BUFFERCACHE(9) OpenBSD Kernel Manual BUFFERCACHE(9)NAME
buffercache, bread, breadn, bwrite, bawrite, bdwrite, getblk, geteblk,
incore, allocbuf, brelse, biodone, biowait - buffer cache interfaces
SYNOPSIS
#include <sys/buf.h>
int
bread(struct vnode *vp, daddr64_t blkno, int size, struct ucred
*cred, struct buf **bpp);
int
breadn(struct vnode *vp, daddr64_t blkno, int size, daddr64_t
rablks[], int rasizes[], int nrablks, struct ucred *cred, struct buf
**bpp);
int
bwrite(struct buf *bp);
void
bawrite(struct buf *bp);
void
bdwrite(struct buf *bp);
struct buf *
getblk(struct vnode *vp, daddr64_t blkno, int size, int slpflag, int
slptimeo);
struct buf *
geteblk(int size);
struct buf *
incore(struct vnode *vp, daddr64_t blkno);
void
allocbuf(struct buf *bp, int size);
void
brelse(struct buf *bp);
void
biodone(struct buf *bp);
int
biowait(struct buf *bp);
DESCRIPTION
The buffercache interface is used by each filesystem to improve I/O
performance using in-core caches of filesystem blocks.
The kernel memory used to cache a block is called a buffer and described
by a buf structure. In addition to describing a cached block, a buf
structure is also used to describe an I/O request as a part of the disk
driver interface.
FUNCTIONS
bread(vp, blkno, size, cred, bpp)
Read a block corresponding to vp and blkno. The buffer is
returned via bpp.
If the buffer is not found (i.e. the block is not cached in
memory), bread() calls getblk() to allocate a buffer with enough
pages for size and reads the specified disk block into it.
The buffer returned by bread() is marked as busy. (The B_BUSY
flag is set.) After manipulation of the buffer returned from
bread(), the caller should unbusy it so that another thread can
get it. If the buffer contents are modified and should be
written back to disk, it should be unbusied using one of the
variants of bwrite(). Otherwise, it should be unbusied using
brelse().
breadn(vp, blkno, size, rablks, rasizes, nrablks, cred, bpp)
Get a buffer as bread(). In addition, breadn() will start read-
ahead of blocks specified by rablks, rasizes, and nrablks. The
read-ahead blocks aren't returned, but are available in cache
for future accesses.
bwrite(bp)
Write a block. Start I/O for write using VOP_STRATEGY(). Then,
unless the B_ASYNC flag is set in bp, bwrite() waits for the I/O
to complete.
bawrite(bp)
Write a block asynchronously. Set the B_ASYNC flag in bp and
simply call VOP_BWRITE(), which results in bwrite() for most
filesystems.
bdwrite(bp)
Delayed write. Unlike bawrite(), bdwrite() won't start any I/O.
It only marks the buffer as dirty (B_DELWRI) and unbusies it.
This routine should be used when the buffer is expected to be
modified again soon, typically a small write that partially
fills a buffer.
getblk(vp, blkno, size, slpflag, slptimeo)
Get a block of requested size size that is associated with a
given vnode and block offset, specified by vp and blkno. If it
is found in the block cache, mark it as having been found, make
it busy and return it. Otherwise, return an empty block of the
correct size. It is up to the caller to ensure that the cached
blocks are of the correct size.
If getblk() needs to sleep, slpflag and slptimeo are used as
arguments for tsleep().
geteblk(size)
Allocate an empty, disassociated block of a given size size.
incore(vp, blkno)
Determine if a block associated with a given vnode and block
offset is in the cache. If it is there, return a pointer to it.
Note that incore() doesn't mark the buffer as busy unlike
getblk().
allocbuf(bp, size)
Expand or contract the actual memory allocated to a buffer. If
the buffer shrinks, the truncated part of the data is lost, so
it is up to the caller to have written it out first if needed;
this routine will not start a write. If the buffer grows, it is
the caller's responsibility to fill out the buffer's additional
contents.
brelse(bp)
Unlock a buffer by clearing the B_AGE, B_ASYNC, B_BUSY,
B_NOCACHE, and B_DEFERRED flags and release it to the free
lists.
biodone(bp)
Mark I/O complete on a buffer. If a callback has been requested
by B_CALL, do so. Otherwise, wake up the waiting processes.
biowait(bp)
Wait for operations on the buffer to complete. When they do,
extract and return the I/O's error value. If the operation on
the buffer is being done via a direct call to a strategy() type
function, then the buffer must be previously initialized with
the B_RAW flag.
CODE REFERENCES
This section describes places within the OpenBSD source tree where actual
code implementing the buffer cache subsystem can be found. All pathnames
are relative to /usr/src.
The buffer cache subsystem is implemented within the file
sys/kern/vfs_bio.c.
SEE ALSOintro(9), vnode(9), VOP_STRATEGY(9)
Maurice J. Bach, The Design of the UNIX Operating System, Prentice Hall,
1986.
Marshall Kirk McKusick, Keith Bostic, Michael J. Karels, and John S.
Quarterman, The Design and Implementation of the 4.4BSD Operating System,
Addison Wesley, 1996.
Leffler, et. al., The Design and Implementation of the 4.3 BSD Unix
Operating System, Addison Wesley, 1989.
BUGS
In the current implementation, bread() and its variants don't use a
specified credential.
OpenBSD 4.9 July 24, 2008 OpenBSD 4.9
