single disk vs. RAID5 (amr)

[Scott Long]

Mikhail Teterin wrote:
> [Subject changed]
> 
> п'ятниця 09 червень 2006 13:20, Scott Long написав:
> 
>>>I never benchmarked the speed on the single drives, other than to compare
>>>with my RAID5 array (which puzzlingly always loses to a single drive, but
>>>that's a different story).
>>
>>All depends on access alignment and cache behaviour.
> 
> 
> Well, this does not tell me much :-( I compare a single disk vs. a RAID5 array 
> of 6 such disks joined using amr(4). I tried to figure out the best 
> FS-parameters for the RAID, but whatever I tried, it was always quite 
> inferior to the the single drive -- on both reading and writing.
> 
> Perhaps, adding the battery-backup option to the RAID card would improve 
> things, but I expected it to be faster even without it.
> 
> 	-mi

THere are two things to consider.  First is that FreeBSD mis-aligns the
filesysytem blocks if you are using an fdisk/MBR header on the disk. 
This misalignment doesn't affect single disks, but it greatly affects
RAID, and especially RAID-5.  An easy way to check this is to compare
the performance on an array that has just the filesystem on it and no
MBR or disklabel information.

The second is that the LSI controllers will turn the cache off if a
battery is not present.  This won't affect read speed much, but it will
greatly affect write speed.

Scott