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NAME

softraid — software RAID

SYNOPSIS

softraid0 at root

DESCRIPTION

The softraid device emulates a Host Bus Adapter (HBA) that provides RAID and other I/O related services. The softraid device provides a scaffold to implement more complex I/O transformation disciplines. For example, one can tie chunks together into a mirroring discipline. There really is no limit on what type of discipline one can write as long as it fits the SCSI model.

softraid supports a number of disciplines. A discipline is a collection of functions that provides specific I/O functionality. This includes I/O path, bring-up, failure recovery, and statistical information gathering. Essentially a discipline is a lower level driver that provides the I/O transformation for the softraid device.

A volume is a virtual disk device that is made up of a collection of chunks.

A chunk is a partition or storage area of fstype “RAID”. disklabel(8) is used to alter the fstype.

Currently softraid supports the following disciplines:

RAID 0

A striping discipline. It segments data over a number of chunks to increase performance. RAID 0 does not provide for data loss (redundancy).

RAID 1

A mirroring discipline. It copies data across more than one chunk to provide for data loss. Read performance is increased, though at the cost of write speed. Unlike traditional RAID 1, softraid supports the use of more than two chunks in a RAID 1 setup.

RAID 4

A striping discipline with a fixed parity chunk. It stripes data across chunks and provides parity to prevent data loss of a single chunk failure. Read performance is increased, though write performance is limited by the parity chunk.

RAID 5

A striping discipline with floating parity across all chunks. It stripes data across chunks and provides parity to prevent data loss of a single chunk failure. Read performance is increased; write performance should be faster than RAID 4.

CRYPTO

An encrypting discipline. It encrypts data on a single chunk to provide for data confidentiality. CRYPTO does not provide redundancy.

CONCAT

A concatenating discipline. It writes data to each chunk in sequence to provide increased capacity. CONCAT does not provide redundancy.

EXAMPLES

An example to create a 3 chunk RAID 1 from scratch is as follows:

Initialize the partition tables of all disks:

# fdisk -iy wd1
# fdisk -iy wd2
# fdisk -iy wd3

Now create RAID partitions on all disks:

# printf "a\n\n\n\nRAID\nw\nq\n\n" | disklabel -E wd1
# printf "a\n\n\n\nRAID\nw\nq\n\n" | disklabel -E wd2
# printf "a\n\n\n\nRAID\nw\nq\n\n" | disklabel -E wd3

Assemble the RAID volume:

# bioctl -c 1 -l /dev/wd1a,/dev/wd2a,/dev/wd3a softraid0

The console will show what device was added to the system:

scsibus0 at softraid0: 1 targets
sd0 at scsibus0 targ 0 lun 0: <OPENBSD, SR RAID 1, 001> SCSI2
sd0: 1MB, 0 cyl, 255 head, 63 sec, 512 bytes/sec, 3714 sec total

It is good practice to wipe the front of the disk before using it:

# dd if=/dev/zero of=/dev/rsd0c bs=1m count=1

Initialize the partition table and create a filesystem on the new RAID volume:

# fdisk -iy sd0
# printf "a\n\n\n\n4.2BSD\nw\nq\n\n" | disklabel -E sd0
# newfs /dev/rsd0a

The RAID volume is now ready to be used as a normal disk device. See bioctl(8) for more information on configuration of RAID sets.

SEE ALSO

bio(4), bioctl(8), disklabel(8), fdisk(8), newfs(8)

HISTORY

The softraid driver first appeared in OpenBSD 4.2.

AUTHORS

Marco Peereboom.

CAVEATS

The driver relies on underlying hardware to properly fail chunks.

The RAID 1 discipline does not initialize the mirror upon creation. This is by design because all sectors that are read are written first. There is no point in wasting a lot of time syncing random data.

The RAID 4 and 5 disciplines do not initialize the parity upon creation. This is due to the scrub functionality not being currently implemented.

Currently there is no automated mechanism to recover from failed disks.

softraid has boot support only for the CRYPTO and RAID 1 disciplines, and only on amd64 and i386 platforms.

Sparc hardware needs to use fstype “4.2BSD” instead of “RAID”.

Certain RAID levels can protect against some data loss due to component failure. RAID is not a substitute for good backup practices.