Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

DRAFT -- Revision 2.0
August 22, 1996Page 44
2-way LVM mirroring of standalone disks
RAID level 0/1 or 1
RAID levels 3 or 5
Data redundancy is lost with the failure of any one disk mechanism with one-
level redundancy. Exposure time is the length of time it takes to replace the
failed mechanism and recovery the data onto the replacement disk.
The need for double data redundancy
Two-level data redundancy can be accomplished with:
3-way LVM mirroring of standalone disks
2-way LVM mirroring of RAID level 0/1, 1, 3, or 5 disk arrays
Although much more expensive due to the additional standalone disks or arrays,
two-level data redundancy provides protection from a double failure. In addition,
it gives the ability to take one side of the mirror offline for backup or data
maintenance and still maintain data redundancy. Some have used this feature
to maintain an original copy of the data that can be restored very quickly in case
of corruption or other data loss.
The need for continuous data redundancy after a disk mechanism has failed and
before it is replaced
Do I require data redundancy during the time that a disk mechanism has failed
and not yet been repaired? This is really an exposure to risk issue; in another
words, is the risk of a second failure before repair of the first failure acceptable?
LVM mirroring supports 3-way mirroring that could meet this requirement. There
is still a primary and backup copy that exists even if one disk in the mirror fails.
HADA disk arrays have a feature to designate one of the disk mechanisms as a
hot standby that will be reassigned by the storage processor in case of a
mechanism failure. This feature continues data protection while waiting for a
failed disk mechanism to be replaced and is most useful in lights-out
environments or when failed disk mechanism replacement has to be scheduled.
Hot standby disks can be reassigned in an LVM mirrored environment.
However, to do this, a series of LVM commands must be used to reassign a
spare standalone disk as described in Technical HPPA Newsletter # 218.