Hi,
here are some comments / man page updates for things I have learned in my
adventures through msdosfs and vfs_bio.c. I have also added the
bread_cluster function to buffercache(9). It would be nice if someone who
knows the buffer/disk stuff could review this.
Hint for the paragraph on block sizes: In ufs/ffs/ffs_vfsops.c there is:
sbp->f_iosize = fs->fs_bsize;
Then look at the use of fs_bsize in ufs/ffs/fs.h
Cheers,
Stefan
diff --git a/share/man/man9/buffercache.9 b/share/man/man9/buffercache.9
index 84df0c06513..0c4c8f1ad96 100644
--- a/share/man/man9/buffercache.9
+++ b/share/man/man9/buffercache.9
@@ -108,6 +108,7 @@
.Sh NAME
.Nm buffercache ,
.Nm bread ,
+.Nm bread_cluster ,
.Nm breadn ,
.Nm bwrite ,
.Nm bawrite ,
@@ -126,6 +127,9 @@
.Fn bread "struct vnode *vp" "daddr_t blkno" "int size" \
"struct buf **bpp"
.Ft int
+.Fn bread_cluster "struct vnode *vp" "daddr_t blkno" "int size" \
+"struct buf **bpp"
+.Ft int
.Fn breadn "struct vnode *vp" "daddr_t blkno" "int size" \
"daddr_t rablks[]" "int rasizes[]" "int nrablks" \
"struct buf **bpp"
@@ -163,6 +167,11 @@ 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.
+.Pp
+The block size used for logical block numbers depends on the type of the
+given vnode.
+For file vnodes, this is f_iosize of the underlying filesystem.
+For block device vnodes, this will usually be DEV_BSIZE.
.\" XXX struct buf, B_ flags, MP locks, etc.
.\" XXX free list, hash queue, etc.
.\" ------------------------------------------------------------
@@ -184,6 +193,10 @@ to allocate a buffer with enough pages for
.Fa size
and reads the specified disk block into it.
.Pp
+.Fn bread
+always returns a buffer, even if it returns an error due to an I/O
+error.
+.Pp
The buffer returned by
.Fn bread
is marked as busy.
@@ -222,6 +235,30 @@ and
The read-ahead blocks aren't returned, but are available in cache for
future accesses.
.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+.It Xo
+.Fo bread_cluster
+.Fa "vp"
+.Fa "blkno"
+.Fa "size"
+.Fa "bpp"
+.Fc
+.Xc
+Read a block of size
+.Fa "size"
+corresponding to
+.Fa vp
+and
+.Fa blkno ,
+with readahead.
+If neither the first block, nor a part of the next MAXBSIZE bytes is already
+in the buffer cache,
+.Fn bread_cluster
+will perform a read-ahead of MAXBSIZE bytes in a single I/O operation.
+This is currently more efficient than
+.Fn breadn .
+The read-ahead data isn't returned, but is available in cache for
+future accesses.
+.\" - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
.It Fn bwrite "bp"
Write a block.
Start I/O for write using
diff --git a/sys/kern/vfs_bio.c b/sys/kern/vfs_bio.c
index 88adfeff237..4804d370f49 100644
--- a/sys/kern/vfs_bio.c
+++ b/sys/kern/vfs_bio.c
@@ -568,6 +568,12 @@ bread_cluster_callback(struct buf *bp)
}
}
+/*
+ * Read-ahead multiple disk blocks, but make sure only one (big) I/O
+ * request is sent to the disk.
+ * XXX This should probably be dropped and breadn should instead be optimized
+ * XXX to do fewer I/O requests.
+ */
int
bread_cluster(struct vnode *vp, daddr_t blkno, int size, struct buf **rbpp)
{
@@ -1023,6 +1029,9 @@ geteblk(size_t size)
/*
* Allocate a buffer.
+ * If vp is given, put it into the buffer cache for that vnode.
+ * If size != 0, allocate memory and call buf_map().
+ * If there is already a buffer for the given vnode/blkno, return NULL.
*/
struct buf *
buf_get(struct vnode *vp, daddr_t blkno, size_t size)
