Re: Benchmarking btrfs on HW Raid ... BAD

[Daniel J Blueman <daniel.blueman@gmail.com>]

On Mon, Sep 28, 2009 at 9:17 AM, Florian Weimer <fweimer@bfk.de> wrote:
> * Tobias Oetiker:
>
>> Running this on a single disk, I get the quite acceptable results.
>> When running on-top of a Areca HW Raid6 (lvm partitioned)
>> then both read and write performance go down by at least 2
>> magnitudes.
>
> Does the HW RAID use write caching (preferably battery-backed)?

I believe Areca controllers have an option for writeback or
writethrough caching, so it's worth checking this and that you're
running the current firmware, in case of errata. Ironically, disabling
writeback will give the OS tighter control of request latency, but
throughput may drop a lot. I still can't help thinking that this is
down to the behaviour of the controller, due to the 1-disk case
working well.

One way would be to configure the array as 6 or 7 devices, and allow
BTRFS/DM to mange the array, then see if performance under write load
is better, and with or without writeback caching...

Daniel
-- 
Daniel J Blueman