[dm-crypt] Reconsidering default options for cryptsetup-reencrypt

DarKRaveR dark at verfeiert.org
Mon Dec 3 01:27:56 CET 2012


Sorry for the late reply,

just a small followup. What you are saying Milan, is absolutely true,
but for misaligned HW Raid setups, a huge block size together with
O_DIRECT will bypass the OS'es caching strategy and the controller
(knowing the actual layout) has a way better chance of compensating
unnecessary IO with it's optimized write back caching (in the linear
read-modify-write case of reenrypt). For rather small block sizes (near
the size of a RAID stripe) in a misaligned case, such a
compensation/elimination of unneeded IO is much harder.

So small blocksize, lack of topology information, misalignment -> approx
50% loss in performance (as surveilled by opener)

Regards

-Sven


On Sat, 2012-11-24 at 21:59 +0100, Milan Broz wrote:
> On 11/24/2012 06:01 PM, Sven Eschenberg wrote:
> 
> > BTW, what exactly are you referring to, when you talk about 64 MB blocksize?
> 
> Here cryptsetup-reencrypt is in principle simple program, it reads a "block"
> and write it back to device with new encryption parameters (and optionally with
> some different offset). So block here is meant as an unit which is handled in one
> reencryption step.
> (But the real atomic unit of encryption is still 512B block of course.)
> 
> There is no requirement this block to be aligned to underlying hw alignment
> (but if it is misaligned, the same performance degradation problems apply of course).
> 
> Milan
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