APFS Bug in macOS High Sierra Can Cause Data Loss When Writing to Disk Images

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Apple's APFS file system included in macOS High Sierra suffers from a disk image vulnerability that in certain circumstances can lead to data loss, according to the creator of Carbon Copy Cloner.

In a blog post last Thursday, software developer Mike Bombich explained that he had uncovered the data writing flaw in the Apple File System, or APFS, through his regular work with "sparse" disk images.


For those who aren't familiar with the term, a sparse disk image is basically a file that macOS mounts on the desktop and treats as if it was a physically attached drive with a classic disk volume structure. The flexibility of sparse disk images means they are commonly used in the course of performing backup and disk cloning operations, hence Bombich's extensive experience with them.
Earlier this week I noticed that an APFS-formatted sparsebundle disk image volume showed ample free space, despite that the underlying disk was completely full. Curious, I copied a video file to the disk image volume to see what would happen. The whole file copied without error! I opened the file, verified that the video played back start to finish, checksummed the file - as far as I could tell, the file was intact and whole on the disk image. When I unmounted and remounted the disk image, however, the video was corrupted. If you've ever lost data, you know the kick-in-the-gut feeling that would have ensued. Thankfully, I was just running some tests and the file that disappeared was just test data.
Two related problems are identified by Bombich, above. The first is that the free space on the APFS-formatted sparse disk image doesn't update as it should when the free space on the underlying physical host disk is reduced. The second problem is the lack of error reports when write requests fail to dynamically grow the disk image, resulting in data being "written" into a void. Bombich tracks both bugs back to macOS's background "diskimages-helper" application service, which he has since reported to Apple.

Bombich's video demonstrating the APFS bug

Every installation of High Sierra on Macs with all-flash storage converts the existing file system to APFS, which is optimized for modern storage systems like solid-state drives. However, as Bombich notes, ordinary APFS volumes like SSD startup disks are not affected by the problem described above, so the vast majority of users won't be affected by it - the flaw is most applicable when making backups to network volumes. Bombich says Carbon Copy Cloner will not support AFPS-formatted sparse disk images until Apple resolves the issue.

The APFS flaw follows the discovery of another bug in Apple's operating systems that received extensive coverage last week. That bug is induced by sending a specific character in the Indian language Telugu, which causes certain apps on iPhones, iPads, and Macs to freeze up and become unresponsive. The Telugu character bug has already been fixed in Apple's upcoming iOS 11.3 and macOS 10.13.4 software updates.

Article Link: APFS Bug in macOS High Sierra Can Cause Data Loss When Writing to Disk Images
 

BaltimoreMediaBlog

macrumors 6502a
Jul 30, 2015
885
1,533
DC / Baltimore / Northeast
Wow, this makes me wonder. Did Apple and APFS destroy my only 2 year old Seagate drive with mostly disk images on it?

I kept wondering, how could my newest standard HD used the least of all fail quicker then the much older ones? The complete failure boggles my mind. Seagate blamed harsh usage, but it was not the system drive, only contained media and music and disk images for specific purposes.
 

JessePinkman

Suspended
Feb 18, 2018
25
3
this happens randomly with steam games on my macbook pro it happened with pillars of eternity was almost crashing my nvidia gpu then i verified game files via steam client and it found few mb corrupted data for no reason (no power loss or resets) shh macos bugs these days...
 
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fermat-au

macrumors 6502
Dec 7, 2009
459
497
Australia
It is bugs such as this, that this what for mission critical task sys admins don't update there system to the newest version for software the day or even week or month it ships.
 

Kabeyun

macrumors 68030
Mar 27, 2004
2,666
4,997
Eastern USA
High Sierra has been a complete disaster for me. My computer locks up for 5-10 seconds every few hours. Probably some memory leaks or something. Either way this is really pathetic, even windows works better than this.
I feel your pain. Had a major memory leak after moving to Sierra, let alone High Sierra. Progressively unresponsive about 10 minutes after reboot. Wasn’t the notorious iTunes leak either. Couldn’t trace it. Went back to El Cap and am quite happy. My only complaint is that Photos can’t pick a new key frame from a live photo. I’ll live.
 
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BaggieBoy

macrumors 6502
May 29, 2012
480
155
UK
I wonder if this is related to high sierra suddenly invalidating my time machine backups to my NAS, and demanding to do a new full. (Loosing two years worth of history). Then doing it again 2 weeks later.
Same happened to me, 2 new full backups required so far.
 
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Eagle_Eye

macrumors member
Oct 10, 2017
33
47
Netherlands
Anyone else noticed strange behaviour when creating a zip archive? There was a moment (when I was running High Sierra, downgraded to Sierra) where I had to re-zip a folder 6 times before APFS gave me the correct files. In all 5 previous versions the zip contained an older version of a Word file.
 
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dannys1

macrumors 68030
Sep 19, 2007
2,593
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UK
Happy to be a paying CCC user; thanks!
Same, Mike is a really good guy and we use CCC every day in work and it's my businesses number 1 tool for deployment, I prefer it over all other deployment tools that Mac Admins usually use.

APFS has lot's of bugs - i've discovered two edge use cases. Rsync with Apples new AP controllers (which come in MacBook Pros built this year and iMac Pro's) runs 4x slower than normal (which makes a 10minute copy a 40 minute copy) - also APFS formatted raid drives when used as a boot volume take minutes to appear in the macOS boot menu.

Both are edge cases that 99% of users will never experience (though the AP controller is a bigger deal and more and more people will come into that as those computers become more available, so it needs sorted with by Apple or the Samba team working on rsync...i've reported it to both)
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“Bombich”... you can’t make this stuff up.
It's his surname?...
 

JessePinkman

Suspended
Feb 18, 2018
25
3
I feel your pain. Had a major memory leak after moving to Sierra, let alone High Sierra. Progressively unresponsive about 10 minutes after reboot. Wasn’t the notorious iTunes leak either. Couldn’t trace it. Went back to El Cap and am quite happy. My only complaint is that Photos can’t pick a new key frame from a live photo. I’ll live.
imo its hardware not software apple have been using cheap memories since tim cook
 

Luigi55

macrumors newbie
Oct 1, 2014
6
0
Switzerland
High Sierra has been a complete disaster for me. My computer locks up for 5-10 seconds every few hours. Probably some memory leaks or something. Either way this is really pathetic, even windows works better than this.
Same problem happens on my MacBook Pro 13" Mid 2014.
 
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