[Lustre-discuss] RAID Stripe alignment

[Kaizaad Bilimorya]

Hello Andreas,

Thanks for your reply.

On Mon, 6 Dec 2010, Andreas Dilger wrote:
> Is your write cache persistent?

Yes. It is 512 MB battery backed.

> One major factor in having Lustre read and write alignment is that any 
> misaligned write will cause read-modify-write, and misaligned reads will 
> cause 2x reads if the RAID layer is doing parity verification.
>
> If your RAID layer is hiding this overhead via cache, you need to be 
> absolutely sure that it is safe in case of crashes and failover of 
> either or both the OSS and RAID controller.

The HP Smart Array P410 controller also has this setting called 
"Accelerator Ratio" which determines the amount of cache devoted to either 
reads or writes. Currently it is set (default) as follows:

  Accelerator Ratio: 25% Read / 75% Write

We can try setting it to one extreme and the other to see what difference 
it makes. This Lustre system is going to be used as /scratch for a broad 
range of HPC code with diverse requirements (large files, small files, 
many files, mostly reading, mostly writing) so I don't know how much we 
can tune this cache setting to help specific access patterns at the 
detriment of others, we are just looking for appropriate middle ground 
here. But for thread completeness, I post the sgpdd_survey results if 
there are any large differences in performance.

> Cheers, Andreas

thanks a bunch
-k