# Changelog

## v1.37.4 - Sun Mar 29 2026

New commands:

- `bcachefs data-read`: O_DIRECT read via BCHFS_IOC_PREAD_RAW with extended
  error reporting (checksum, IO, decompression, and EC errors). Supports
  `--no-poison-check` for reading poisoned extents.
- `bcachefs unpoison`: Clear poison flags on file extents.

Bug fixes:

- Fix shell completion generation panic
- Fix group subcommand dispatch off-by-one

Tools improvements:

- Migrated to clap derive for subcommand dispatch with typed Cli structs
- Enabled clap suggestions and color help, stripped debuginfo from release builds

Kernel source updates:

- Fix segfault in bch2_stripe_new_buckets_del()
- Fix reconcile checksum rewrite skipping cached pointers
- Fix use-after-free in ec_block_endio()
- Fix cached pointer handling in data update
- Fix init_new_stripe_from_old() copying parity blocks
- Fix torn write of path->l[0].b in btree_path_copy()
- Fix linking error on i586
- BCH_SB_MEMBER_INVALID pointers don't count as written or unwritten
- Don't add cached pointer devices to devs_have
- Don't reuse stripes when live data would overflow into parity
- Detect and repair non-zero parity blockcounts
- Plumb EC reconstruct messages to read path
- Add BCHFS_IOC_PREAD_RAW and BCHFS_IOC_UNPOISON ioctls
- Read path error reporting infrastructure
- Don't start reconcile unless we're really going rw
- Add timestats for btree node/key cache shrinkers
- Improve bch2_bio_to_text(), include bio on BLK_STS_INVAL read errors
- Improve error message when autofix blocked by errors policy
- Automatically advance rewind_seq when journal_rewind_discard_buffer_percent=0

Package CI:

- Publish to release suite for tagged commits
- Atomic publish via staging directory + rsync (fixes apt hash mismatch, #543)

## v1.37.3 - Fri Mar 20 2026

New option: opts.journal_rewind_discard_buffer_percent

This allows the size of the discard buffer for journal rewind to be adjusted -
tiering setups with significantly mismatched device sizes will want to turn this
down, or off.

- Ensure we don't accidentally create cached erasure coded pointers, which
  aren't supported yet
- Fix buffer overflows when padding extents with `BCH_SB_MEBER_INVALID` pointers
- Fix a spurious -EAGAIN in the write path
- Fix a few bugs on 32bit x86
- Fix ppc64le build failures

## v1.37.2 - Mon Mar 16 2026

Bugfix release - fix an oops in mount from incorrect zeroing of
bch_btree_ptr_v2.mem_ptr, and a stripe repair assert.

## v1.37.1 - Sun Mar 15 2026

Bugfix release - fix compatibility issues with bch_sb_field_ext options.

## v1.37.0 - Sun Mar 15 2026

bcachefs_metadata_version_erasure_coding

Highlights:

- Erasure coding is no longer experimental; all the core functionality is
  complete.

- Major update to the Principles of Operation - abbreviated PoO, or simply poo;
  instead of "RTFM", you may now say "Have you checked your poo?".

  It's now at 100 pages, organized into introductory, feature overview and
  subsystem reference sections, and should be thoroughly comprehensive.

- New subcommands (subvolume list, list-snapshots, reflink-option-propagate)

- Journal rewind is now fully safe to use (the filesystem tracks how far back we
  can safely rewind)

- Automatic recovery from devices with bad flush/fua support

- Faster recovery from unclean shutdowns

- Better perforance on multidevice filesystems: saner defaults for buffered
  readahead, controllable by the new `dev_readahead` option.

- Linux 7.0 support

### Erasure coding

- Erasure coding is now hooked up to reconcile: degraded stripes are now
  automatically repaired, like other degraded data, and can be reshaped as
  needed. Tiering setups, and setups with mixed device sizes should work -
  erasure coding will create the biggest stripes possible.

- Erasure coding is no longer hidden behind `CONFIG_BCACHEFS_ERASURE_CODING`,
  but one significant item is still remaining - stripe allocation needs to
  allocate blocks on different devices at similar LBAs, to avoid seeking when
  resilvering an array. This should land in 1.38.

### Subcommands

- **`subvolume list`** (`bcachefs subvolume ls`): List subvolumes with
  filtering and sorting. Uses userspace ioctl helpers for batch queries.

- **`subvolume list-snapshots`** (`bcachefs subvolume ls-snap`): List
  snapshots as a tree with disk usage information.

- **`reflink-option-propagate`**: Propagates a file's IO options
  (compression, checksum, replicas, targets) to its extents, including
  reflinked extents. Respects a new per-pointer permission flag
  (`MAY_UPDATE_OPTIONS`) to prevent unprivileged users from altering
  shared data they don't own.

- **`fs top` TUI mode**: `fs top` and `reconcile wait` now use the
  alternate screen for a proper terminal UI experience; fs top also shows
  per-device stats.

- **Elastic tabstops**: Tabular output (fs usage, show-super, etc.)
  now uses elastic tabstop alignment for cleaner, consistently aligned
  columns.

### Journal rewind, automatic recovery from bad flush/fua:

- We now buffer discards, up to a small percentage of the device size, and track
  in the journal how far back we can safely rewind (i.e. which old buckets have
  not been discarded yet). Rewind is also now transactionally consistent - if we
  crash mid rewind, we remember the previous in-progress rewind.

- The new `scrub_recent_journal_entries`, enabled by default after unclean
  shutdowns, runs a targeted scrub during recovery on the data that was written
  and committed just before crash or shutdown. On checksum error, indicating the
  data wasn't actually written, an immediate repair will be queued up (on
  replicated filesystems) - or if the data is not recoverable we'll automatically
  rewind to the last good state. By default, we won't rewind more than 10
  seconds, controlled by the `scrub_journal_max_rewind_secs` option.

### Bug fixes

- Fix stdout buffering when piped (output now flushed properly)
- Fix utilization percentage in `fs usage` to use bucket counts
- Fix `copy_fs` write truncation
- Fix `readlink` c_char portability for arm64/ppc64el
- Fix `format` to create `sb_field_ext` before setting options
- Fix docgen command ordering
- Fix `escape_latex` mangling `--` flags as en-dashes
- Device evacuate: check filesystem version before starting

### Build system

- Package CI: cached build environments, cross-compilation fixes,
  2-hour build timeout, architecture documentation
- Exclude `debian/` from C source discovery in Makefile
- Remove GitHub Actions build workflow (migrated to package-ci)

### Rust conversion progress

The userspace component of bcachefs has now been converted to Rust. Among other
things, this means we finally have bash autocompletions available, courtesy of
Clap.

Cleanup work is still ongoing - unsafe reduction, "Rusty" APIs to replace C
style ones. This is the test and staging ground for conversion of the kernel
side code to Rust, which will start happening as soon as Rust support is
sufficiently widespread in distro kernels.

This also enables formal verification, in Verus - work here has already started,
with proofs for eytzinger tree operations (search, inorder traversal, roundtrip
bijection), snapshot skiplist construction, snapshot tree invariants, and extent
overwrite conservation. 124+ verified properties.

## v1.36.1 - Fri Feb  6 2026

### New `bcachefs fs timestats` command

Interactive TUI for monitoring various filesystem internals, slowpaths and
device performance, with duration and frequency tracking for various events.
Helpful for diagnosing performance issues.

- `encoded_extent_max` default bumped to 256k; new filesystems now initialize
  `BCH_SB_EXTENT_BP_SHIFT` to 16, so higher settings won't require rebuilding
  backpointers

- `--rotational` flag now works correctly during format

- Copygc now waits until a device is less than 20% free before starting

- Improved `bcachefs reconcile status` output

- Large batch of erasure coding cleanup and hardening: better error reporting
  for EC reconstruct reads, fix a race between EC and data moves, and various
  other EC bugfixes

- Fix write buffer `move_keys_from_inc_to_flushing()` regression, which was
  causing occasional oopses under load for some users

- Snapshot deletion is now much faster when deleting large numbers of snapshos;
  we now use an eytzinger tree for the list of nodes being deleted

- Fix sporadic superblock checksum failures during device scan

And many smaller bugfixes.

## v1.36.0 - Sat Jan 31 2026

bcachefs_metadata_version_no_sb_user_data_replicas

This requires an incompatible upgrade to enable, and once enabled we'll no
longer store replicas entries in the superblock for user data, which are used
for deciding whether we an do a degraded mount without data loss - instead, we
defer that and use the accounting btree to check, in early recovery.

This is a performance/scalability fix: on filesystems with large numbers of
drives (a 50 device filesystem was the original bug report), the superblock
writes needed to add and delete replicas entries become a bottleneck.

Replicas entries for metadata (btree and journal) can still be an issue, and
another bug report indicated that these will have to be addressed soon - a
single slow (or dying) device in a large multidevice will cause all superblock
writes to slow to a degree that can cause major problems. Metadata replicas
entries will however require a different approach to solve, so expect that in a
future update.

- Some fairly involved fixes for the data update path: it turns out, the data
  update path was dropping replicas to devices being evacuated (which are
  considered to have durability of 0) before the extent was sufficiently
  replicated on other devices. This caused data loss for a few users,
  unfortunately, but the new code is much more rigorous when reconciling the
  exsiting extent with newly written replicas and deciding which replicas to
  keep and which can be dropped.

- Fix various codepaths that were (incorrectly) causing the filesystem to go
  emergency read-only when finding a pointer to an invalid device, instead of
  continuing so it could be repaired or flagging the filesystem as needing
  repair. We now should only go emergency read-only on pointer to invalid device
  when that would indicate a runtime bug, not filesystem corruption.

- Reconcile will now shut down correctly (when the filesystem is going read-only
  or unmounting) when processing the reconcile_*_phys btrees.

- Multiple other smaller reconcile fixes; various users report that issues where
  reconcile did not seem to be finding pending work seem to be resolved.

- Degraded btree nodes are no longer un-degraded synchronously; now that we have
  reconcile this is no longer necessary, and forcing them to be un-degraded
  synchronously was prone to causing deadlocks on open_bucket allocation.

- The 'allocator stuck' log message now provides improved information, and
  internally has been re-plumbed to have access to the original 'struct
  alloc_request', so if necessary for future debugging we can easily provide as
  much information about how the allocation was attempted as required.

## v1.35.2 - Tue Jan 20 2026

- Linux v6.19 is now supported

- Reconcile now considers the amount of durability we have available among
  online devices when dropping extra replicas (because the replicas setting was
  changed), and won't let the online durability go below the replicas setting.

- Fix a race in the nocow write path when checking if we need to fall back to a
  normal COW write

- Fix a livelock when walking btree roots in reconcile and elsewhere

- Journal discards are now done asynchronously instead of being done by the
  journal reclaim thread, and we try to keep more of the journal discarded to
  avoid journal writes having to block and do discards synchronously

- Fix several bugs with copygc <-> reconcile interaction, and copygc should no
  longer spin when a device is completely full with no fragmented buckets for it
  to evacuate.

- Fix propagating the incompressible bit in the data update path: sometimes this
  would be lost, leading to spurious "extent with bad/missing reconcile options"
  errors.

## v1.35.1 - Fri Jan 16 2026

- Self healing for the new stripe refcount field in `bch_alloc_v4`

  This fixes issues upgrading to 1.35 with (still experimental) erasure coding
  feature.

- Major allocator refactoring, simplifying the central control flow. Prep work
  for failure domains.

- Erasure coding can now delete stripes from triggers; this gives better
  behaviour when data being deleted with no other activity to cause stripes to
  be deleted.

- Fix a deadlock in device add when allocating journal on the new device; this
  fixes a regression from the watermark cleanup.

- Fedora builds are working again

## v1.35.0 - Mon Jan 12 2026

bcachefs_metadata_version_bucket_stripe_index

- The requirement that devices must have matched bucket sizes to be members of
  the same stripes has been removed.

- Stripes may be reshaped (number of blocks increased or decreased), as needed;
  this improves EC's handling of device failures.

- Significantly improved evacuate, rereplicate performance on rotating disks: we
  now launch one thread per device being read from (i.e. every device that
  shared data with the device going away); each device is read from in parallel
  with reads across the whole device done in sorted order.

- `backpointer_scan_iter`, for improved performance for code doing backpointer
  -> extent walks, including but not limited to reconcile; this is quite
  significant on systems with metadata on rotating disk and relatively limited
  memory.

- The bug with reconcile where btree roots wouldn't be processed has been fixed.

- A few bugs with reconcile's handling of cached data have been fixed.

- The reconcile tracepoints, especially `reconcile_set_pending`, now give
  significantly more information.

- Reconcile now knows how to wait on copygc when a device it wants to write to
  is full, rather than (incorrectly) marking the extent as pending.

- Fixed several memory reclaim recursion bugs; performance under memory pressure
  should be improved.

- Various allocation watermark fixes; btree updates now only run with high
  priority watermarks when necessary. This fixes some allocator deadlocks on
  open bucket allocation.

- 'encoded_extent_max` settings of 1MB and greater now work properly;
  previously, this could cause backpointer issues if compression was enabled.

Along with numerous other bugfixes.

## v1.34.0 - Sat Dec 27 2025

bcachefs_metadata_version_extended_key_type_error

- `KEY_TYPE_error` keys new include a field that indicates the reason and
  codepath they were created

- We now run `check_snapshots` before deleting interior snapshot nodes, after
  observing a bug where bad skiplist entries were created due to prior
  corruption of the snapshot depth field.

- The compression code now always bounces the source buffer if it may have been
  mapped to userspace; this should solve reports of corruption with zstd

- `str_hash` (dirents and xattrs) repair now handles keys in different snapshots
  correctly

## v1.33.4 - Thu Dec 25 2025

- Fix a critical bug with interior snapshot node deletion:

  Interior snapshot nodes can't be fully deleted at runtime while the filesystem
  is in use, since snapshot tree fixups can require adjustments to arbitrarily
  many nodes and can't be done atomically, so we defer them until the next mount
  (all the heavy lifting of deleting/moving keys that refer to those snapshot
  nodes is done at runtime).

  But, incorrectly, we were doing interior snapshot node deletion before going
  RW: before going RW, transaction commits use a different path that queues up
  updates to the list of updates for journal replay - and this path doesn't run
  in-memory triggers, but snapshots use an in-memory trigger for keeping the
  in-memory snapshots table in sync with the snapshots btree - this broke
  `snapshot_is_ancestor()`

  Affected users would see filesystem corruption that disappeared on the next
  remount.

  This is fixed by now doing interior snapshots deletion just after going RW,
  but before starting processes that require snapshots lookups.

- New mode for verifying the result of data compression, before writing
  compressed data out to disk.

  There's been sporadic reports of corruption when zstd is in use; to track this
  down, there's a new `verify_compress` module parameter. When enabled, we
  decompress data immediately after compressing and verify the result with
  memcmp(). On mismatch, we mark the extent as incompressible and print an error
  with the file, offset and length; this will let us find the exact data that
  caused the error and do further testing.

- Reconcile no longer runs when the filesystem is mounted read only.

  When a filesystem is mounted read only, we will still go read-write internally
  if we need to fsck or do journal replay. There are two main background tasks
  we start when going read-write for background data processing: copygc and
  reconcile. Copygc is required to run when we're read-write for the allocator
  to be guaranteed to make forward progress, but reconcile is not.

- We no longer include durability=0 devices when calculating filesystem
  capacity.

## v1.33.3 - Mon Dec 22 2025

- More snapshot deletion fixes, old interior snapshot nodes should finally be
  getting cleaned up correctly

- We now run `check_snapshots` on every mount; there have been some bugs which
  result in snapshot tree corruption in the depth/skiplist fields, breaking
  `snapshot_is_ancestor()`. We can't efficiently detect this kind of corruption
  at runtime, but `check_snapshots` is no more expensive than `read_snapshots`;
  if we still have bugs in snapshot deletion, this will render them harmless.

- Some obscure repair paths are now more robust - str_hash mismatch repair,
  inode reconstruction.

- Btree node rewrites no longer run at `BCH_WATERMARK_btree` by default; this
  should solve some deadlocks that started happening when reconcile started
  moving around a lot more btree nodes.

- When we get a ZSTD decompression error, the specific error code from zstd will
  now be reported in the error message.

## v1.33.2 - Wed Dec 17 2025

(Almost) bugfixes only:

- Fix multiple bugs involving deleting interior snapshot nodes

- Fix an assertion pop caused by leftover rebalance scan cookies, from
  pre-1.33.0

- Fix mmap-involved page cache inconsistency/corruption, users generally noticed
  this as files that seemed to be corrupted by the cp afterwards

- Fix a topology inconsistency caused by a transaction commit merging a node we
  were updating a key for in the same transaction; we now have stricter topology
  checks

- Online fsck now understands `-o recovery_passes`

- Copygc (and elsewhere) now correctly uses the 'fragemented' counter under
  `dev_data_type` accounting; intricacies of compressed data accounting mean
  that `buckets * bucket_size - sectors` does not work for this, and may
  underflow.

- New recovery pass: `kill_i_generation_keys`. Modern filesystems do not use
  `KEY_TYPE_i_generation` for implementing NFS inode generation numbers, and old
  filesystems may have significant amounts of wasted space in the inodes btree
  from these. Must be run manually, and can be run online.

- Subvolumes and snaapshot trees are now viewable in debugfs, along with the
  per-snapshot accounting. These should be considered prototype interfaces, to
  give users something to look at and comment on before the real interfaces are
  designed.

- Snapshot accounting is no longer kept in-memory; this fixes slow
  `accouting_read` on filesystems with huge numbers of snapshots.

## v1.33.1 - Thu Dec 11 2025

### Recovery passes will now be run in the background when possible

When a scheduled recovery pass and all scheduled passes that depend on it can be
run online, we'll now run it in the background instead of blocking mount.

This means that upgrades to 1.33 from previous versions will now happen in the
background.

### Bugfixes:

- We now avoid blocking on memory reclaim when allocating btree node buffers; it
  was discovered that under memory pressure it can take > 10 seconds to satisfiy
  a single allocation due to compaction. We'll now fall back to vmalloc much
  quicker.

  This should help with the SRCU lock hold time warnings that have still been
  popping up.

  There's a new btree node cache statistic to track the number of vmalloc
  allocations; if we notice that this is now too high we may want to add a
  background task to allocate physically contiguous buffers to replace the
  vmalloc allocations (vmalloc memory is a bit slower than physically contiguous
  memory).

- Fix a "pending incorrectly set" ERO

- Fix checking for device rebalance scan cookies, this will eliminate some
  spurious "extent with incorrect/missing reconcile opts" errors.

- Snapshot deletion fixes; when multiple leaves were being deleted
  simultaneously and interior nodes needed to be deleted too, the interior nodes
  often wouldn't get cleaned up - and in rare situations keys could get moved to
  the incorrect snapshot node, due to a DFS iteration bug.

## v1.33.0 - Thu Dec  4 2025

`bcachefs_metadata_version_reconcile` (formerly known as rebalance_v2)

### Reconcile

An incompatible upgrade is required to enable reconcile.

Reconcile now handles all IO path options; previously only the background target
and background compression options were handled.

Reconcile can now process metadata (moving it to the correct target,
rereplicating degraded metadata); previously rebalance was only able to handle
user data.

Reconcile now automatically reacts to option changes and device setting
changes, and immediately rereplicates degraded data or metadata

This obsoletes the commands `data rereplicate`, `data job
drop_extra_replicas`, and others; the new commands are `reconcile status` and
`reconcile wait`.

The recovery pass `check_reconcile_work` now checks that data matches the
specified IO path options, and flags an error if it does not (if it wasn't due
to an option change that hasn't yet been propagated).

Additional improvements over rebalance and implementation notes:

We now have a separate index for data that's scheduled to be processed by
reconcile but can't (e.g. because the specified target is full),
`BTREE_ID_reconcile_pending`; this solves long standing reports of rebalance
spinning when a filesystem has more data than fits on the specified background
target.

This also means you can create a single device filesystem with replicas=2, and
upon adding a new device data will automatically be replicated on the new
device, no additional user intervention required.

There's a separate index for "high priority" reconcile processing -
`BTREE_ID_reconcile_hipri`. This is used for degraded extents that need to be
rereplicated; they'll be processed ahead of other work.

Rotating disks get special handling. We now track whether a disk is rotational
(a hard drive, instead of an SSD); pending work on those disks is additionally
indexed in the `BTREE_ID_reconcile_work_phys` and
`BTREE_ID_reconcile_hipri_phys` btrees so they can be processed in physical
LBA order, not logical key order, avoiding unnecessary seeks.

We don't yet have the ability to change the rotational setting on an existing
device, once it's been set; if you discover you need this, please let us know so
it can be bumped up on the list (it'll be a medium sized project).

`BCH_MEMBER_STATE_failed` has been renamed to `BCH_MEMBER_STATE_evacuating`;
as the name implies, reconcile automatically moves data off of devices in the
evacuating state. In the future, when we have better tracking and monitoring
of drive health, we'll be able to automatically mark failing devices as
evacuating: when this lands, you'll be able to load up a server with disks and
walk away - come back a year later to swap out the ones that have been failed.

Reconcile was a massive project: the short and simple user interface is
deceptive, there was an enormous amount of work under the hood to make
everything work consistently and handle all the special cases we've learned
about over the past few years with rebalance.

There's still reconcile-related work to be done on disk space accounting when
devices are read-only or evacuating, and in the future we want to reserve space
up front on option change, so that we can alert the user if they might be doing
something they don't have disk space for.

### Other improvements and changes:

- Degraded data is now always properly reported as degraded (by `bcachefs fs
  usage`); data is considered degraded any time the durability on good
  (non-evacuating devices) is less than the specified replication level.

- Counters (shown by `bcachefs fs top` and tracepoints have gotten a giant
  cleanup and rework: every counter has a corresponding tracepoint. This makes
  it easy to drill down and investigate when a filesystem is doing something
  unusual and unexpected.

  Under the hood, the conversion of tracepoints to printbufs/pretty printers has
  now been completed, with some much improved helpers. This makes it much easier
  to add new counters and tracepoints or add additional info to existing
  tracepoints, typically a 5-20 line patch. If there's something you're
  investigating and you need more info, just ask.

  We now make use of type information on counters to display data rates in
  `bcachefs fs top` where applicable, and many counters have been converted to
  data rates. This makes it much easier to correlate different counters (e.g.
  `data_update`, `data_update_fail`) to check if the rates of slowpath events
  should be a cause for concern.

- Logging/error message improvements

  Logging has been a major area of focus, with a lot of under the hood
  improvements to make it ergonomic to generate messages that clearly explain
  what the system is doing an why: error messages should not include just the
  error, but how it was handled (soft error or hard error) and all actions taken
  to correct the error (e.g. scheduling self healing or recovery passes).

  When we receive an IO error from the block layer we now report the specific
  error code we received (e.g. `BLK_STS_IOERR`, `BLK_STS_INVAL`).

  The various write paths (user data, btree, journal) now report one error
  message for the entire operation that includes all the sub-errors for the
  individual replicated writes and the status of the overall operation (soft
  error (wrote degraded data) vs. hard error), like the read paths.

  On failure to mount due to insufficient devices, we now report which device(s)
  were missing; we remember the device name and model in the superblock from the
  last time we saw it so that we can give helpful hints to the user about what's
  missing.

  When btree topology repair recovers via btree node scan, we now report which
  node(s) it was able to recover via scan; this helps with determining if data
  was actually lost or not.

  We now ratelimit soft and hard errors separately, in the data/journal/btree
  read and write paths, ensuring that if the system is being flooded with soft
  errors the hard errors will still be reported.

  All error ratelimiting now obeys the `no_ratelimit_errors` option.

  All recovery passes should now have progress indicators.

- New options:

  `mount_trusts_udev`: there have been reports of mounting by UUID failing due
  to known bugs in libblkid. Previously this was available as an environment
  variable, but it now may be specified as a mount option (where it should also
  be much easier to find). When specified, we only use udev for getting the list
  of the system's block devices; we do all the probing for filesystem members
  ourself.

  `writeback_timeout`: if set, this overrides the `vm.dirty_writeback*` sysctls
  for the given filesystem, and may be set persistently. Useful for setting a
  lower writeback timeout for removeable media.

- Other smaller user-visible improvements

  The `mi_btree_bitmap` field in the member info section of the superblock now
  has a recovery pass to clean it up and shrink it; it will be automatically
  scheduled when we notice that there is significantly more space on a device
  marked as containing metadata than we have metadata on that device.

  The member-info btree bitmap is used by btree node scan, for disaster recovery
  repair; shrinking the bitmap reduces the amount of the device that has to be
  scanned if we have to recover from btree nodes that have become unreadable or
  lost despite replication. You don't ever want to need it, but if you do need
  it it's there.

- Promotes are now ratelimited; this resolves an issue with spinning up far too
  many kworker threads for promotes that wouldn't happen due to the target being
  busy.

- An issue was spotted on a user filesystem where btree node merging wasn't
  happening properly on the `reconcile_work` btree, causing a very slow upgrade.
  Btree node merging has now seen some improvements; btree lookups can now kick
  off asynchronous btree node merges when they spot an empty btree node, and the
  btree write buffer now does btree merging asynchronously, which should be a
  noticeable improvement on system performance under heavy load for some users -
  btree write buffer flushing is single threaded and can be a bottleneck.

  There's also a new recovery pass, `merge_btree_nodes`, to check all btrees for
  nodes that can be merged. It's not run automatically, but can be run if
  desired by passing the `recovery_passes` option to an online fsck.

- And many other bug fixes.

### Notable under-the-hood codebase work:

A lot of codebase modernization has been happening over the past six months,
to prepare for Rust. With the latest features recently available in C and in
the kernel, we can now do incremental refactorings to bring code steadily more
in line with what the Rust version will be, so that the future conversion will
be mostly syntactic - and not a rewrite. The big enabler here was CLASS(),
which is the kernel's version of pseudo-RAII based on `__cleanup()`; this
allows for the removal of goto based error handling (Rust notably does not
have goto).

We're now down to ~600 gotos in the entire codebase, down from ~2500 when the
modernization started, with many files being complete.

Other work includes avoiding open coded vectors; bcachefs uses DARRAY(), which
is decently close to Rust/C++ vectors, and the try() macro for forwarding
errors, stolen from Rust. These cleanups have deleted thousands of lines from
the codebase over the past months.
