Re: [dm-crypt] md-raid5+lvm+dm-crypt+kvm: one streaming write starves all reads

[Arno Wagner <arno@wagner.name>]

On Thu, Sep 17, 2009 at 02:36:39AM +0200, Christian Pernegger wrote:
> > Hmm. So without dm-crypt no problem? The main effect of dm-crypt
> > is to make the disk appear slower, because of the encryption.
> 
> Unfortunately it's not *just* slower. Yes, the arrays would be 2x-3x
> as fast without encryption, but that's not the point. It's that I/O
> bandwidth isn't shared fairly at all. As if all crypt requests went
> into one big FIFO ... (But then, maybe they do.) The stream with the
> most/biggest requests then gets all the bandwidth.

Yes. And that may be some s"synergetic" effect triggered by the 
slowness. At least that is one possible explanation.
 
> > You could try this with a dummy-cipher that does not take encryption time.
> 
> Excellent idea, what's the cipher string for that? I couldn't find
> anything obvious in /proc/crypto or under
> /lib/modules/.../kernel/crypto/.

I have not tried this myself, but hit candidates are
"ciper_null" and "ecb(ciper_null)". I have both (kernel 
2.6.30.5)

Arno
-- 
Arno Wagner, Dr. sc. techn., Dipl. Inform., CISSP -- Email: arno@wagner.name 
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