Dell PowerVault MD3000i Manuale di sintonizzazione - Pagina 8

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Dell™ PowerVault MD3000 and MD3000i Array Tuning Best Practices

4.1.1 Selecting a RAID Level - High Write Mix Scenario

In random I/O applications with a >10% mix of write operations and a low degree

of burstiness, RAID 1/10 provides the best overall performance for redundant

disk groups.

RAID 1/10 performance can be >20% better than RAID 5 in these environments,

but has the highest disk cost; for instance, more physical disks must be

purchased. RAID 5 provides protection and minimizes disk cost for net capacity

but is heavily impacted by the write performance overhead of parity updates.

RAID 6, while providing better protection than RAID 5 with minimal disk cost, is

more heavily impacted by write performance overhead for the double parity

calculations it requires.

In sequential I/O applications with relatively small write transfer sizes, RAID level

does not make a large difference. With medium transfer sizes, RAID 1/10 can

provide an advantage over RAID 5/6, again with a higher associated disk-cost.

In very large sequential writes, RAID 5 can perform equal or better than RAID

1/10 especially when factoring in disk cost for equivalent capacity. Additionally,

better performance is always achieved when the application or OS is capable of

buffering or coalescing writes to fill a whole segment or stripe.

In random I/O applications with a low mix (<10%) of write operations, RAID 5

offers approximately the same performance as RAID 1/10 in small transfer sizes

but at a lower disk cost. RAID 0 provides somewhat better performance than 5

or 1/10 but offers no data protection. In larger transfer size environments, RAID

1/10 performances can be slightly better than RAID 5 but has a much higher disk

cost.

When optimizing performance, there are many factors to consider, such as drive

type, capacity, and the number of drives.

The following general guidelines can be used in grouping drives in a disk group:

• Separate random and sequential workloads on different disks groups –

segregating I/O traffic to minimize sharing of disk groups between virtual

disks.

• Choose faster drives – in general, a single 15K RPM drive yields

approximately 20% more performance than 10K for mixed sequential

and random operations. Please refer to the manufacturer's data sheets

to better determine the optimally suitable drive.

• Adding more drives to a disk group while holding the stripe size fixed

can increase I/O rate for sequential I/O, up to the point of controller

saturation – more drives mean more spindles to service I/O.

December 2008 – Revision A01

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