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authorru <ru@FreeBSD.org>2002-05-29 14:11:17 +0000
committerru <ru@FreeBSD.org>2002-05-29 14:11:17 +0000
commitb2b3f82524aad49cf7f09630ba0fb063287e33a3 (patch)
tree39f47798f75be29e0c1c6ed6346af25040430a91 /share
parentc17d65090d4962d81a9002d7ec3c85b383885c93 (diff)
downloadFreeBSD-src-b2b3f82524aad49cf7f09630ba0fb063287e33a3.zip
FreeBSD-src-b2b3f82524aad49cf7f09630ba0fb063287e33a3.tar.gz
mdoc(7) police: tidy up the markup.
Diffstat (limited to 'share')
-rw-r--r--share/man/man4/ccd.4106
1 files changed, 65 insertions, 41 deletions
diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4
index 4eae7b7..879d761 100644
--- a/share/man/man4/ccd.4
+++ b/share/man/man4/ccd.4
@@ -48,41 +48,43 @@ The
driver provides the capability of combining one or more disks/partitions
into one virtual disk.
.Pp
-This document assumes that you're familiar with how to generate kernels,
+This document assumes that you are familiar with how to generate kernels,
how to properly configure disks and devices in a kernel
configuration file, and how to partition disks.
.Pp
Note that the
-.Sq raw
+.Dq raw
partitions of the disks
-.Pa should not
-be combined. The kernel will only allow component partitions of type
-FS_BSDFFS (type
-.Dq Bx 4.2
-as shown as
+.Em should not
+be combined.
+The kernel will only allow component partitions of type
+.Dv FS_BSDFFS (type
+.Dq Li 4.2BSD
+as shown by
.Xr disklabel 8 ) .
.Pp
-In order to compile in support for the ccd, you must add a line similar
+In order to compile in support for the
+.Nm ,
+you must add a line similar
to the following to your kernel configuration file:
-.Bd -unfilled -offset indent
-device ccd # concatenated disk devices
-.Ed
.Pp
-The count argument is how many
-.Nm Ns 's
-memory is allocated for a boot time. In this example, no more than 4
-.Nm Ns s
-may be configured. As of the
+.Dl "device ccd # concatenated disk devices"
+.Pp
+As of the
.Fx 3.0
release, you do not need to
-configure your kernel with ccd but may instead use it as a kernel loadable
-module. Simply running
-.Nm ccdconfig
+configure your kernel with
+.Nm
+but may instead use it as a kernel loadable
+module.
+Simply running
+.Xr ccdconfig 8
will load the module into the kernel.
.Pp
A
.Nm
-may be either serially concatenated or interleaved. To serially
+may be either serially concatenated or interleaved.
+To serially
concatenate the partitions, specify the interleave factor of 0.
Note that mirroring may not be used with an interleave factor of 0.
.Pp
@@ -97,35 +99,46 @@ If a
is interleaved correctly, a
.Dq striping
effect is achieved, which can increase sequential read/write
-performance. The interleave factor is expressed in units of DEV_BSIZE
-(usually 512 bytes). For large writes, the optimum interleave factor
+performance.
+The interleave factor is expressed in units of
+.Dv DEV_BSIZE
+(usually 512 bytes).
+For large writes, the optimum interleave factor
is typically the size of a track, while for large reads, it is about a
quarter of a track.
(Note that this changes greatly depending on the
-number and speed of disks.) For instance, with eight 7,200 RPM drives
+number and speed of disks.)
+For instance, with eight 7,200 RPM drives
on two Fast-Wide SCSI buses, this translates to about 128 for writes
-and 32 for reads. A larger interleave tends to work better when the
+and 32 for reads.
+A larger interleave tends to work better when the
disk is taking a multitasking load by localizing the file I/O from
-any given process onto a single disk. You lose sequential performance when
+any given process onto a single disk.
+You lose sequential performance when
you do this, but sequential performance is not usually an issue with a
multitasking load.
.Pp
An interleave factor must be specified when using a mirroring configuration,
even when you have only two disks (i.e. the layout winds up being the same
-no matter what the interleave factor). The interleave factor will determine
+no matter what the interleave factor).
+The interleave factor will determine
how I/O is broken up, however, and a value 128 or greater is recommended.
.Pp
-CCD has an option for a parity disk, but does not currently implement it.
+.Nm
+has an option for a parity disk, but does not currently implement it.
.Pp
The best performance is achieved if all component disks have the same
-geometry and size. Optimum striping cannot occur with different
+geometry and size.
+Optimum striping cannot occur with different
disk types.
.Pp
For random-access oriented workloads, such as news servers, a larger
-interleave factor (e.g., 65,536) is more desirable. Note that there
-isn't much
+interleave factor (e.g., 65,536) is more desirable.
+Note that there
+is not much
.Nm
-can do to speed up applications that are seek-time limited. Larger
+can do to speed up applications that are seek-time limited.
+Larger
interleave factors will at least reduce the chance of having to seek
two disk-heads to read one directory or a file.
.Ss Disk Mirroring
@@ -133,15 +146,22 @@ You can configure the
.Nm
to
.Dq mirror
-any even number of disks. See
+any even number of disks.
+See
.Xr ccdconfig 8
-for how to specify the necessary flags. For example, if you have a ccd
+for how to specify the necessary flags.
+For example, if you have a
+.Nm
configuration specifying four disks, the first two disks will be mirrored with
-the second two disks. A write will be run to both sides of
-the mirror. A read will be run to either side of the mirror depending
-on what the driver believes to be most optimal. If the read fails,
+the second two disks.
+A write will be run to both sides of
+the mirror.
+A read will be run to either side of the mirror depending
+on what the driver believes to be most optimal.
+If the read fails,
the driver will automatically attempt to read the same sector from the
-other side of the mirror. Currently
+other side of the mirror.
+Currently
.Nm
uses a duel seek zone model to optimize reads for a multi-tasking load
rather then a sequential load.
@@ -153,7 +173,8 @@ to recover the failed disk.
.Pp
Note that a one-disk
.Nm
-is not the same as the original partition. In particular, this means
+is not the same as the original partition.
+In particular, this means
if you have a filesystem on a two-disk mirrored
.Nm
and one of the disks fail, you cannot mount and use the remaining
@@ -170,9 +191,12 @@ fails, the entire
filesystem will be lost unless you are mirroring the disks.
.Pp
If one of the disks in a mirror is lost, you should still
-be able to backup your data. If a write error occurs, however, data
-read from that sector may be non-deterministic. It may return the data
-prior to the write or it may return the data that was written. When a
+be able to back up your data.
+If a write error occurs, however, data
+read from that sector may be non-deterministic.
+It may return the data
+prior to the write or it may return the data that was written.
+When a
write error occurs, you should recover and regenerate the data as soon
as possible.
.Pp
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