&KDSWHU&4XLFN6HWXS
5$,'/HYHO
7\SHRI'LVN$UUD\
0LQLPXPQXPEHURIKDUGGLVNV
5$,'
GDWDVWULSLQJ
5$,'
GLVNPLUURULQJ
5$,'
GDWDVWULSLQJZLWKSDULW\GULYH
5$,'
GDWDVWULSLQJZLWKVWULSHGSDULW\
5$,'
GDWDVWULSLQJDQGPLUURULQJ
The desired usable disk space of the disk array as well as the following two issues have a
direct impact on the number of physical hard disks needed.
&&
:
:KKLLFFK
K//HHYYHHOORRII55HHGGXXQQGGDDQQFF\\LLVV11HH
HHGGHHG
G""
To come straight to the point,
RAID 0
(data striping) does not imply any redundancy at all
(the
R
in front of the
AID
is rather misleading). On the other hand, a RAID 0 disk array is
pretty fast, since no parity information is required. With
RAID 1
(disk mirroring), the data is
100% redundant because it is mirrored. This is definitely the highest level of redundancy,
but the most expensive one, too. An interesting combination of RAID levels 0 and 1 is
RAID
10.
Two RAID 0 stripe sets are simply mirrored. If one drive fails, the data are still available
on the mirrored drive. With
RAID 4
(data striping with dedicated drive) and
RAID 5
(data
striping with striped parity), parity information is calculated from the present data with a
simple mathematical operation (eXclusive OR, XOR), and stored either to one dedicated
drive (RAID 4) or to all drives (RAID 5). If one drive should fail, the data of the defective
drive can be reconstructed on the basis of the normal user data and the previously calcu-
lated parity data. RAID levels 4, 5 and 10 can tolerate the failure of one drive just as RAID 1,
but in comparison to the latter, RAID 4, RAID 5 or RAID 10 are less expensive.
As already mentioned before, the entire disk array controlling function is carried out at con-
troller level and therefore does not load the host computer.
Let us have a look at the following table which explains the correlations between the RAID
level, usable disk capacity and number of physical hard disks. To make things easier, we
consider identical 1 GB hard disks:
8VDEOHVWRUDJHFDSDFLW\RIWKHGLVNDUUD\
5$,'/HYHO
KDUGGLVNV
KDUGGLVNV
KDUGGLVNV
KDUGGLVNV
5$,'
*%
*%
*%
*%
5$,'
*%
*%
*%
*%
5$,'
*%
*%
*%
5$,'
*%
*%
*%
5$,'
*%
It is quite obvious that the redundancy of level RAID 1 soon becomes very expensive when
more than 2 hard disks are used. Only with RAID 4 and RAID 5 have you a reasonable rela-
tion between storage capacity and expenses for the disk array.
&&
''R
RZ
ZH
H11HH
HHG
G++RRWW))LL[[GGUULLYYHHVV""
In other words: Should RAIDYNE automatically reconstruct the lost data after a hard disk
failure ?
One of the reasons that have led you to choose
R
AID disk arrays definitely lies with the
r
e-
dundancy, that is, the data security you still preserve even in the event of disk failure, thus
resting assured against loss of data and time.