This appendix provides an example of how the maximum capacity of a DA can be calculated.

Ignoring overhead for checksum data and some internal VDisks (for example, loghome), the total raw capacity is the mount of PDisks times the capacity per drive. For example, assume 1 TB disks are available. For a fully utilized DA, this following calculation is used:

Depending on the block size of the VDisks, the amount of data per VDisk track varies. Per VDisk track, a fixed check sum trailer of 4 K is added at each PDisk segment. Therefore, the overhead can be estimated by using the following calculations (depending on RAID level and block size):

Therefore, the maximum usable space for a VDisk that is built on 1 TB drives is less than 134 TB. For our example, we continue with 134 TB.

Taking a fully utilized DA (174 PDisks in a GL6), with a VDisk 8+2p and a capacity of 134 TB as an example, the data that is affected by a single failure is 10/174 * 134 TB ~ 7.8 TB.

Only 8/10 out of this 7.8 TB is user data, while the rest is parity. Also, only one segment per VDisk track is affected and needs to be rebuilt. Finally, 1/10 x 7.8 TB = 0.78 TB must be rebuilt. By having a reserved hot spare capacity of six drives, enough room is available to allocate the disk space required for rebuilding.