performance problem with gstripe

[Ivan Voras]

Joel Jacobson wrote:
> i have a bit of a weird issue, which i suspect is a configuration
> problem, and was looking for a little advice.  i have an LSI JBOD box
> with a bunch of SAS drives that i would like to gstripe together.  each
> drive individually seems to be able to do about 80 MB/sec streaming
> write, and doing parallel dd's gives me the 160 MB/sec i would expect. 
> if i gstripe them together with a 256k stripe width, i only see 80
> MB/sec, though.

How do you measure this? If with dd, what block size (bs) do you use?

> 
> if, however, i newfs/mount it as ufs and then dd myself a big file, that
> gets me about 120-130 MB/sec.
> 
> why does mounting matter?

I'd guess because of write caching that enables you to make use of
multiple stripes at once, if your dd bs is smaller than 2*stripe size.

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