From b2b3f82524aad49cf7f09630ba0fb063287e33a3 Mon Sep 17 00:00:00 2001 From: ru Date: Wed, 29 May 2002 14:11:17 +0000 Subject: mdoc(7) police: tidy up the markup. --- share/man/man4/ccd.4 | 106 +++++++++++++++++++++++++++++++-------------------- 1 file changed, 65 insertions(+), 41 deletions(-) (limited to 'share') 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 -- cgit v1.1