.\" $OpenBSD: buffercache.9,v 1.7 2008/07/24 14:19:24 jmc Exp $ .\" $NetBSD: buffercache.9,v 1.13 2004/06/25 15:31:37 wiz Exp $ .\" .\" Copyright (c)2003 YAMAMOTO Takashi, .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" .\" following copyright notices are from sys/kern/vfs_bio.c. .\" they are here because i took some comments from it. yamt@NetBSD.org .\" .\" .\"/*- .\" * Copyright (c) 1982, 1986, 1989, 1993 .\" * The Regents of the University of California. All rights reserved. .\" * (c) UNIX System Laboratories, Inc. .\" * All or some portions of this file are derived from material licensed .\" * to the University of California by American Telephone and Telegraph .\" * Co. or Unix System Laboratories, Inc. and are reproduced herein with .\" * the permission of UNIX System Laboratories, Inc. .\" * .\" * Redistribution and use in source and binary forms, with or without .\" * modification, are permitted provided that the following conditions .\" * are met: .\" * 1. Redistributions of source code must retain the above copyright .\" * notice, this list of conditions and the following disclaimer. .\" * 2. Redistributions in binary form must reproduce the above copyright .\" * notice, this list of conditions and the following disclaimer in the .\" * documentation and/or other materials provided with the distribution. .\" * 3. Neither the name of the University nor the names of its contributors .\" * may be used to endorse or promote products derived from this software .\" * without specific prior written permission. .\" * .\" * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" * SUCH DAMAGE. .\" * .\" * @(#)vfs_bio.c 8.6 (Berkeley) 1/11/94 .\" */ .\" .\"/*- .\" * Copyright (c) 1994 Christopher G. Demetriou .\" * .\" * Redistribution and use in source and binary forms, with or without .\" * modification, are permitted provided that the following conditions .\" * are met: .\" * 1. Redistributions of source code must retain the above copyright .\" * notice, this list of conditions and the following disclaimer. .\" * 2. Redistributions in binary form must reproduce the above copyright .\" * notice, this list of conditions and the following disclaimer in the .\" * documentation and/or other materials provided with the distribution. .\" * 3. All advertising materials mentioning features or use of this software .\" * must display the following acknowledgement: .\" * This product includes software developed by the University of .\" * California, Berkeley and its contributors. .\" * 4. Neither the name of the University nor the names of its contributors .\" * may be used to endorse or promote products derived from this software .\" * without specific prior written permission. .\" * .\" * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" * SUCH DAMAGE. .\" * .\" * @(#)vfs_bio.c 8.6 (Berkeley) 1/11/94 .\" */ .\" .\" .\" ------------------------------------------------------------ .Dd $Mdocdate: July 24 2008 $ .Dt BUFFERCACHE 9 .Os .Sh NAME .Nm buffercache , .Nm bread , .Nm breadn , .Nm bwrite , .Nm bawrite , .Nm bdwrite , .Nm getblk , .Nm geteblk , .Nm incore , .Nm allocbuf , .Nm brelse , .Nm biodone , .Nm biowait .Nd buffer cache interfaces .\" ------------------------------------------------------------ .Sh SYNOPSIS .In sys/buf.h .Ft int .Fn bread "struct vnode *vp" "daddr64_t blkno" "int size" \ "struct ucred *cred" "struct buf **bpp" .Ft int .Fn breadn "struct vnode *vp" "daddr64_t blkno" "int size" \ "daddr64_t rablks[]" "int rasizes[]" "int nrablks" \ "struct ucred *cred" "struct buf **bpp" .Ft int .Fn bwrite "struct buf *bp" .Ft void .Fn bawrite "struct buf *bp" .Ft void .Fn bdwrite "struct buf *bp" .Ft struct buf * .Fn getblk "struct vnode *vp" "daddr64_t blkno" "int size" \ "int slpflag" "int slptimeo" .Ft struct buf * .Fn geteblk "int size" .Ft struct buf * .Fn incore "struct vnode *vp" "daddr64_t blkno" .Ft void .Fn allocbuf "struct buf *bp" "int size" .Ft void .Fn brelse "struct buf *bp" .Ft void .Fn biodone "struct buf *bp" .Ft int .Fn biowait "struct buf *bp" .\" ------------------------------------------------------------ .Sh DESCRIPTION The .Nm interface is used by each filesystem to improve I/O performance using in-core caches of filesystem blocks. .Pp The kernel memory used to cache a block is called a buffer and described by a .Em buf structure. In addition to describing a cached block, a .Em buf structure is also used to describe an I/O request as a part of the disk driver interface. .\" XXX struct buf, B_ flags, MP locks, etc. .\" XXX free list, hash queue, etc. .\" ------------------------------------------------------------ .Sh FUNCTIONS .Bl -tag -width compact .It Fn bread "vp" "blkno" "size" "cred" "bpp" Read a block corresponding to .Fa vp and .Fa blkno . The buffer is returned via .Fa bpp . .Pp If the buffer is not found (i.e. the block is not cached in memory), .Fn bread calls .Fn getblk to allocate a buffer with enough pages for .Fa size and reads the specified disk block into it. .Pp The buffer returned by .Fn bread is marked as busy. (The .Dv B_BUSY flag is set.) After manipulation of the buffer returned from .Fn 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 .Fn bwrite . Otherwise, it should be unbusied using .Fn brelse . .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Xo .Fo breadn .Fa "vp" .Fa "blkno" .Fa "size" .Fa "rablks" .Fa "rasizes" .Fa "nrablks" .Fa "cred" .Fa "bpp" .Fc .Xc Get a buffer as .Fn bread . In addition, .Fn breadn will start read-ahead of blocks specified by .Fa rablks , .Fa rasizes , and .Fa nrablks . The read-ahead blocks aren't returned, but are available in cache for future accesses. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn bwrite "bp" Write a block. Start I/O for write using .Fn VOP_STRATEGY . Then, unless the .Dv B_ASYNC flag is set in .Fa bp , .Fn bwrite waits for the I/O to complete. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn bawrite "bp" Write a block asynchronously. Set the .Dv B_ASYNC flag in .Fa bp and simply call .Fn VOP_BWRITE , which results in .Fn bwrite for most filesystems. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn bdwrite "bp" Delayed write. Unlike .Fn bawrite , .Fn bdwrite won't start any I/O. It only marks the buffer as dirty .Pq Dv 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. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn getblk "vp" "blkno" "size" "slpflag" "slptimeo" Get a block of requested size .Fa size that is associated with a given vnode and block offset, specified by .Fa vp and .Fa 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. .Pp If .Fn getblk needs to sleep, .Fa slpflag and .Fa slptimeo are used as arguments for .Fn tsleep . .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn geteblk "size" Allocate an empty, disassociated block of a given size .Fa size . .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn 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 .Fn incore doesn't mark the buffer as busy unlike .Fn getblk . .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn 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 .Em 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. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn brelse "bp" Unlock a buffer by clearing the .Dv B_AGE , .Dv B_ASYNC , .Dv B_BUSY , .Dv B_NOCACHE , and .Dv B_DEFERRED flags and release it to the free lists. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn biodone "bp" Mark I/O complete on a buffer. If a callback has been requested by .Dv B_CALL , do so. Otherwise, wake up the waiting processes. .\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - .It Fn 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 .Fn strategy type function, then the buffer must be previously initialized with the .Dv B_RAW flag. .El .\" ------------------------------------------------------------ .Sh CODE REFERENCES This section describes places within the FabBSD source tree where actual code implementing the buffer cache subsystem can be found. All pathnames are relative to .Pa /usr/src . .Pp The buffer cache subsystem is implemented within the file .Pa sys/kern/vfs_bio.c . .Sh SEE ALSO .Xr intro 9 , .Xr vnode 9 , .Xr VOP_STRATEGY 9 .Rs .%A Maurice J. Bach .%B "The Design of the UNIX Operating System" .%I "Prentice Hall" .%D 1986 .Re .Rs .%A Marshall Kirk McKusick .%A Keith Bostic .%A Michael J. Karels .%A John S. Quarterman .%B "The Design and Implementation of the 4.4BSD Operating System" .%I "Addison Wesley" .%D 1996 .Re .Rs .%A Leffler, et. al. .%B "The Design and Implementation of the 4.3 BSD Unix Operating System" .%I Addison Wesley .%D 1989 .Re .\" ------------------------------------------------------------ .Sh BUGS In the current implementation, .Fn bread and its variants don't use a specified credential.