Tony, Since you are the 3rd person to ask, I'll answer publicly, again:
MD is a kernel subsystem (& utilities/tools) which allows you to map several phy
sical
disk devices (partitions) into one larger drive, whose capacity is the sum of al
l
included devices.
MD supports two modes, linear and striped. The linear (or concatenated) mode al
lows
several partitions to combine serially into one larger device. Access (or LBA
assignement) is sequential.
For example assume: /dev/sda4 = 20MB, /dev/sdb5 = 60MB and /dev/sdc9 = 100MB.
In linear mode, all will be concatenated into /dev/md0 which will have a total
capacity of 180MB. If you write a 21 MB file to a clean file system, you will g
et
the first 20MB (or so) on /dev/sda4 and the last 1MB will spill into the start o
f
/dev/sdb5.
In striped (RAID-) mode, you will be wise to have all partitions the same size,
so
for example, on my system I have /dev/sdc5 = 256MB, /dev/sdd5 = 256MB, /dev/sde5
=
256MB, /dev/sdf5 = 256MB and /dev/sdg5 = 256MB.
When defining /dev/md0, I used stripes of 32K. Using the same simple-minded fil
e,
If you create a file of 512KB, the first 32K will be in /dev/sdc5, the 2nd 32K i
n
/dev/sdd5, etc.
Once setup (very simple), all this is transparent. You just use /dev/md? instea
d of
/dev/sd?x or /dev/hd?x.
Performance is pretty good (not as good as a true-blue RAID controller) and the
ability to create very large file systems is very important to some of us.
Hope this answers it.
Sincerely Yours,
Simon
Simon Shapiro i-Connect.Net, a Division of iConnect Corp.
Shimon@i-Connect.Net 13455 SW Allen Blvd., Suite 140
503.677.2911 Beaverton, OR 97008