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Diffstat (limited to 'share/man/man4/ccd.4')
-rw-r--r-- | share/man/man4/ccd.4 | 51 |
1 files changed, 47 insertions, 4 deletions
diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4 index 0bdc790..169a41d 100644 --- a/share/man/man4/ccd.4 +++ b/share/man/man4/ccd.4 @@ -72,12 +72,17 @@ The count argument is how many .Nm ccds memory is allocated for a boot time. In this example, no more than 4 .Nm ccds -may be configured. +may be configured. As of the FreeBSD 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 +will load the module into the kernel. .Pp A .Nm ccd 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 There is a run-time utility that is used for configuring .Nm ccds . @@ -96,7 +101,18 @@ 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 on two Fast-Wide SCSI buses, this translates to about 128 for writes -and 32 for reads. +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 +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 +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. .Pp The best performance is achieved if all component disks have the same geometry and size. Optimum striping cannot occur with different @@ -116,7 +132,18 @@ to .Dq mirror any even number of disks. See .Xr ccdconfig 8 -for how to specify the necessary flags. In an event of a disk +for how to specify the necessary flags. For example, if you have a ccd +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 driver will automatically attempt to read the same sector from the +other side of the mirror. Currently +.Nm ccd +uses a duel seek zone model to optimize reads for a multi-tasking load +rather then a sequential load. +.Pp +In an event of a disk failure, you can use .Xr dd 1 to recover the failed disk. @@ -129,11 +156,26 @@ if you have a filesystem on a two-disk mirrored and one of the disks fail, you cannot mount and use the remaining partition as itself; you have to configure it as a one-disk .Nm ccd. +You cannot replace a disk in a mirrored +.Nm ccd +partition without first backing up the partition, then replacing the disk, +then restoring the partition. .Sh WARNINGS If just one (or more) of the disks in a .Nm ccd fails, the entire file system 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 +write error occurs, you should recover and regenerate the data as soon +as possible. +.Pp +Changing the interleave or other parameters for a +.Nm ccd +disk usually destroys whatever data previously existed on that disk. .Sh FILES /dev/{,r}ccd* - ccd device special files. .Pp @@ -148,4 +190,5 @@ Utah. .Xr fsck 8 , .Xr MAKEDEV 8 , .Xr mount 8 , -.Xr newfs 8 . +.Xr newfs 8 , +.Xr vinum 8 . |