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author | asami <asami@FreeBSD.org> | 1997-01-25 02:10:43 +0000 |
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committer | asami <asami@FreeBSD.org> | 1997-01-25 02:10:43 +0000 |
commit | 5db90170359062a77843cf3f36a7710cfd8cdf83 (patch) | |
tree | f146f2276986cae1d91b99e3922429c9a02da3e0 | |
parent | 09dd134c9400d86db83af6043b65ea6598caa241 (diff) | |
download | FreeBSD-src-5db90170359062a77843cf3f36a7710cfd8cdf83.zip FreeBSD-src-5db90170359062a77843cf3f36a7710cfd8cdf83.tar.gz |
Explain more about the interleave factor. Mention mirroring, and add
a reminder that you can't mount half of a mirrored two-disk ccd.
-rw-r--r-- | share/man/man4/ccd.4 | 60 |
1 files changed, 47 insertions, 13 deletions
diff --git a/share/man/man4/ccd.4 b/share/man/man4/ccd.4 index c581bf4..7f31d71 100644 --- a/share/man/man4/ccd.4 +++ b/share/man/man4/ccd.4 @@ -74,31 +74,64 @@ may be configured. .Pp A .Nm ccd -may be either serially concatenated or interleaved. If a +may be either serially concatenated or interleaved. To serially +concatenate the partitions, specify the interleave factor of 0. +.Pp +There is a run-time utility that is used for configuring +.Nm ccds . +See +.Xr ccdconfig 8 +for more information. +.Ss The Interleave Factor +If a .Nm ccd is interleaved correctly, a .Dq striping -effect is achieved, which can increase performance. The optimum interleave -factor is typically the size of a track. Since the interleave factor -is expressed in units of DEV_BSIZE, one must account for sector sizes -other than DEV_BSIZE in order to calculate the correct interleave. -The kernel will not allow an interleave factor less than the size -of the largest component sector divided by DEV_BSIZE. +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 +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. .Pp -Note that best performance is achieved if all component disks have the same +The best performance is achieved if all component disks have the same geometry and size. Optimum striping cannot occur with different disk types. .Pp -There is a run-time utility that is used for configuring -.Nm ccds . -See +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 +.Nm ccd +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 +You can configure the +.Nm ccd +to +.Dq mirror +any even number of disks. See .Xr ccdconfig 8 -for more information. +for how to specify the necessary flags. In an event of a disk +failure, you can use +.Xr dd 1 +to recover the failed disk. +.Pp +Note that a one-disk +.Nm ccd +is not the same as the original partition. In particular, this means +if you have a filesystem on a two-disk mirrored +.Nm ccd +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. .Sh WARNINGS If just one (or more) of the disks in a .Nm ccd fails, the entire -file system will be lost. +file system will be lost unless you are mirroring the disks. .Sh FILES /dev/{,r}ccd* - ccd device special files. .Pp @@ -106,6 +139,7 @@ file system will be lost. The concatenated disk driver was originally written at the University of Utah. .Sh SEE ALSO +.Xr dd 1 , .Xr MAKEDEV 8 , .Xr ccdconfig 8 , .Xr config 8 , |