xref: /dflybsd-src/share/man/man9/bufcache.9 (revision 9d835656908d66f619bd14af2cabde55cf2bf450)

.\" Copyright (c) 2005 The DragonFly Project.  All rights reserved.
.\"
.\" This code is derived from software contributed to The DragonFly Project
.\" by Hiten Pandya <hmp@dragonflybsd.org>.
.\"
.\" 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 DragonFly Project 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 COPYRIGHT HOLDERS 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
.\" COPYRIGHT HOLDERS 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.
.\"
.Dd December 27, 2018
.Dt BUFCACHE 9
.Os
.Sh NAME
.Nm bufinit ,
.Nm bread ,
.Nm bwrite
.Nd Buffer Cache Functions
.Sh SYNOPSIS
.In sys/param.h
.In sys/systm.h
.In sys/buf.h
.In sys/buf2.h
.Ft int
.Fo bread
.Fa "struct vnode *vp"
.Fa "off_t loffset"
.Fa "int size"
.Fa "struct buf **bpp"
.Fc
.Ft int
.Fo bwrite
.Fa "struct buf *bp"
.Fc
.Sh DESCRIPTION
The buffer cache functions are at the heart of all storage file systems;
they are used for reading from and writing to the underlying storage.
The
.Fn bread
and
.Fn bwrite
functions observe most activity in the kernel from file systems, but other
functions such as
.Fn breadn
are also used.
.Pp
At boot time, the
.Fn bufinit
function is invoked to initialize various accounting code.
It also initializes
.Va nbuf
number of buffers and inserts them into the empty queue
.Dv BQUEUE_EMPTY .
The variable
.Va nbuf
is a global variable in the kernel that is tunable at boot time using
the
.Xr loader 8 .
.Sh FUNCTIONS
.Bl -tag -width compact
.It Fn bread "*vp" "loffset" "size" "**bpp"
Retrieve a buffer with specified data.
An internal function,
.Fn getblk
is called to check whether the data is available in cache or if it
should be read from the
.Fa vp .
If the data is available in cache, the
.Dv B_CACHE
flag will be set otherwise
.Fa size
bytes will be read starting at block number
.Fa loffset
from the block special device vnode
.Fa vp .
.Pp
In case when the buffer is not in cache or not cacheable this
function will put the calling process or thread to sleep, using
.Fa bp
as the wait channel and
.Ql "biord"
as the wait message.
.Pp
On successful return, the
.Va b_data
field of
.Fa bp
will point to valid data address and
.Va b_count
will contain the number of bytes read.
.It Fn bwrite "*bp"
Write a buffer back to the device pointed to by
.Va b_dev
field.
Until the write operation is complete, the calling thread or
process will be put to sleep by the kernel using
.Fa bp
as the wait channel and
.Ql "biowr"
as the wait message.
.Pp
Before calling this function, the following fields are the least
to be set:
.Bl -tag -width compact
.It Va b_data
This field should be set to a valid data buffer to be written by
.Fn bwrite .
.It Va b_bcount
Size of buffer to be written, analogous to the
.Fa size
argument of
.Fn bread .
.It Va b_blkno
Logical block number at which the buffer should be written.
.It Va b_dev
This can be set by using the
.Fn vn_todev
function on the device vnode.
.It Va b_vp
This should be set to the vnode of the device to which the buffer
will be written.
.El
.Pp
This function will put the calling process or thread to sleep if the
data cannot be written when operating synchronously, using
.Fa bp
as the wait channel and
.Ql "biowr"
as the wait message.
On successful return the
.Va b_resid
field of
.Fa bp
will be set to the value zero, thus indicating a successful write.
.El
.Sh CODE REFERENCES
The file system code, located under
.Pa sys/vfs
directory are the main source of reference.
.Sh SEE ALSO
.Xr buf 9 ,
.Xr VFS 9
.Sh AUTHORS
This manual page was written by
.An Hiten Pandya Aq Mt hmp@freebsd.org .