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BUFFERCACHE(9) NetBSD Kernel Developer's Manual BUFFERCACHE(9)
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by Kimmo Suominen.
Based on man-cgi by Panagiotis Christias.
buffercache, bread, breadn, bwrite, bawrite, bdwrite, getblk, geteblk,
incore, allocbuf, brelse -- buffer cache interfaces
bread(struct vnode *vp, daddr_t blkno, int size, int flags, buf_t **bpp);
breadn(struct vnode *vp, daddr_t blkno, int size, daddr_t rablks,
int rasizes, int nrablks, int flags, buf_t **bpp);
getblk(struct vnode *vp, daddr_t blkno, int size, int slpflag,
incore(struct vnode *vp, daddr_t blkno);
allocbuf(buf_t *bp, int size, int preserve);
brelse(buf_t *bp, int set);
The buffercache interface is used by each filesystems to improve I/O per-
formance 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
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
If the buffer is not found (i.e. the block is not cached in mem-
ory), 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 writ-
ten 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.
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
Write a block asynchronously. Set the B_ASYNC flag in bp and
simply call VOP_BWRITE(), which results in bwrite() for most
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. Other-
wise, return an empty block of the correct size. It is up to
the caller to ensure that the cached blocks are of the correct
If getblk() needs to sleep, slpflag and slptimeo are used as
arguments for cv_timedwait().
Allocate an empty, disassociated block of a given size size.
Determine if a block associated to a given vnode and block off-
set 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
Unbusy a buffer and release it to the free lists.
The buffer cache subsystem is implemented within the file
Maurice J. Bach, The Design of the UNIX Operating System, Prentice Hall,
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.
NetBSD 9.0 April 11, 2017 NetBSD 9.0