode. There are two more: DA (device managed, no zoned information exported to
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the system), HA (host aware, can be used as regular disk but zoned writes
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improve performance). There are not many devices available at the moment, the
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information about exact zoned mode is hard to find, check data sheets or
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community sources gathering information from real devices.
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Note: zoned mode won't work with DM-SMR disks.
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- Ultrastar® DC ZN540 NVMe ZNS SSD (`product
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brief <https://documents.westerndigital.com/content/dam/doc-library/en_us/assets/public/western-digital/collateral/product-brief/product-brief-ultrastar-dc-zn540.pdf>`__)
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Emulated: null_blk
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""""""""""""""""""
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The driver *null_blk* provides memory backed device and is suitable for
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testing. There are some quirks setting up the devices. The module must be
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loaded with *nr_devices=0* or the numbering of device nodes will be offset. The
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*configfs* must be mounted at */sys/kernel/config* and the administration of
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the null_blk devices is done in */sys/kernel/config/nullb*. The device nodes
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are named like */dev/nullb0* and are numbered sequentially. NOTE: the device
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name may be different than the named directory in sysfs!
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Setup:
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.. code-block:: bash
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modprobe configfs
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modprobe null_blk nr_devices=0
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Create a device *mydev*, assuming no other previously created devices, size is
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2048MiB, zone size 256MiB. There are more tunable parameters, this is a minimal
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example taking defaults:
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.. code-block:: bash
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cd /sys/kernel/config/nullb/
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mkdir mydev
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cd mydev
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echo 2048 > size
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echo 1 > zoned
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echo 1 > memory_backed
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echo 256 > zone_size
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echo 1 > power
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This will create a device */dev/nullb0* and the value of file *index* will
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match the ending number of the device node.
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Remove the device:
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.. code-block:: bash
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rmdir /sys/kernel/config/nullb/mydev
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Then continue with *mkfs.btrfs /dev/nullb0*, the zoned mode is auto-detected.
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For convenience, there's a script wrapping the basic null_blk management operations
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https://github.com/kdave/nullb.git, the above commands become:
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.. code-block:: bash
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nullb setup
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nullb create -s 2g -z 256
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mkfs.btrfs /dev/nullb0
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...
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nullb rm nullb0
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Emulated: TCMU runner
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"""""""""""""""""""""
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TCMU is a framework to emulate SCSI devices in userspace, providing various
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backends for the storage, with zoned support as well. A file-backed zoned
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device can provide more options for larger storage and zone size. Please follow
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the instructions at https://zonedstorage.io/projects/tcmu-runner/ .
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Compatibility, incompatibility
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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- the feature sets an incompat bit and requires new kernel to access the
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filesystem (for both read and write)
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- superblock needs to be handled in a special way, there are still 3 copies
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but at different offsets (0, 512GiB, 4TiB) and the 2 consecutive zones are a
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ring buffer of the superblocks, finding the latest one needs reading it from
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the write pointer or do a full scan of the zones
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- mixing zoned and non zoned devices is possible (zones are emulated) but is
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recommended only for testing
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- mixing zoned devices with different zone sizes is not possible
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- zone sizes must be power of two, zone sizes of real devices are e.g. 256MiB
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or 1GiB, larger size is expected, maximum zone size supported by btrfs is
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8GiB
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Status, stability, reporting bugs
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^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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The zoned mode has been released in 5.12 and there are still some rough edges
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and corner cases one can hit during testing. Please report bugs to
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https://github.com/naota/linux/issues/ .
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References
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^^^^^^^^^^
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- https://zonedstorage.io
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- https://zonedstorage.io/projects/libzbc/ -- *libzbc* is library and set
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of tools to directly manipulate devices with ZBC/ZAC support
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- https://zonedstorage.io/projects/libzbd/ -- *libzbd* uses the kernel
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provided zoned block device interface based on the ioctl() system calls
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- https://hddscan.com/blog/2020/hdd-wd-smr.html -- some details about exact device types
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- https://lwn.net/Articles/853308/ -- *Btrfs on zoned block devices*
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- https://www.usenix.org/conference/vault20/presentation/bjorling -- Zone
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Append: A New Way of Writing to Zoned Storage
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