2
RAIDTypes
2-7
RAID Types and Tradeoffs
For each write to a RAID3 Group, the storage system
1. Calculates the parity data.
2. Writes the new user and parity data.
RAID 1 Mirrored Pair A RAID 1 Group consists of two disks that are mirrored
automatically by the storage-system hardware.
RAID 1 hardware mirroring within the storage system is not the same
as software mirroring or hardware mirroring for other kinds of disks.
Functionally, the difference is that you cannot manually stop
mirroring on a RAID 1 mirrored pair, and then access one of the
images independently. If you want to use one of the disks in such a
mirror separately, you must unbind the mirror (losing all data on it),
rebind the disk in as the type you want, and software format the
newly bound LUN.
With a storage system, RAID1 hardware mirroring has the following
advantages:
•automatic operation (you do not have to issue commands to
initiate it)
•physical duplication of images
•a rebuild period that you can select during which the SP recreates
the second image after a failure
With a RAID 1 mirrored pair, the storage system writes the same data
to both disks, as follows.
Figure2-4 RAID 1 Mirrored Pair
EMC1817
Second disk
…
…
First disk
User data
1
1
2
2
3
3
4
4
0
0