[dm-crypt] aes-xts-plain with aes_x86_64 makes my SSD 5x slower than my encrypted HD

Arno Wagner arno at wagner.name
Sun Jul 22 23:47:57 CEST 2012


On Sun, Jul 22, 2012 at 01:39:29PM -0700, Marc MERLIN wrote:
> On Sun, Jul 22, 2012 at 09:47:32PM +0200, Yves-Alexis Perez wrote:
> > > Any suggestions would be appreciated. 
> > 
> > I'm using Debian sid (so still at 3.2 kernel), currently using a 256G
> > Samsung SSD. What I get is:

SID? That would be "unstable", whit possible assorted problems.

[...]
> gandalfthegreat:~# dd if=/dev/mapper/ssdcrypt of=/dev/null bs=1M count=1024 
> 1073741824 bytes (1.1 GB) copied, 44.3302 s, 24.2 MB/s
> 
> atop shows dd isn't really pegging a single core:
> THR    SYSCPU   USRCPU     RDDSK   WRDSK   ST  EXC   S  CPUNR    CPU  CMD 
>   1     0.60s    0.01s    226.2M      0K   --    -   D      3     6%  dd

It would not, as AES-NI (AFAIK) does need very little CPU
assistance. AES-NI may be the problem though. Can you try with
the normal AES module? I think unloading the AES-NI module 
may be enough for that, but I am not sure. 

Maybe AES-NI needs very long for something it needs to do each 
sector. Google("aes-ni slow") found at least some indications that
aes-ni may still have problems.
 
[...]
> It shouldn't be a hardware problem since I do see 400MB/s before encryption.
>
> I'm a bit lost here. I'll try other kernels just in case, but that shouldn't
> make a difference.

It could. I remember that some time ago, quite a few people
had issues with slow crypto due to some problems in the 
device-mapper layers. You might have hit that.

Here is one benchmark from me, 1GB luks-file via /dev/loop, 
residing on a 3-way RAID1 with 2 HDDs as write-mostly
and one older SSD. Kernel is 3.3.8, self-compiled on top of
Debian squeeze, pure software AES on AMD quad-core. (Yes,
I know this is complicated, but apparently works well, see 
below ;-)

/dev/mapper/x1:
 Timing buffered disk reads: 612 MB in  3.00 seconds = 203.80 MB/sec

And the raw SSD:
/dev/sdd:
 Timing buffered disk reads: 618 MB in  3.01 seconds = 205.56 MB/sec

Arno
-- 
Arno Wagner,    Dr. sc. techn., Dipl. Inform.,   Email: arno at wagner.name 
GnuPG:  ID: 1E25338F  FP: 0C30 5782 9D93 F785 E79C  0296 797F 6B50 1E25 338F
----
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are stupid, and those with any imagination and understanding are filled 
with doubt and indecision. -- Bertrand Russell 


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