HPFOCUS

======Posted by Jon======

Can anyone provide guidance on a time-per-GB for RAID rebuild times for a P420i with 2GB cache ?

Approximate or min and max would be fine.

I need this for both 6Gb 10K SAS HDD (eg 900GB drive) for a RAID5 set and also for 12Gb 800GB SSD drives in a RAID1 set as well.

=======Posted by Robert=======

There are several factors that affect rebuild performance.

Drive speed is the main limit.

• A HDD is limited by its rpm and bit density.
• An SSD is limited by how many flash cells the drive has in parallel. Ideally, they can saturate their interface link rate.
• Writes to SSDs are usually slower than reads from SSDs.

Other bottlenecks:

• As the RAID array gets bigger, more drives must be read to rebuild the data. 7+1 RAID-5 will outperform 15+1 RAID-5.
• Controller memory bandwidth (XOR engine). Use a 64-bit wide cache module for best results (2 GB qualifies).
• Not using all the controller phys (e.g., putting all drives behind one x4 port rather than scattering them across both x4 ports)
• Slower SAS links between the controller and drive (e.g., 12 Gb/s drive in a 6 Gb/s backplane or JBOD). The P420i is 6 Gb/s, so won't ever attain 12 Gb/s from that SSD.

Some things help rebuild performance:

• Smart Array divides the logical drive in 16 regions and only rebuilds the sections that have been written