Dell PERC 4/SI - Manual (Page 44)

Spanned Drives
You can arrange arrays sequentially with an identical number of drives so that the drives in the different arrays are spanned. Spanned drives can be treated
as one large drive. Data can be striped across multiple arrays as one logical drive. The maximum number of spans is eight.
You can create spanned drives using your array management software, which is the BIOS Configuration Utility.
Storage in an Array with Drives of Different Sizes
For RAID levels 0 and 5, data is striped across the disks. If the hard drives in an array are not the same size, data is striped across all the drives until one or
more of the drives is full. After one or more drives are full, disk space left on the other disks cannot be used. Data cannot be written to that disk space
because other drives do not have corresponding disk space available.
Figure 4 -2 shows an example of storage allocation in a RAID 5 array. The data is striped, with parity, across the three drives until the smallest drive is full. The
remaining storage space in the other hard drives cannot be used because not all of the drives have disk space for redundant data.
Figure 4-2. Storage in a RAID 5 Array
Storage in RAID 10 and RAID 50 Arrays
You can span RAID 1 and 5 arrays to create RAID 10 and RAID 50 arrays, respectively. For RAID levels 10 and 50, you can have some arrays with more storage
space than others. After the storage space in the smaller arrays is full, you can use the additional space in larger arrays can store data.
Figure 4 -3 shows the example of a RAID 50 span with three RAID 5 arrays of different sizes. (Each array can have from three to 14 hard disks.) Data is striped
across the three RAID 5 arrays until the smallest array is full. The data is striped across the remaining two RAID 5 arrays until the smaller of the two arrays is
full. Finally, data is stored in the additional space in the largest array.
Figure 4-3. Storage in a RAID 50 Array
NOTE: Using hard disk drives of different sizes is not recommended.