I have played with various RAID-5 setups though I'd post some speed results and interesting findings

SetupRAID LevelRead MB/per secWrite MB/per secHDD Count

AMD/ASUS/Windows 7/Onboard, Not NASRAID-5175MB60MB4

ReadyNAS with ARM ProcessorRAID-540MB12MB4

AMD/ASUS/OpenFiler V2.99RAID-595MB60MB4

AMD/ASUS/unRAID V5.0-Beta14 OSunRAID95MB70MB3

 

Something I noticed with unRAID, unlike all other tests I have done with RAID-5, while I am writing a file to Disk 1, only Disk 1 and the parity drive spin up. Disk 2 is spun down the whole time. How is unRAID calculating the parity without reading Disk 2. It is Reading Disk 1 and Parity, then working out what Disk 2 is, changing Disk 1 and writing the new parity, if so, it is ingenouse and suprisingly fast.

This isn't interesting at all since this is covered in multiple topics already in the forums, use the search to find these other topics.

 

The benefit to unRAID is it only needs to spin up the Parity drive and the drive being written to for updates. Also when reading it only needs to spin up the drive being read from.

 

Think of it as RAID-4 but with a dedicated drive for parity. In order to update a drive, unraid uses the following formula:

 

A1) Read Data Old Sector N

A2) Read Parity Old Sector N

B) Compute Parity Formula removing Data Old Sector N from Parity Old Sector N, then Compute Parity Formula using Data NEW Sector N to get Parity  NEW Sector N

C1) Write Data NEW Sector N

C2) Write Parity NEW Sector N

 

I believe the parity formula is a simple XOR with even parity result. It is simplistic and extremely fast to calculate.

 

The write speeds of unRAID are limited by the latency of the slowest drive in the mix and because it has to do 4 I/O operations per write.

What I'm interested in is how you managed to get 70MB/s writes out of unRAID 

This is pretty non scientific findings since my raid5's perform at better then 400MB/s read/write.

 

@fast_ant. Cache drive?

What I'm interested in is how you managed to get 70MB/s writes out of unRAID 

 

As am I.  a writeread10gb test bursts at 55MB/s on the local machine and slowly drops GB by GB to 35MB/s after 10GB.

 

For me the burst (Kernel tuning) is what I need as I usually access allot of small files over the network.

If the drives are all up and spinning write access is as fast as other machines on my lan for small files.

 

70MB/s, I'm gonna have to revisit some tunings to see how to achieve that.