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mdoc(7) police: tidy up the markup.
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Ruslan Ermilov
parent
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commit
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Notes:
svn2git
2020-12-20 02:59:44 +00:00
svn path=/head/; revision=97459
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@ -48,41 +48,43 @@ The
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driver provides the capability of combining one or more disks/partitions
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into one virtual disk.
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.Pp
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This document assumes that you're familiar with how to generate kernels,
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This document assumes that you are familiar with how to generate kernels,
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how to properly configure disks and devices in a kernel
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configuration file, and how to partition disks.
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.Pp
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Note that the
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.Sq raw
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.Dq raw
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partitions of the disks
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.Pa should not
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be combined. The kernel will only allow component partitions of type
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FS_BSDFFS (type
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.Dq Bx 4.2
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as shown as
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.Em should not
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be combined.
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The kernel will only allow component partitions of type
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.Dv FS_BSDFFS (type
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.Dq Li 4.2BSD
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as shown by
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.Xr disklabel 8 ) .
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.Pp
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In order to compile in support for the ccd, you must add a line similar
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In order to compile in support for the
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.Nm ,
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you must add a line similar
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to the following to your kernel configuration file:
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.Bd -unfilled -offset indent
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device ccd # concatenated disk devices
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.Ed
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.Pp
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The count argument is how many
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.Nm Ns 's
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memory is allocated for a boot time. In this example, no more than 4
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.Nm Ns s
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may be configured. As of the
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.Dl "device ccd # concatenated disk devices"
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.Pp
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As of the
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.Fx 3.0
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release, you do not need to
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configure your kernel with ccd but may instead use it as a kernel loadable
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module. Simply running
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.Nm ccdconfig
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configure your kernel with
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.Nm
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but may instead use it as a kernel loadable
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module.
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Simply running
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.Xr ccdconfig 8
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will load the module into the kernel.
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.Pp
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A
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.Nm
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may be either serially concatenated or interleaved. To serially
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may be either serially concatenated or interleaved.
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To serially
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concatenate the partitions, specify the interleave factor of 0.
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Note that mirroring may not be used with an interleave factor of 0.
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.Pp
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@ -97,35 +99,46 @@ If a
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is interleaved correctly, a
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.Dq striping
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effect is achieved, which can increase sequential read/write
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performance. The interleave factor is expressed in units of DEV_BSIZE
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(usually 512 bytes). For large writes, the optimum interleave factor
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performance.
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The interleave factor is expressed in units of
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.Dv DEV_BSIZE
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(usually 512 bytes).
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For large writes, the optimum interleave factor
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is typically the size of a track, while for large reads, it is about a
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quarter of a track.
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(Note that this changes greatly depending on the
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number and speed of disks.) For instance, with eight 7,200 RPM drives
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number and speed of disks.)
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For instance, with eight 7,200 RPM drives
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on two Fast-Wide SCSI buses, this translates to about 128 for writes
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and 32 for reads. A larger interleave tends to work better when the
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and 32 for reads.
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A larger interleave tends to work better when the
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disk is taking a multitasking load by localizing the file I/O from
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any given process onto a single disk. You lose sequential performance when
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any given process onto a single disk.
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You lose sequential performance when
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you do this, but sequential performance is not usually an issue with a
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multitasking load.
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.Pp
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An interleave factor must be specified when using a mirroring configuration,
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even when you have only two disks (i.e. the layout winds up being the same
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no matter what the interleave factor). The interleave factor will determine
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no matter what the interleave factor).
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The interleave factor will determine
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how I/O is broken up, however, and a value 128 or greater is recommended.
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.Pp
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CCD has an option for a parity disk, but does not currently implement it.
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.Nm
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has an option for a parity disk, but does not currently implement it.
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.Pp
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The best performance is achieved if all component disks have the same
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geometry and size. Optimum striping cannot occur with different
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geometry and size.
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Optimum striping cannot occur with different
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disk types.
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.Pp
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For random-access oriented workloads, such as news servers, a larger
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interleave factor (e.g., 65,536) is more desirable. Note that there
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isn't much
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interleave factor (e.g., 65,536) is more desirable.
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Note that there
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is not much
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.Nm
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can do to speed up applications that are seek-time limited. Larger
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can do to speed up applications that are seek-time limited.
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Larger
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interleave factors will at least reduce the chance of having to seek
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two disk-heads to read one directory or a file.
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.Ss Disk Mirroring
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@ -133,15 +146,22 @@ You can configure the
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.Nm
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to
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.Dq mirror
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any even number of disks. See
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any even number of disks.
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See
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.Xr ccdconfig 8
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for how to specify the necessary flags. For example, if you have a ccd
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for how to specify the necessary flags.
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For example, if you have a
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.Nm
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configuration specifying four disks, the first two disks will be mirrored with
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the second two disks. A write will be run to both sides of
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the mirror. A read will be run to either side of the mirror depending
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on what the driver believes to be most optimal. If the read fails,
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the second two disks.
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A write will be run to both sides of
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the mirror.
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A read will be run to either side of the mirror depending
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on what the driver believes to be most optimal.
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If the read fails,
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the driver will automatically attempt to read the same sector from the
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other side of the mirror. Currently
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other side of the mirror.
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Currently
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.Nm
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uses a duel seek zone model to optimize reads for a multi-tasking load
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rather then a sequential load.
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@ -153,7 +173,8 @@ to recover the failed disk.
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.Pp
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Note that a one-disk
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.Nm
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is not the same as the original partition. In particular, this means
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is not the same as the original partition.
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In particular, this means
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if you have a filesystem on a two-disk mirrored
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.Nm
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and one of the disks fail, you cannot mount and use the remaining
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@ -170,9 +191,12 @@ fails, the entire
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filesystem will be lost unless you are mirroring the disks.
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.Pp
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If one of the disks in a mirror is lost, you should still
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be able to backup your data. If a write error occurs, however, data
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read from that sector may be non-deterministic. It may return the data
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prior to the write or it may return the data that was written. When a
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be able to back up your data.
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If a write error occurs, however, data
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read from that sector may be non-deterministic.
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It may return the data
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prior to the write or it may return the data that was written.
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When a
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write error occurs, you should recover and regenerate the data as soon
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as possible.
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.Pp
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