3 HD's running in Raid 5 most efficent?

[magik20]

i was reading a article in Maxium PC that 3 HD's running in Raid 5 was the fastest , with a 4th HD slowing the Raid system down.

any truth to this? I was thinking of running 3 HD's in Raid 5 for my Gaming / Data Storage folder and running 2 HD's in raid 0 for my OS

 

[UICompE02]

In my experience (I'll have to look around and see if I can find any benchmarks to back this up) adding more disks to a RAID 5 volume will increase read speed and possibly decrease write speed.

Here's why:

Doing a read from a RAID 5 volume is a lot like reading from a RAID 0 volume. The data is striped across several disks, so you can benefit from the increased parallelism of having a striped volume. (The main difference between a RAID 0 and RAID 5 read, of course, is that the RAID 5 volume will also have to read the parity information for the data as well.) Therefore, depending on your RAID controller's power, and the bandwidth of the drive interface and system interface, adding more drives to RAID 5 will increase read performance due to striping.

When writing to a RAID5 volume, the controller must first read from the stripe of data you are writing to, perform an XOR of that data with the new data you are writing, and then commit that data back to the disk. So, as you can see, this requires many more operations, and the more disks you have, the more data you must read before the write to calculate the parity on. Of course, if you are writing full stripes only, this performance is much better since no data must be read from the disks to calculate the parity, you can do that with the write data only, therefore, this overhead isn't is great of a penalty if you are writing long continuous files rather than smaller noncontiguous files. Most higher-end RAID adapters will be smart enough in their caching algorithms to exploit this whenever possible to reduce the number of reads that must be done on a write.

So, I guess in short, the impact of adding more drives to a RAID 5 volume beyond the minimum three depends on what type of data accesses you will be doing and how intelligent your RAID controller is.

 

[Ice Czar]

how intelligent your RAID controller is.

and thats a huge factor, in higher end cards as UICompE02 will tell you
you often have large RAM caches (up to 256MB) and dedicated Processors for parity calculations

was Maxium PC running hardware RAID or Software RAID (Dynamic Disks with the parity calculations done by the CPU?)