Time Machine backups to NAS fail with "Failed to eject" and "Resource busy"

Back in 2020 I had already opened a thread about problems backing up to an SMB share on my NAS; it was so slow it would never catch up with all the snapshots my system creates every hour. I had been able to solve this by blacklisting all those many small files that don’t really need to be backed up, like browser cache etc.

This had all been working well for the last 10 months, but now I’m again at a point where TM backups don’t work anymore.

I am using a Synology NAS and an SMB share. My OS is the latest update to Big Sur. The following problems occur:

1. At night the connection to my NAS gets lost, so that backups do not complete. This problem has just started occurring at some point, no change to my configuration, neither on the client nor on the server side had been made. I can circumvent this with Amphetamine.app if necessary. Not elegant, but works.

2. Backups are never started and eventually end with an error message telling me that they will start once my backup disk has become available again. This is the problem I cannot solve. Details below.

Whenever a backup start my logs exhibit an endless sequence of these messages:

Code:

2021-10-29 13:13:11.398916+0200  localhost backupd[47011]: (TimeMachine) [com.apple.TimeMachine:General] Failed to eject 'disk2' using ioctl, error: 16 Resource busy
2021-10-29 13:13:16.409130+0200  localhost backupd[47011]: (TimeMachine) [com.apple.TimeMachine:General] Failed to eject 'disk2' using DiskArb, dissenter: {
    DAStatus = 49168;
} with status: 49168 (null)

I have searched the net for advice how to deal with this, but without success. Rebooting the NAS, my MacBook or both has not shown any success.

I tried using afp instead of smb. This avoids the above problem but is so slow that my list of unprocessed local snapshots keeps growing. Back to smb, back to “Resource busy”.

All this has started happening without me having made any changes to the configuration. However I need to mention that at some point we had a power loss here, so that my NAS had a hard stop.

Anyone got an idea what to do here?

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