Why are encrypted DMGs so slow? OS X bug?

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Level 1

49 points

Why are encrypted DMGs so slow? OS X bug?

I appreciate [Mountain] Lion's ability to encrypt entire volumes, and I make good use of that feature. There seems to be little to no performance penalty for encrypting a volume. I frequently copy files to external drives and can achieve copy speeds in the low-30's MB/s range (USB 2) regardless of whether or not the drive is encrypted.

But when I copy files to an encrypted DMG the performance is terrible. I can't even get up to 10 MB/s copying the same files to the same drive. What gives? Presumably the same system of encryption is being used, right? Why is copying a file to an encrypted DMG over three times slower than copying a file to an encrypted volume??

This makes a pretty big difference to me since I routinely copy a 30GB file around (virtual machine image) so it's basically the difference between the operation being done in 15 minutes vs. having to wait almost an entire hour.

Mac mini, OS X Mountain Lion (10.8.2), 2012, 2.3GHz i7

Posted on Jan 11, 2013 7:14 PM

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Answer 1

Han73528

Level 1

0 points

Apr 27, 2014 7:37 AM in response to motrek

I'm facing the exact same problem. I'm not using virtual machines. I simply copy files to an encrypted volume. Copying a couple of GBs takes hours (not exaggerating... It is literally hours). I have to copy files overnight if I want to keep them encrypted. This cannot be how it is supposed to work, can it? What am I doing wrong?

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Answer 2

etresoft

Level 8

46,098 points

Apr 27, 2014 7:45 AM in response to Han73528

What image format did you use when creating the file? I think if you used read/write disk image, it tries to keep the DMG small and compressed. If you use one of the sparse formats, it should work much better.

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Answer 3

motrek Author

Level 1

49 points

Apr 27, 2014 11:00 AM in response to etresoft

I've only used sparse formats, since I use them for drive backups and they need to be hundreds of gigabytes.

They used to be fast in previous versions of OS X, now they are ridiculously slow.

In the meantime, I've switched to using disk encryption, so I'm writing to encrypted partitions instead of encrypted DMGs and it works fine. But I would still rather use encrypted DMGs since they would be a little more convenient in my case (easier to organize).

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