Added another disk to raid 5 array, performance slightly decreased?

[Red Squirrel]

I have a raid 5 with 5 1TB drives and decided to add another disk. They are all the same brand and model. I actually saw a slight decrease in performance. I would normaly do about 150-200MB/sec writes, now I'm only getting about 100MB/sec. Is there a point with raid 5 where adding more drives no longer adds a performance increase? I'm using mdadm raid. Should I maybe stick to 5 drives, and just create separate arrays instead?

 

[greenhawk]

did you give it enough time to do the change over to a 6 disk array? Not farmilar with linux based raid (mdadm IIRC is for linux), but if the raid is being expanded in the background, any performance will be held up while this change occurs.

 

[Emulex]

raid expansion typically is done at a low priority as it restripes everything. and remember stripe size is important - i've found with some raid systems the stripe size is not the same with 3 drive as with 8 drive (max i'd do with raid-5).

 

[Red Squirrel]

Oh it's done, it was a 800 minute process so I let it go overnight.

And good point about chunk size... I should probably increase that. It's currently 64k

Edit: It just occurred to me, the numbers I was thinking of were when I was playing with values when benchmarking. By default the numbers are not as high but as I start playing with the chunk sizes I get better readings. I never really kept track of what values I used, so I can't really compare now.

I just hit 254MB/sec playing with different values with dd (did not try bonnie++ yet). I should have kept track of the values I used before. Flawed benchmarking. 😛 I can easily confirm I never saw anything past 200 before though, so guess performance DID increase.