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

Marc MERLIN marc at merlins.org
Mon Jul 23 08:28:51 CEST 2012


On Sun, Jul 22, 2012 at 11:47:57PM +0200, Arno Wagner wrote:
> 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 was worth a shot, thanks for the suggestion.

gandalfthegreat:~# lsmod | grep aes
aes_x86_64             16796  34 
gandalfthegreat:~# hdparm -t -T /dev/mapper/cryptroot

/dev/mapper/cryptroot:
 Timing cached reads:   15802 MB in  2.00 seconds = 7909.98 MB/sec
 Timing buffered disk reads:  68 MB in  3.04 seconds =  22.39 MB/sec
gandalfthegreat:~# 

Unfortunately, speed is exactly the same.

with aes_x86_64:
gandalfthegreat:/var/local/VirtualBox VMs/w2k_virtual# dd if=test of=/dev/null
4192640+0 records in
4192640+0 records out
2146631680 bytes (2.1 GB) copied, 12.4848 s, 172 MB/s

I then rebooted with aesni_intel and got this:
gandalfthegreat:/var/local/VirtualBox VMs/w2k_virtual# dd if=test of=/dev/null
4192640+0 records in
4192640+0 records out
2146631680 bytes (2.1 GB) copied, 8.42506 s, 255 MB/s

So those speeds are expected/believable, but I still don't know why
hdparm is so slow on /dev/mapper/cryptroot.

Mmmh....

Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/


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