xref: /dflybsd-src/sys/vfs/hammer2/TODO (revision 65cacacfb9005fe8dd541b0e830c2466c0ab8453)

* Need backend synchronization / serialization when the frontend detaches
  a XOP.  modify_tid tests won't be enough, the backend may wind up executing
  the XOP out of order after the detach.

* xop_start - only start synchronized elements

* See if we can remove hammer2_inode_repoint()

* FIXME - logical buffer associated with write-in-progress on backend
  disappears once the cluster validates, even if more backend nodes
  are in progress.

* FIXME - backend ops need per-node transactions using spmp to protect
  against flush.

* FIXME - modifying backend ops are not currently validating the cluster.
  That probably needs to be done by the frontend in hammer2_xop_start()

* modify_tid handling probably broken w/ the XOP code for the moment.

* embedded transactions in XOPs - interlock early completion

* remove current incarnation of EAGAIN

* mtx locks should not track td_locks count?.  They can be acquired by one
  thread and released by another.  Need API function for exclusive locks.

* Convert xops and hammer2_update_spans() from cluster back into chain calls

* syncthr leaves inode locks for entire sync, which is wrong.

* recovery scan vs unmount.  At the moment an unmount does its flushes,
  and if successful the freemap will be fully up-to-date, but the mount
  code doesn't know that and the last flush batch will probably match
  the PFS root mirror_tid.  If it was a large cpdup the (unnecessary)
  recovery pass at mount time can be extensive.  Add a CLEAN flag to the
  volume header to optimize out the unnecessary recovery pass.

* More complex transaction sequencing and flush merging.  Right now it is
  all serialized against flushes.

* adding new pfs - freeze and force remaster

* removing a pfs - freeze and force remaster

* bulkfree - sync between passes and enforce serialization of operation

* bulkfree - signal check, allow interrupt

* bulkfree - sub-passes when kernel memory block isn't large enough

* bulkfree - limit kernel memory allocation for bmap space

* bulkfree - must include any detached vnodes in scan so open unlinked files
	     are not ripped out from under the system.

* bulkfree - must include all volume headers in scan so they can be used
	     for recovery or automatic snapshot retrieval.

* bulkfree - snapshot duplicate sub-tree cache and tests needed to reduce
	     unnecessary re-scans.

* Currently the check code (bref.methods / crc, sha, etc) is being checked
  every single blasted time a chain is locked, even if the underlying buffer
  was previously checked for that chain.  This needs an optimization to
  (significantly) improve performance.

* flush synchronization boundary crossing check and current flush chain
  interlock needed.

* snapshot creation must allocate and separately pass a new pmp for the pfs
  degenerate 'cluster' representing the snapshot.  This theoretically will
  also allow a snapshot to be generated inside a cluster of more than one
  node.

* snapshot copy currently also copies uuids and can confuse cluster code

* hidden dir or other dirs/files/modifications made to PFS before
  additional cluster entries added.

* transaction on cluster - multiple trans structures, subtrans

* inode always contains target cluster/chain, not hardlink

* chain refs in cluster, cluster refs

* check inode shared lock ... can end up in endless loop if following
  hardlink because ip->chain is not updated in the exclusive lock cycle
  when following hardlink.

cpdup /build/boomdata/jails/bleeding-edge/usr/share/man/man4 /mnt/x3


        * The block freeing code.  At the very least a bulk scan is needed
          to implement freeing blocks.

        * Crash stability.  Right now the allocation table on-media is not
          properly synchronized with the flush.  This needs to be adjusted
          such that H2 can do an incremental scan on mount to fixup
          allocations on mount as part of its crash recovery mechanism.

        * We actually have to start checking and acting upon the CRCs being
          generated.

        * Remaining known hardlink issues need to be addressed.

        * Core 'copies' mechanism needs to be implemented to support multiple
          copies on the same media.

        * Core clustering mechanism needs to be implemented to support
          mirroring and basic multi-master operation from a single host
          (multi-host requires additional network protocols and won't
          be as easy).

* make sure we aren't using a shared lock during RB_SCAN's?

* overwrite in write_file case w/compression - if device block size changes
  the block has to be deleted and reallocated.  See hammer2_assign_physical()
  in vnops.

* freemap / clustering.  Set block size on 2MB boundary so the cluster code
  can be used for reading.

* need API layer for shared buffers (unfortunately).

* add magic number to inode header, add parent inode number too, to
  help with brute-force recovery.

* modifications past our flush point do not adjust vchain.
  need to make vchain dynamic so we can (see flush_scan2).??

* MINIOSIZE/RADIX set to 1KB for now to avoid buffer cache deadlocks
  on multiple locked inodes.  Fix so we can use LBUFSIZE!  Or,
  alternatively, allow a smaller I/O size based on the sector size
  (not optimal though).

* When making a snapshot, do not allow the snapshot to be mounted until
  the in-memory chain has been freed in order to break the shared core.

* Snapshotting a sub-directory does not snapshot any
  parent-directory-spanning hardlinks.

* Snapshot / flush-synchronization point.  remodified data that crosses
  the synchronization boundary is not currently reallocated.  see
  hammer2_chain_modify(), explicit check (requires logical buffer cache
  buffer handling).

* on fresh mount with multiple hardlinks present separate lookups will
  result in separate vnodes pointing to separate inodes pointing to a
  common chain (the hardlink target).

  When the hardlink target consolidates upward only one vp/ip will be
  adjusted.  We need code to fixup the other chains (probably put in
  inode_lock_*()) which will be pointing to an older deleted hardlink
  target.

* Filesystem must ensure that modify_tid is not too large relative to
  the iterator in the volume header, on load, or flush sequencing will
  not work properly.  We should be able to just override it, but we
  should complain if it happens.

* Kernel-side needs to clean up transaction queues and make appropriate
  callbacks.

* Userland side needs to do the same for any initiated transactions.

* Nesting problems in the flusher.

* Inefficient vfsync due to thousands of file buffers, one per-vnode.
  (need to aggregate using a device buffer?)

* Use bp->b_dep to interlock the buffer with the chain structure so the
  strategy code can calculate the crc and assert that the chain is marked
  modified (not yet flushed).

* Deleted inode not reachable via tree for volume flush but still reachable
  via fsync/inactive/reclaim.  Its tree can be destroyed at that point.

* The direct write code needs to invalidate any underlying physical buffers.
  Direct write needs to be implemented.

* Make sure a resized block (hammer2_chain_resize()) calculates a new
  hash code in the parent bref

* The freemap allocator needs to getblk/clrbuf/bdwrite any partial
  block allocations (less than 64KB) that allocate out of a new 64K
  block, to avoid causing a read-before-write I/O.

* Check flush race upward recursion setting SUBMODIFIED vs downward
  recursion checking SUBMODIFIED then locking (must clear before the
  recursion and might need additional synchronization)

* There is definitely a flush race in the hardlink implementation between
  the forwarding entries and the actual (hidden) hardlink inode.

  This will require us to associate a small hard-link-adjust structure
  with the chain whenever we create or delete hardlinks, on top of
  adjusting the hardlink inode itself.  Any actual flush to the media
  has to synchronize the correct nlinks value based on whether related
  created or deleted hardlinks were also flushed.

* When a directory entry is created and also if an indirect block is
  created and entries moved into it, the directory seek position can
  potentially become incorrect during a scan.

* When a directory entry is deleted a directory seek position depending
  on that key can cause readdir to skip entries.

* TWO PHASE COMMIT - store two data offsets in the chain, and
  hammer2_chain_delete() needs to leave the chain intact if MODIFIED2 is
  set on its buffer until the flusher gets to it?


				OPTIMIZATIONS

* If a file is unlinked buts its descriptors is left open and used, we
  should allow data blocks on-media to be reused since there is no
  topology left to point at them.