NAME
buffercache,
bread,
breadn,
bwrite,
bawrite,
bdwrite,
getblk,
geteblk,
incore,
allocbuf,
brelse —
buffer cache interfaces
SYNOPSIS
#include <sys/buf.h>
int
bread(
struct
vnode *vp,
daddr_t
blkno,
int size,
int flags,
buf_t **bpp);
int
breadn(
struct
vnode *vp,
daddr_t
blkno,
int size,
daddr_t rablks[],
int rasizes[],
int nrablks,
int flags,
buf_t **bpp);
int
bwrite(
buf_t
*bp);
void
bawrite(
buf_t
*bp);
void
bdwrite(
buf_t
*bp);
buf_t *
getblk(
struct
vnode *vp,
daddr_t
blkno,
int size,
int slpflag,
int slptimeo);
buf_t *
geteblk(
int
size);
buf_t *
incore(
struct
vnode *vp,
daddr_t
blkno);
void
allocbuf(
buf_t
*bp,
int size,
int preserve);
void
brelse(
buf_t
*bp,
int set);
DESCRIPTION
The
buffercache interface is used by each filesystems 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,
flags, bpp)
- Read a block corresponding to vp and
blkno. The buffer is returned via
bpp. The units of blkno are
specifically the units used by the VOP_STRATEGY()
routine for the vp vnode. For device special files,
blkno is in units of
DEV_BSIZE
and both blkno and
size must be multiples of the underlying device's
block size. For other files, blkno is in units
chosen by the file system containing vp.
If the buffer is not found (i.e. the block is not cached in memory),
bread() allocates 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
variants of bwrite(). Otherwise, it should be unbusied
using brelse().
-
-
- breadn(vp,
blkno, size,
rablks, rasizes,
nrablks, flags,
bpp)
- Get a buffer as bread(). In addition,
breadn() will start read-ahead of blocks specified by
rablks, rasizes,
nrablks.
-
-
- 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 (
BO_DELWRI
) and unbusy it.
-
-
- 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, 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
cv_timedwait().
-
-
- geteblk(size)
- Allocate an empty, disassociated block of a given size
size.
-
-
- incore(vp,
blkno)
- Determine if a block associated to a given vnode and block
offset is in the cache. If it is there, return a pointer to it. Note that
incore() doesn't busy the buffer unlike
getblk().
-
-
- allocbuf(bp,
size, preserve)
- Expand or contract the actual memory allocated to a buffer.
If preserve is zero, the entire data in the buffer
will be lost. Otherwise, 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 callers responsibility to fill out the
buffer's additional contents.
-
-
- brelse(bp,
set)
- Unbusy a buffer and release it to the free lists.
CODE REFERENCES
The buffer cache subsystem is implemented within the file
sys/kern/vfs_bio.c.
SEE ALSO
intro(9),
vnode(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.