man/man9/wapbl.9

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.Dd March 26, 2015
.Dt WAPBL 9
.Os
.Sh NAME
.Nm WAPBL ,
.Nm wapbl_start ,
.Nm wapbl_stop ,
.Nm wapbl_begin ,
.Nm wapbl_end ,
.Nm wapbl_flush ,
.Nm wapbl_discard ,
.Nm wapbl_add_buf ,
.Nm wapbl_remove_buf ,
.Nm wapbl_resize_buf ,
.Nm wapbl_register_inode ,
.Nm wapbl_unregister_inode ,
.Nm wapbl_register_deallocation ,
.Nm wapbl_jlock_assert ,
.Nm wapbl_junlock_assert
.Nd write-ahead physical block logging for file systems
.Sh SYNOPSIS
.In sys/wapbl.h
.Vt typedef void (*wapbl_flush_fn_t)(struct mount *, daddr_t *, int *, int) ;
.Ft int
.Fn wapbl_start "struct wapbl **wlp" "struct mount *mp" "struct vnode *devvp" \
        "daddr_t off" "size_t count" "size_t blksize" \
        "struct wapbl_replay *wr" \
        "wapbl_flush_fn_t flushfn" "wapbl_flush_fn_t flushabortfn"
.Ft int
.Fn wapbl_stop "struct wapbl *wl" "int force"
.Ft int
.Fn wapbl_begin "struct wapbl *wl" "const char *file" "int line"
.Ft void
.Fn wapbl_end "struct wapbl *wl"
.Ft int
.Fn wapbl_flush "struct wapbl *wl" "int wait"
.Ft void
.Fn wapbl_discard "struct wapbl *wl"
.Ft void
.Fn wapbl_add_buf "struct wapbl *wl" "struct buf *bp"
.Ft void
.Fn wapbl_remove_buf "struct wapbl *wl" "struct buf *bp"
.Ft void
.Fn wapbl_resize_buf "struct wapbl *wl" "struct buf *bp" "long oldsz" \
       "long oldcnt"
.Ft void
.Fn wapbl_register_inode "struct wapbl *wl" "ino_t ino" "mode_t mode"
.Ft void
.Fn wapbl_unregister_inode "struct wapbl *wl" "ino_t ino" "mode_t mode"
.Ft void
.Fn wapbl_register_deallocation "struct wapbl *wl" "daddr_t blk" "int len"
.Ft void
.Fn wapbl_jlock_assert "struct wapbl *wl"
.Ft void
.Fn wapbl_junlock_assert "struct wapbl *wl"
.Sh DESCRIPTION
.Nm ,
or
.Em write-ahead physical block logging ,
is an abstraction for file systems to write physical blocks in the
.Xr buffercache 9
to a bounded-size log first before their real destinations on disk.
The name means:
.Bl -tag -width "physical block" -offset abcd
.It logging
batches of writes are issued atomically via a log
.It physical block
only physical blocks, not logical file system operations, are stored in
the log
.It write-ahead
before writing a block to disk, its new content, rather than its old
content for roll-back, is recorded in the log
.El
.Pp
When a file system using
.Nm
issues writes (as in
.Xr bwrite 9
or
.Xr bdwrite 9 ) ,
they are grouped in batches called
.Em transactions
in memory, which are serialized to be consistent with program order
before
.Nm
submits them to disk atomically.
.Pp
Thus, within a transaction, after one write, another write need not
wait for disk I/O, and if the system is interrupted, e.g. by a crash or
by power failure, either both writes will appear on disk, or neither
will.
.Pp
When a transaction is full, it is written to a circular buffer on
disk called the
.Em log .
When the transaction has been written to disk, every write in the
transaction is submitted to disk asynchronously.
Finally, the file system may issue new writes via
.Nm
once enough writes submitted to disk have completed.
.Pp
After interruption, such as a crash or power failure, some writes
issued by the file system may not have completed.
However, the log is written consistently with program order and before
file system writes are submitted to disk.
Hence a consistent program-order view of the file system can be
attained by resubmitting the writes that were successfully stored in
the log using
.Xr wapbl_replay 9 .
This may not be the same state just before interruption \(em writes in
transactions that did not reach the disk will be excluded.
.Pp
For a file system to use
.Nm ,
its
.Xr VFS_MOUNT 9
method should first replay any journal on disk using
.Xr wapbl_replay 9 ,
and then, if the mount is read/write, initialize
.Nm
for the mount by calling
.Fn wapbl_start .
The
.Xr VFS_UNMOUNT 9
method should call
.Fn wapbl_stop .
.Pp
Before issuing any
.Xr buffercache 9
writes, the file system must acquire a shared lock on the current
.Nm
transaction with
.Fn wapbl_begin ,
which may sleep until there is room in the transaction for new writes.
After issuing the writes, the file system must release its shared lock
on the transaction with
.Fn wapbl_end .
Either all writes issued between
.Fn wapbl_begin
and
.Fn wapbl_end
will complete, or none of them will.
.Pp
File systems may also witness an
.Em exclusive
lock on the current transaction when
.Nm
is flushing the transaction to disk, or aborting a flush, and invokes a
file system's callback.
File systems can assert that the transaction is locked with
.Fn wapbl_jlock_assert ,
or not
.Em exclusively
locked, with
.Fn wapbl_junlock_assert .
.Pp
If a file system requires multiple transactions to initialize an
inode, and needs to destroy partially initialized inodes during replay,
it can register them by
.Vt ino_t
inode number before initialization with
.Fn wapbl_register_inode
and unregister them with
.Fn wapbl_unregister_inode
once initialization is complete.
.Nm
does not actually concern itself whether the objects identified by
.Vt ino_t
values are
.Sq inodes
or
.Sq quaggas
or anything else \(em file systems may use this to list any objects
keyed by
.Vt ino_t
value in the log.
.Pp
When a file system frees resources on disk and issues writes to reflect
the fact, it cannot then reuse the resources until the writes have
reached the disk.
However, as far as the
.Xr buffercache 9
is concerned, as soon as the file system issues the writes, they will
appear to have been written.
So the file system must not attempt to reuse the resource until the
current
.Nm
transaction has been flushed to disk.
.Pp
The file system can defer freeing a resource by calling
.Fn wapbl_register_deallocation
to record the disk address of the resource and length in bytes of the
resource.
Then, when
.Nm
next flushes the transaction to disk, it will pass an array of the disk
addresses and lengths in bytes to a file-system-supplied callback.
(Again,
.Nm
does not care whether the
.Sq disk address
or
.Sq length in bytes
is actually that; it will pass along
.Vt daddr_t
and
.Vt int
values.)
.Sh FUNCTIONS
.Bl -tag -width abcd
.It Fn wapbl_start wlp mp devvp off count blksize wr flushfn flushabortfn
Start using
.Nm
for the file system mounted at
.Fa mp ,
storing a log of
.Fa count
disk sectors at disk address
.Fa off
on the block device
.Fa devvp
writing blocks in units of
.Fa blksize
bytes.
On success, stores an opaque
.Vt "struct wapbl *"
cookie in
.Li * Ns Fa wlp
for use with the other
.Nm
routines and returns zero.
On failure, returns an error number.
.Pp
If the file system had replayed the log with
.Xr wapbl_replay 9 ,
then
.Fa wr
must be the
.Vt "struct wapbl_replay *"
cookie used to replay it, and
.Fn wapbl_start
will register any inodes that were in the log as if with
.Fn wapbl_register_inode ;
otherwise
.Fa wr
must be
.Dv NULL .
.Pp
.Fa flushfn
is a callback that
.Nm
will invoke as
.Fa flushfn ( Fa mp , Fa deallocblks , Fa dealloclens , Fa dealloccnt )
just before it flushes a transaction to disk, with the an exclusive
lock held on the transaction, where
.Fa mp
is the mount point passed to
.Fn wapbl_start ,
.Fa deallocblks
is an array of
.Fa dealloccnt
disk addresses, and
.Fa dealloclens
is an array of
.Fa dealloccnt
lengths, corresponding to the addresses and lengths the file system
passed to
.Fn wapbl_register_deallocation .
If flushing the transaction to disk fails,
.Nm
will call
.Fa flushabortfn
with the same arguments to undo any effects that
.Fa flushfn
had.
.It Fn wapbl_stop wl force
Flush the current transaction to disk and stop using
.Nm .
If flushing the transaction fails and
.Fa force
is zero,
return error.
If flushing the transaction fails and
.Fa force
is nonzero, discard the transaction, permanently losing any writes in
it.
If flushing the transaction is successful or if
.Fa force
is nonzero,
free memory associated with
.Fa wl
and return zero.
.It Fn wapbl_begin wl file line
Wait for space in the current transaction for new writes, flushing it
if necessary, and acquire a shared lock on it.
.Pp
The lock is not exclusive: other threads may acquire shared locks on
the transaction too.
The lock is not recursive: a thread may not acquire it again without
calling
.Fa wapbl_end
first.
.Pp
May sleep.
.Pp
.Fa file
and
.Fa line
are the file name and line number of the caller for debugging
purposes.
.It Fn wapbl_end wl
Release a shared lock on the transaction acquired with
.Fn wapbl_begin .
.It Fn wapbl_flush wl wait
Flush the current transaction to disk.
If
.Fa wait
is nonzero, wait for all writes in the current transaction to
complete.
.Pp
The current transaction must not be locked.
.It Fn wapbl_discard wl
Discard the current transaction, permanently losing any writes in it.
.Pp
The current transaction must not be locked.
.It Fn wapbl_add_buf wl bp
Add the buffer
.Fa bp
to the current transaction, which must be locked, because someone has
asked to write it.
.Pp
This is meant to be called from within
.Xr buffercache 9 ,
not by file systems directly.
.It Fn wapbl_remove_buf wl bp
Remove the buffer
.Fa bp ,
which must have been added using
.Fa wapbl_add_buf ,
from the current transaction, which must be locked, because it has been
invalidated (or XXX ???).
.Pp
This is meant to be called from within
.Xr buffercache 9 ,
not by file systems directly.
.It Fn wapbl_resize_buf wl bp oldsz oldcnt
Note that the buffer
.Fa bp ,
which must have been added using
.Fa wapbl_add_buf ,
has changed size, where
.Fa oldsz
is the previous allocated size in bytes and
.Fa oldcnt
is the previous number of valid bytes in
.Fa bp .
.Pp
This is meant to be called from within
.Xr buffercache 9 ,
not by file systems directly.
.It Fn wapbl_register_inode wl ino mode
Register
.Fa ino
with the mode
.Fa mode
as commencing initialization.
.It Fn wapbl_unregister_inode wl ino mode
Unregister
.Fa ino ,
which must have previously been registered with
.Fa wapbl_register_inode
using the same
.Fa mode ,
now that its initialization has completed.
.It Fn wapbl_register_deallocation wl blk len
Register
.Fa len
bytes at the disk address
.Fa blk
as ready for deallocation, so that they will be passed to the
.Fa flushfn
that was given to
.Fn wapbl_start .
.It Fn wapbl_jlock_assert wl
Assert that the current transaction is locked.
.Pp
Note that it might not be locked by the current thread: this assertion
passes if
.Em any
thread has it locked.
.It Fn wapbl_junlock_assert wl
Assert that the current transaction is not exclusively locked by the
current thread.
.Pp
Users of
.Nm
observe exclusive locks only in the
.Fa flushfn
and
.Fa flushabortfn
callbacks to
.Fn wapbl_start .
Outside of such contexts, the transaction is never exclusively locked,
even between
.Fn wapbl_begin
and
.Fn wapbl_end .
.Pp
There is no way to assert that the current transaction is not locked at
all \(em i.e., that the caller may acquire a shared lock on the
transaction with
.Fn wapbl_begin
without danger of deadlock.
.El
.Sh CODE REFERENCES
The
.Nm
subsystem is implemented in
.Pa sys/kern/vfs_wapbl.c ,
with hooks in
.Pa sys/kern/vfs_bio.c .
.Sh SEE ALSO
.Xr buffercache 9 ,
.Xr vfsops 9 ,
.Xr wapbl_replay 9
.Sh BUGS
.Nm
works only for file system metadata managed via the
.Xr buffercache 9 ,
and provides no way to log writes via the page cache, as in
.Xr VOP_GETPAGES 9 ,
.Xr VOP_PUTPAGES 9 ,
and
.Xr ubc_uiomove 9 ,
which is normally used for file data.
.Pp
Not only is
.Nm
unable to log writes via the page cache, it is also unable to defer
.Xr buffercache 9
writes until cached pages have been written.
This manifests as the well-known garbage-data-appended-after-crash bug
in FFS: when appending to a file, the pages containing new data may not
reach the disk before the inode update reporting its new size.
After a crash, the inode update will be on disk, but the new data will
not be \(em instead, whatever garbage data in the free space will
appear to have been appended to the file.
.Nm
exacerbates the problem by increasing the throughput of metadata
writes, because it can issue many metadata writes asynchronously that
FFS without
.Nm
would need to issue synchronously in order for
.Xr fsck 8
to work.
.Pp
The criteria for when the transaction must be flushed to disk before
.Fn wapbl_begin
returns are heuristic, i.e. wrong.
There is no way for a file system to communicate to
.Fn wapbl_begin
how many buffers, inodes, and deallocations it will issue via
.Nm
in the transaction.
.Pp
.Nm
mainly supports write-ahead, and has only limited support for rolling
back operations, in the form of
.Fn wapbl_register_inode
and
.Fn wapbl_unregister_inode .
Consequently, for example, large writes appending to a file, which
requires multiple disk block allocations and an inode update, must
occur in a single transaction \(em there is no way to roll back the
disk block allocations if the write fails in the middle, e.g. because
of a fault in the middle of the user buffer.
.Pp
.Fn wapbl_jlock_assert
does not guarantee that the current thread has the current transaction
locked.
.Fn wapbl_junlock_assert
does not guarantee that the current thread does not have the current
transaction locked at all.
.Pp
There is only one
.Nm
transaction for each file system at any given time, and only one
.Nm
log on disk.
Consequently, all writes are serialized.
Extending
.Nm
to support multiple logs per file system, partitioned according to an
appropriate scheme, is left as an exercise for the reader.
.Pp
There is no reason for
.Nm
to require its own hooks in
.Xr buffercache 9 .
.Pp
The on-disk format used by
.Nm
is undocumented.