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

Edward Ned Harvey <blu at> writes:

>> From: Derek Atkins [mailto:warlord at MIT.EDU]
>> Sent: Friday, August 26, 2011 11:19 AM
>> > Simply write a file.  Eliminate the possibility of external drive
> slowdown.
>> > time dd if=/dev/zero of=10Gfile bs=1024k count=10240
>> I did this a few times with various count sizes and noticed that the
>> speed declined significantly once I started writing more than my RAM
>> cache size data:
>> [warlord at mocana mocana]$ time dd if=/dev/zero
>> of=/home/warlord/TestDataWrite bs=1k count=20000
>> 20000+0 records in
>> 20000+0 records out
>> 20480000 bytes (20 MB) copied, 0.0662049 s, 309 MB/s
>> 0.002u 0.063s 0:00.10 60.0%	0+0k 128+40000io 2pf+0w
> hehehe, yes, of course.  :-)  The number I suggested above was around 10G.
> That was not based on anything, and it may need to be bigger on your system,
> depending on your system specs.  Really this test should be as large as you
> can bear to let it be.  But don't go over approx 50% of the drive, or else
> you might start getting hurt by fragmentation etc.
> Hint:  Any benchmark you complete in 0.06 seconds isn't going to be very
> useful.   ;-)  Perhaps try something that runs at least 5-10 minutes,
> minimally.

Did you miss attempts 2 and three, which were 200MB and 2GB tests?  Yes,
I know that a 0.06s test is irrelevant.  I included it for completeness.
That's why I also did two more tests with increasing dataset sizes to
more accurately get my disk write speed.

>> Still, 50MB/s is a SIGNIFICANT reduction in I/O throughput from what I
>> think I should be seeing w/o encryption.
> You're also using a 1k blocksize.  Try increasing that, at least 128k.  I
> usually say 1024k.  Given that "dd" is actually topping your cpu charts,
> you're probably only generating your data at 50 MB/s.  

I don't think it's the blocksize.  Just to make you happy, here's the
same 2GB test with a 1MB blocksize:

time dd if=/dev/zero of=/home/warlord/TestDataWrite bs=1024k count=2048
2048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 49.2395 s, 43.6 MB/s
0.000u 2.064s 0:49.57 4.1%	0+0k 1376+4194304io 15pf+0w

See?  There's my 40-50MB/s again.

> Try running dd directly from /dev/zero into /dev/null, and see how your
> blocksizes affect it.  That way you can ensure you're at least running dd
> efficiently...  And then you can write something to disk.  Are you familiar
> with pv?  It's useful to stick into your pipeline, so you can see what's
> going on.

And verified, it's not a generation problem:

time dd if=/dev/zero of=/dev/null bs=1024k count=20482048+0 records in
2048+0 records out
2147483648 bytes (2.1 GB) copied, 0.127949 s, 16.8 GB/s
0.000u 0.127s 0:00.30 40.0%	0+0k 256+0io 4pf+0w

> I agree, 50 MB/sec is not stellar.  Any typical 7200rpm sata drive should
> sustain 1Gbit/sec.  SSD's should sustain about the same throughput, but much
> faster IOPS.

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.  Unfortunately I don't have any non-encrypted space available
on the disk.  At least nothing sufficiently large enough to get a good
sample to see if it's the disk or dm-crypt.

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.

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

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