Spanning Sets - Impediment

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					                    Large Arrays with ‘Spanning’
Mylex support ‘spanning’ that offers a construction of very large arrays with
improved performance and higher fault tolerance.

Spanning drive packs provides the ability to configure multiple or parts or multiple drive packs
as one large system drive. This allows more than 16 physical disk drives to be configured as a
system drive. A system drive can span up to 16 drive packs. The controller creates the
spanned system drive during the array configuration process. Data is striped across the drive
packs of the spanned system drive.

        Figure 1. The System Drive Spanning illustrates spanning across several drive packs.

Note: Spanning and M.O.R.E.TM (Mylex Online RAID Expansion) operations are mutually exclusive.
       A request to perform a M.O.R.E. operation will be rejected if any system drive is spanned across
       multiple drive packs.

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Mylex offers the full suite of RAID levels (00, 10, 30, 0+1+0, & 50) by ‘spanning’

When creating a disk array, the number of disks that can be organized into the array is limited
by the RAID level chosen. For instance, RAID level 1 limits the size of the array to 2 drives.
With RAID level 5, it is normal to want to limit the number of drives that are protected by a single
parity. Not only does too large of a RAID 5 array affect performance but it also increases the
possibility that a double drive failure will cause loss of data.

                                      RAID Level 1

Figure 2. RAID level 1 configures two disk drives as a mirrored pair. All data is 100% duplicated on an
          equivalent disk drive. Fault tolerance is provided by complete duplication of data. RAID level
          1 is configured from two disk drives, no more, any less.

If multiple disk arrays could be interleaved at a low level, so that disk operations are spread
evenly over all of the disk drives, then the limitations on disk array size could be removed. This
is precisely the motivation for and the objective of RAID level 10 and RAID level 50.

With Mylex RAID 10, up to sixteen RAID 1 arrays can be interleaved to create one drive
array with up to 32 drives. Each of the individual RAID 1 arrays retains its independent
fault protection so that, in certain circumstances, up to 16 drive failures can be tolerated.
But the RAID 10 array of 32 drives, give the full balanced performance across all 32
drives. This results in a fault tolerance arrays of 32 drives for capacities of over 2TB
(based on 73GB drive).

Note: The principle of spanning applies to all of Mylex RAID level supported. For example, RAID level
       0, by spanning you obtain RAID level 00.

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The advantages of Mylex RAID level 50

The Mylex RAID 5 array will support up to 16 drives. By using Mylex RAID 50, the array
size may be increased to an array size of 256 drives to build larger arrays of very high
capacities of over 18TB! Again, each of the individual RAID 5 arrays has independent
fault protection, so that a RAID 50 array comprised of 16 RAID 5 arrays will be able, in
many situations, to tolerate 16 drive failures. Also, the number of these arrays that are
interleaved to form the RAID 50 array magnifies the performance of the individual RAID 5

                                         RAID Level 5

Figure 3. RAID level 5 data is striped across several disk drives. Parity protection is used for data
          redundancy and fault tolerance.

RAID levels 10 and 50 can tolerate multiple drive failures, as long as there is no more
than one failure per pack (physical array). There is also a performance advantage since
files can be spread across a high number of spindles.

    Spanning allows construction of very large arrays with improved
                performance and higher fault tolerance.

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