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The atacontrol command can also be used to create purely software

RAID arrays in systems that do NOT have a "real" hardware RAID card

such as a Highpoint or Promise card.  A common scenario is a 1U

server such as the HP DL320 G4 or G5.  These servers contain a SATA

controller that has 2 channels that can contain 2 disks per channel,

but the servers are wired to only place a single SATA drive on each

channel.  These servers do have a "pseudo" RAID BIOS but it uses

a proprietary format that is not compatible with the ata driver, and

thus their RAID bios must be switched off.  Another common scenario

would be a Promise UDMA100 

controller card that did not contain the Fasttrack RAID BIOS, but

did contain 2 UDMA channels. One channel would be put on 1 disk

and the other channel on the other disk.  It is NOT recommended to

create such arrays on a Primary/Secondary pair on a SINGLE channel

since the throughput of the mirror would be severley compromised,

the ability to rebuild the array in the event of a disk failure

would be greatly complicated, and if a disk controller electronics

failed it could wedge the channel and take both disks in the

mirror offline. (which would defeat the purpose of having a mirror

in the first place)

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A quick and dirty way to create such a mirrored array on a new

system is to boot off the FreeBSD install CD, do a minimal scratch

install, abort out of the post install questions, and at the command

line issue the command:

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.Dl "atacontrol create RAID1 ad4 ad6"

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then immediately issue a reboot and boot from the installation CD

again, and during the installation, you will now see "ar0" listed

as a disk to install on, and install on that instead of ad4, ad6, etc.

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To get information about the status of a RAID array in the system

use the command line:

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.Dl "atacontrol status ar0"

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A typical output showing good health on a RAID array might be as

follows:

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.Dl "ar0: ATA RAID1 subdisks: ad4 ad6 status: READY"

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If a disk drive in a RAID1 array dies the system will mark the disk

in a DOWN state and change the array status to DEGRADED.  This can

ALSO happen in rare instances due to a power fluctuation or other event

causing the system to not shutdown properly.  In that case the output

will look like the following:

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.Dl "ar0: ATA RAID1 subdisks: ad4 DOWN status: DEGRADED"

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For a mirrored RAID1 system the server WILL ALLOW you to remove a

dead SATA disk drive (if the drive is in a hot-swap tray) without

freezing up the system, so you can remove the disk and while your

obtaining a replacement the server can run off of the active disk.

The only caveat is that if the active disk is ad6, the system most

likely will NOT be able to be rebooted since most systems only

support booting off of the first disk drive.

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To deactivate the DOWN disk ad6 to allow for it to be ejected, use

the following:

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.Dl "atacontrol detach ata3"

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then eject or remove the disk. Note that this only works if the 2 disks

in the mirror are on separate channels (which is the standard setup for

1-U servers like the HP DL320)  When the new disk drive is obtained,

make sure it is blank, then shut the system down.  At this point, if

the system has a RAID array card like a Hipoint or Promise controller,

you may then boot it into BIOS of the card and use the manufacturers

RAID array rebuild utilities to rebuild the array.

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If the system has a pure software array and is not using a "real" ATA

RAID controller, then shut the system down, make sure that the disk

that was still working is moved to the bootable position (channel 0

or whatever the BIOS allows the system to boot from) and the blank disk

is placed in the secondary position, then boot the system into 

single-user mode and issue the command:

.Pp

.Dl "atacontrol addspare ar0 ad6"

.Dl "atacontrol rebuild ar0"

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If the disk drive did NOT fail and the RAID array became unmirrored due

to a software glitch or improper shutdown, then a slightly different

process must be followed.  Begin by issuing the detach command (this

shows the detach for disk ad6, the primary master on channel 3):

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.Dl "atacontrol detach ata3"

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then reboot the system into single-user mode.  (don't just init the system,

reboot it so that both disks get probed)  You will probably see TWO mirrored

RAID arrays appear during the boot messages, ar0 and ar1.  Issue the

command:

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.Dl "atacontrol delete ar1"

.Dl "atacontrol addspare ar0 ad6"

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Now a status command will show the array rebuilding.

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