Input/output error during apfs_extentref check on Fusion Drive

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Level 1

14 points

Input/output error during apfs_extentref check on Fusion Drive

On a self made fusion-drive consisting of a 480GB SSD and an 16TB HDD being used as time-machine-backup, I get the following error message:

Checking the object map

Checking the snapshot metadata tree

Checking the snapshot metadata

Checking snapshot 1 of 8 (com.apple.TimeMachine.2024-12-23-040323.backup)

error: (oid 0x6fbbec9) apfs_extentref: btn: dev_read_finish(117161673, 1): Input/output error

Snapshot is invalid

The volume /dev/rdisk10s2 was found to be corrupt and cannot be repaired

Verifying allocated space

The volume /dev/rdisk10s2 could not be verified completely

File system check exit code is 8

Restoring the original state found as mounted

Error: -69845: File system verify or repair failed

Underlying error: 8

Deleting this snapshot shows the next shapshot as defect.

It looks like a defective drive, but this hardware is brand new and dd shows no problems:

% sudo dd bs=8m if=/dev/rdisk6 of=/dev/null

Password:

57232+1 records in

57232+1 records out

480103981056 bytes transferred in 1142.978742 secs (420046291 bytes/sec)

% sudo dd bs=8m if=/dev/rdisk7 of=/dev/null

Password:

1907456+0 records in

1907456+0 records out

16000900661248 bytes transferred in 85313.804252 secs (187553477 bytes/sec)

I already recreated this fusion-drive from scratch, made a new backup (6,5TB) but got the same error message.

Any ideas?

[Re-Titled by Moderator]

Mac mini, macOS 12.7

Posted on Dec 23, 2024 8:11 AM

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Answer 1

John Galt

Level 10

147,312 points

Dec 23, 2024 12:59 PM in response to Flugor

The reason is that dd is not intended to be used as a drive integrity verification tool. It copies files, which it did, over the course of a day.

Apple's details regarding eligible backup drives are somewhat cursory: Backup disks you can use with Time Machine - Apple Support, but with backups it has been my experience that it's best to keep things simple. Simplicity and redundancy are essential features of a reliable backup strategy. A fusion drive introduces an element of complexity and detracts from that reliability.

Seriously, don't bother with building a fusion drive. May I ask why you need one that large though? What is the capacity of the source volume? For example I would expect 16 TB to be appropriate for a 5 to 8 TB source volume, and even then I would use one monolithic physical hard disk drive.

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Answer 2

John Galt

Level 10

147,312 points

Dec 23, 2024 10:02 AM in response to Flugor

Yes. Forget about implementing a fusion drive as a Time Machine backup. The processing overhead required of Time Machine completely obliterates any speed advantage a SSD might convey. It's a waste.

An input / output error means the system was unable to read or write data to or from a storage device. That error will not appear until the process already tried and failed several times.

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Answer 3

Flugor Author

Level 1

14 points

Dec 23, 2024 12:17 PM in response to John Galt

Ok, thanks.

I thought about using a fusion-drive as a volume for time-machine because of a chance of higher IOPs.

But why does macos report an input/output error, if dd tells me that the physical drives are ok?

Another Fusion drive (used as "Macintosh-HD") build with the same components works flawless.

This makes a software-bug less reasonable.

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