Selecting A Raid Level - High Write Mix Scenario; Selecting A Raid Level - Low Write Mix Scenario; Choosing The Number Of Drives In A Disk Group - Dell PowerVault MD3000i Tuning Manual

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.

4.1.2 Selecting a RAID Level - Low Write Mix Scenario

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.

4.2 Choosing the Number of Drives in a Disk Group

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 Page 8