[Discuss] dm-crypt overhead (was Re: TrueCrypt with SSD)

[Derek Atkins]

Edward Ned Harvey <blu at nedharvey.com> writes:

>> From: Derek Atkins [mailto:warlord at MIT.EDU]
>> 
>> This is a spinning disk, not SSD, but as you say it should be able to
>> sustain 1Gb/s.  It's not.  I'm only getting 400Mb/s to the disk through
>> dm-crypt.  
>
> Well, I only made a generalization.  ;-)  What does your drive mfgr publish
> for specs on that drive?   Nevermind.  Try this...
>
> Use dd to read from /dev/sda (or whatever) dump to /dev/null.  This will
> prove the sequential hardware read speed of the disk without encryption.
>
> Then create a large file (repeat the above dd command, but read from
> /dev/zero and write to a file.)  If you feel like it, reboot just to ensure
> nothing is cached or buffered.  Read the file.  Now the only thing you've
> done is add filesystem overhead and encryption overhead.
>
> That should be a pretty good test, to see if encryption is really the
> bottleneck for you...  At least for reading.  But as you said, without any
> free space on the drive, it's hard to test writing without encryption.
>
>
>> The disk in this machine is the same model as the disk in the other
>> machine where I was seeing full-speed data without dm-crypt.  Alas I did
>> change both hardware type and added dm-crypt at the same time so I don't
>> know if it's the ThinkPad vs. Dell or no-encryption v. dm-crypt that's
>> slowing down my disk I/O.
>
> There are a million things it could be... firmware, drivers, etc.  One thing
> that's simple to check is your disk mode.  ACHI vs ATA.

Well, I finally migrated off that Dell and back onto a ThinkPad.  For
kicks I tried my tests again using 2GB and 20GB write tests through the
file system and got 118MB/s and 108MB/s, both of which are much more
reasonable throughput.

Granted, I'm using a different model HDD here (a 7200RPM Hitachi instead
of the 5400RPM WD) however even the raw dd read off the WD went much
faster on this laptop.  So I think it might be controller related.

*shrugs*

I consider this closed for now.  Thanks!

-derek

-- 
       Derek Atkins, SB '93 MIT EE, SM '95 MIT Media Laboratory
       Member, MIT Student Information Processing Board  (SIPB)
       URL: http://web.mit.edu/warlord/    PP-ASEL-IA     N1NWH
       warlord at MIT.EDU                        PGP key available