The IBM RAID Controller provides the Server 95A with a
data redundancy technique that distributes data across all the drives in
the array. In the event of a single drive failure, read and write
requests are satisfied by the remaining disks in the array, without loss
of data. The defunct hard disk drive appears to be operating correctly,
if at reduced performance, despite its total failure.
The Server 95A supports RAID levels 0, 1, and 5. Following are descriptions of these three RAID levels.
RAID Level 0
RAID level 0 stripes the data across all of the drives of the array.
Warning: Drive failure results in loss of data in logical drives assigned RAID level 0.
This RAID level offers substantial speed enhancement, but allows for no data redundancy; therefore, a hard disk failure within the array results in loss of data in the logical drive assigned RAID level 0, but only in that logical drive.
Note: Although in this case logical drives assigned RAID level 1 or 5 in the same array do not suffer loss of data, the status of these drives is Critical and they cannot sustain another hard disk drive failure. They operate at reduced performance, and the defunct hard disk drive should be replaced and rebuilt promptly to avoid a multiple hard disk drive failure.
When the defunct drive is replaced, all the logical drives assigned
RAID levels 5 and 1 are rebuilt onto that hard disk drive; the level-0
logical drive is defined, but the data in the level-0 logical drive is
Ed. To run individual
drives (one or two) on a RAID controller, define them as RAID 0 Array.
You are limited to two arrays on either the Passplay or Cheetah RAID controller.
RAID Level 1
RAID level 1 supported by the IBM RAID Controller provides an enhanced feature for disk mirroring that stripes data and copies of the data across all the drives of the array. The first stripe is the data stripe, and the second stripe is the mirror (copy) of the first data stripe but shifted one drive. Because the data is mirrored, the capacity of the logical drive when assigned level 1 is 50 percent of the physical capacity of the grouping of hard disk drives in the array.
If you have three or more drives in the array and you want data redundancy, it's a good idea to choose level 5 first. To learn why this might be to your advantage, see RAID Level 5.
RAID Level 1 Illustration:
DRIVE 1 DRIVE 2 DRIVE 3
The above illustrates three drives in an array, with one logical drive
assigned RAID level 1.
RAID Level 5
RAID level 5 stripes data and parity of the data across all the drives of the array. The capacity of the logical drive when assigned level 5 is reduced by one drive (for data parity storage).
Level 5 is generally the most desirable choice because it offers both data protection and increased throughput. It gives you higher capacity than level 1, but level 1 offers higher performance. If after using level 5 you are dissatisfied with the performance and can tolerate lower capacity, you can either redefine the level 5 to a level 1 (refer to Redefining Space in an Array), or you can use a logical drive that you have assigned level 1.
If you want to have a hot-spare drive and also assign level 5, you must have at least four hard disk drives and no logical drives assigned RAID level 0. To maintain capacity, the size of the additional drive can be larger but must be no smaller than the size of the three drives that came with your server. All the drives in an array are configured to the capacity of the smallest.
RAID 5 Rebuild Process Explained for Non-Techno-Dweebs
Helmut P. Einfalt speaks in a way we can understand:
Actually, for six HDs (one hot spare) the data are chopped up into chunks of userdefinable size (4k, 8k, 16k) and written to the disks in the following order:
HD1 HD2 HD3 HD4 HD5 SSS
CCC is the checksum, SSS in this case is the hot spare. That one isn't
If anything goes wrong, say, with HD2 the situation will be:
HD1 HD2 HD3 HD4 HD5 SSS
At that point the RAID will automatically calculate the missing data from "checksum minus available data" and insert the calculated data into the hot spare:
HD1 HD2 HD3 HD4 HD5 HDS
The dead HD2 will be marked as dead, and the array will work on with:
HD1 DDD HD3 HD4 HD5 HD2
Spare now has become HD2, while the original HD2 is marked as defunct
HD1 DDD HD3 HD4 HD5 HD2 RAID
RAID in this case is the controller supplying the missing data. In the penultimate case it is not required to do so, since only the checksum is missing and that one isn't what the system wants anyway.
If something goes wront at *that* instance, however, you're stuck...
I've run the last situation on my -B0C with 5x2,1 DFHS for a week before
The nice thing is that you're not limited to 6 drives -- you can go up to 15 max on the Cheetah, which makes the "loss" proportionally smaller. If I could get an SCSI enclosure to match the 9595-B0C (was it 3512?), I'd set up my system that way and keep *all* data in there, not only the critical ones...