Specifications

Best Practices for Virtualizing and Managing Exchange 2013

Striped storage spaces can be used for the following:

 Delivering the overall best performance in terms of reads and writes.

 Balancing the overall storage load across all physical drives.

 Backing up disks to increase backup throughput or to distribute the use of space across disks.

Mirror spaces: This data layout process uses the concept of mirroring to create copies of data on multiple

physical disks. A logical virtual disk is created by combining two or more sets of mirrored disks. Mirror

storage spaces are resilient in nature because in the event of failure, if one copy is lost, the other is still

available. To make them resilient from disk failures, mirror spaces are configured to at least one (two-way

mirror) or two (three-way mirror) concurrent physical disks.

In Figure 8, 512 KB of data needs to be written to the storage space. For the first stripe of data (A1),

Storage Spaces writes 256 KB of data to the first column, which is written in duplicate to the first two

disks. For the second stripe of data (A2), Storage Spaces writes 256 KB of data to the second column,

which is written in duplicate to the next two disks. The column-to-disk correlation of a two-way mirror is

1:2, while for a three-way mirror, the correlation is 1:3. Reads on mirror spaces are very fast because they

are done from either of the two copies of data. If disks 1 and 3 are busy servicing another request, the

needed data can be read from disks 2 and 4.

Figure 8: Mirror storage space across four disks

Mirror storage spaces are used for the following:

 Enabling faster reads on data.

 Increasing resiliency and protection from disk failures.

Parity spaces: Parity storage spaces store parity-bit information that helps in reconstructing data from a

failed disk. This can be useful in providing data recovery capabilities. Storage Spaces uses rotating parity

that stores data and parity information by rotating from stripe to stripe across different disks. Parity

spaces tend to have lower write performance than mirror spaces because each parity block takes time in

updating itself to the corresponding modified data block. Parity is more cost efficient than mirroring

because it requires only one additional disk per virtual disk, instead of double or triple the total number of

disks in an array.

In Figure 9, for the first stripe of data, 768 KB is written across disks 1 through 3 (A1, A2, A3), while the

corresponding parity bit (AP) is placed on disk 4. For the second stripe of data, Storage Spaces writes the

data on disks 1, 2, and 4, thereby rotating the parity to disk 3 (BP). Because parity is striped across all

disks, it provides good read performance and resiliency to single disk failure.