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larry666 Author

Level 1

28 points

Time Machine and kernel_task transferring large amounts of data

macOS 11.5.1

Synology NAS DS213+ running DSM 6.2.4-25556

OK, so I don't know if this is an issue with macOS or Synology NAS but I'll ask here and again in the Synology forums - I have upgraded operating systems on both recently.

I do Time Machine backups to a NAS and I use the NAS only for Time Machine backups.

Currently the NAS has 2 x 4 TB HDDs (both less than a year old) set up as a single RAID volume - the useable space on this volume is about 3.7 TB with around 60% (2.22 TB) used, the remainder free.

Some months ago, I don't recall exactly when, I noticed that Time Machine backups, even small incremental backups seemed to be taking a very long time. Investigating further, and watching Activity Monitor, this seems to be associated with kernel_task.

I have just now watched, on Activity Monitor, Time Machine do a 156 MB incremental backup. As Time Machine started preparing the backup, kernel_task kicked in. Time Machine, from the icon in the menu bar seemed to think it had finished the backup within a few minutes.

However, kernel_task kept running. When I checked the Synology NAS, Time Machine was still logged in as a user. Activity Monitor shows kernel_task as having sent 618 MB and received 8.6 GB (!) over the network and read/written 2.69 GB/2.61 GB to the internal HDD. However, kernel_task isn't taking much CPU time - kernel_task was taking about 2% of CPU time.

I read that one of the commoner reasons for kernel_task running a lot is to control CPU usesage, to ensure the CPU doesn't overheat. Well, while this Time Machine incremental backup was running, the PECI CPU temperature was only 47 ºC and the fans were not running hard (iStatistica sensors).

My concerns are:

While all this is going on, my LAN is slowed down.
Repeated reading/writing of large amounts of data to the NAS HDDs is going to significantly shorten their lifetime.

Ideas and suggestions please 😊

iMac 21.5″, macOS 11.5

Posted on Aug 10, 2021 2:13 AM

Top-ranking reply

larry666 Author

Level 1

28 points

Posted on Aug 21, 2021 6:28 AM

Solved 😀

Having decided to give it one more try to get Time Machine working properly, I looked up the instructions from Synology for backing up from Time Machine to a Synology NAS.

I've previously done a complete new Time Machine backup to the NAS, about 6 - 8 weeks ago, deleting all the existing Time Machine files, which hadn't solved the problem

But then I realised that while I'd deleted all the files, I hadn't deleted the Time Machine share folder, which was originally created some years ago, perhaps even before the AFPS was introduced.

So, I deleted the Time Machine share folder and created a new share folder. Went to Time Machine and set it up to backup to the new share on the NAS.

Problem gone!

No idea what changed in a recent Big Sur or Synology DSM update to cause the problem but that doesn't worry me.

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Time Machine and kernel_task transferring large amounts of data

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