Brocade Communications Systems StorageWorks 4400 - Enterprise Virtual Array Instrukcja Użytkownika Pobierz pdf (Strona 8)

To determine the number of spindles required for performance, the following simple formula can be used:

Number of drives needed =

(Total IOPs * RAID penalty * write %) + (Total IOPs * read %)

Raw performance of the disk drive

The RAID penalty represents the I/O overhead associated with a RAID level; that is, the number of I/Os to

the physical disks incurred by a write operation coming in from a host. Table 1 lists the penalties for every

supported RAID level on the EVA.

Table 1. RAID-level penalties

Raid Level

I/O Penalty

RAID 0

RAID 1

RAID 5

RAID 6

Note: When data is replicated remotely, application performance is not

necessarily improved by increasing the number of disks in a disk group, because

response time for application writes includes the time for replication. In

synchronous mode, performance will likely be limited by replication before it is

limited by the number of disks.

Note: Sequential access (read or write) is limited by the per-disk performance

rather than by the number of disks in the disk group. Consider approx 10 MB/s

per physical disk.

EVA disk group capacity planning

After determining the amount of capacity needed for the virtual machines operating system files and data, it

is important to understand how much formatted capacity you will get from a physical drive and what RAID

overhead will be applied to the group of disks. In the industry, disk drive capacity is expressed and

advertised using the decimal representation. This is a source of much confusion since most software

including operating systems or the EVA firmware use a binary representation. For example a 300 GB disk

once configured in the EVA has a formatted capacity expressed in binary terms of only 279.39 GB or

approximately 7% difference.

The EVA uses active sparing for data protection. The spare space for disk failure protection is actually

spread across the entire disk group. This provides instant availability to the EVA to transfer data and rebuild

a failed drive. Doing so ensures that there is always enough available capacity and no possibility that the

disk spare could go bad, leaving you without protection. The sparing capacity equates to two times the

largest hard drive in the disk group for single sparing and four times for double sparing.

The following formula is used to determine the number of physical disks needed to deliver the required

capacity.

Number of drives needed =

Data Size + (Data Size* Raid overhead)

+ Sparing Capacity

Formatted Capacity

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 37 38

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Brocade Communications Systems StorageWorks 4400 - Enterprise Virtual Array Instrukcja Użytkownika Strona 8