Extremely slow writing on encrypted USB sticks?

Question

Extremely slow writing on encrypted USB sticks?

This is weird, and I have no good explanation for it: formatted an external USB stick (SanDisk Ultra SDCZ048, 64 GB) with encryption. This combination is lethal to writing speeds: just writing a large file with dd shows speeds 10x slower than if the drive is not encrypted, which yields usually around 30MB/s. Worse: unmounting such a stick can then take a few minutes. This is running 10.12.6 on a 2013 MacBook Air, 8GB RAM.

I tried it with another stick of the same kind, same results. The solution is weird: do not encrypt the stick, create an encrypted sparse bundle with roughly the same size and, wouldn't you know it, one can write to latter with almost the same speed as in the fully unencrypted scenario.

I've been living with the slow write speeds for over a year now, but just finally decided to dig into it. Anyone has a clue of what MacOS is doing wrong here or what can be tuned? The CPU is always idle when things slow down dramatically, so it must be something else in the OS layer.

And having encrypted USB sticks shouldn't be an exotic usage scenario, quite the contrary ...

MacBook Air (13-inch Mid 2013), OS X El Capitan (10.11.4)

Posted on Jul 28, 2017 11:32 AM

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Answer 1

Jul 29, 2017 2:19 PM in response to Barney-15E

I thought so too, but I checked: encryption was complete, drive is idle. Also this issue remained after having the stick being plugged in for hours and even days, so even if there was out-of-monitor background encryption catch-up happening it should have been done eventually. Unless ... this catch-up only happens when there's activity, interfering somehow with the actual I/O work, but that's speculation.

Again my test procedure:

Unmount whatever former disk is mounted, in Disk Utility.
Use low level sudo dd if=/dev/zero of=/dev/rdiskX bs=1m wipe of stick to set everything to zero.
Write speed reported is around 30MB/s. This should be the raw/fastest speed achievable.
Unplug.
Plug back in. "Disk not initialized..." message.
Format the drive in Disk Utility with encryption turned on.
Disk Utility completes after roughly a minute, drive is supposed to be ready.
Drive unmounts very fast, no disk I/O show in Activity Monitor after that.

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Answer 2

Barney-15E

Level 10

122,588 points

Jul 29, 2017 7:31 AM in response to Community User

Has it finished encrypting? If you right-click on it in the Sidebar, does it say it is encrypting or offer to Decrypt it?

I tried to encrypt a 4GB USB stick and it seems to be taking a long time. I'm wondering if it will ever finish.

It is very slow while encrypting as it has to move everything around as it creates a CoreStorage volume.

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Answer 3

Jul 29, 2017 10:11 AM in response to Barney-15E

It's not about encrypting the drive itself. The stick started as empty, as a matter of fact containing nothing else than zeros. I then erased or formatted it respectively with Disk Utility for it to be encrypted. That happens really quickly because of no former data which has to be encrypted obviously. But using such a drive for everyday purpose is extremely slow, writing at least that is. It's not the drive itself, because just using sparse volumes on the same unencrypted counterpart performs really well. Hence the mystery.

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Answer 4

Barney-15E

Level 10

122,588 points

Jul 29, 2017 1:29 PM in response to Community User

My point was it may have never finished encrypting and is therefore incapable of efficient operation.

After erasing and encrypting with Disk Utility as you describe, it takes about 6 minutes to copy 1.2 GB which is not very fast.

This appears consistent with what I am seeing: macbook - Extremely slow write speed to encrypted external drive on Mavericks - Ask Different

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