Last time I explained the basic premise of a spanned set. I used the example of a clients RAID that contained 3 36 GB SCSI drives. In a standard set each one of the drives would use one partiton for the entire drive and then the set would be mounted contiguously. The particular spanned set that I worked with had two spanned sets defined across the three drives.
In a soft spanned set the key to maintaining the array is using the Master Boot Records set of four possible partition records to define the size of each chunk of the array. In this case the first drive in the set defined three partitions. The following are the numbers for this drive. I only use three numbers from the partition record. The partition type, the relative sectors, and the total sectors.
The partition type tells the operating system either the file system type, or how this particular chunk of sectors is to be handled in relation to the others. The following is an example of the partition types for the spanned set I worked on.
Next, is the Partition Relative Sectors. This value designates how many sectors from the partition definition table the actual partition starts. Finally, the Partition Total Sectors should be self explanatory, as it is the total size of the partition.
As an example we can take a look at drive 0 and see that there are two defined partitions. The first partition is type 86. As an aside, all partition types are defined as hex. Type 86 is defined as the first drive in the spanned set. Normally a file system type is placed in the partition type field however the file system type in this case is defined by the the OS Boot Record found at sector 63. The second partition for drive zero is also defined as 86. So we know that this is the first partition chunk of a second spanned set. This partition starts at sector 55295730 and is 16868250 sectors in size.
Now that we have seen how the first drive is defined, and have seen that partition type 86 defines the beginning of a spanned volume, next time I will explain the partition types of 05, and 87, and how they relate to this spanned set.
Until next time…