[dm-crypt] unusual writing speeds when benchmark speeds are high enough

Matthias Schniedermeyer ms at citd.de
Mon Mar 18 11:43:59 CET 2013


On 18.03.2013 11:01, Heinz Diehl wrote:
> On 18.03.2013, Matthias Schniedermeyer wrote: 
> 
> > The SSD i have can reach (with encryption) about 400MB/s, but that is 
> > best-case performance when all moons are aligned. I haven't really 
> > benchmarked the real-world performance, the dd-example (with conv=fsync) 
> > only got 88MB/s.
> 
> On all my SSD disks, cryptsetup performance is not even near that. I
> have about 450 MB/s without encryption, and max. 120 MB/s with. 

SSD performance has more 'it depends' than anything else.
It isn't funny. ;-)

But as i don't need my SSDs for write-heavy linear performance (much), 
the much better random-write rate than HDDs still make them worthwile. 
And the much better (random) read performance is the icing.


Overwriting appers to have better performance, but that is nothing i 
would do regularly.
dd if=/dev/zero of=zero bs=1M count=10k oflag=direct
10737418240 bytes (11 GB) copied, 116.629 s, 92.1 MB/s

dd if=/dev/zero of=zero bs=1M count=10k oflag=direct conv=notrunc
10737418240 bytes (11 GB) copied, 50.3905 s, 213 MB/s

dd if=/dev/zero of=zero bs=1M count=10k oflag=direct conv=notrunc
10737418240 bytes (11 GB) copied, 51.0608 s, 210 MB/s


My previous SSD with a Sandforce controller had much worse "worst case" 
performance as the Sandforce controller really doesn't like 
incompressible data and encrypted data looks like (or at least should 
look like) incompressible random-data.



-- 

Matthias


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