MARS Backup failed to access the shadow copy because of inadequate disk space on the volume (0x08174)

Bullshevic 0

Hello world!

Our MARS backup agent has started to throw this message from a scheduled backup. The total amount of the backup is around ~9.2TB and there is plenty of space on the data drive and OS drive (7.03TB, 657GB respectively). I was able to make it work again by selecting less folders from the data drive, amounting to about 5TB of data but if I select the rest of the data, MARS starts throwing errors. Looks like there is a limit somewhere but I can't see where that is.
The data drive is a 15.7TB with 7.03TB free. OS is 700GB with 657 free. Also, I moved the scratch/cache folder onto another drive where there is 2.35TB free.

Why is MARS complaining that I do not have enough disk space when I try to backup all the data totaling to about 9.1TB?

SadiqhAhmed-MSFT 44,336 Reputation points Microsoft Employee

2024-09-06T20:59:29.3633333+00:00

@Bullshevic Greetings!

The issue you are encountering with MARS agent backup is related to disk space allocation for the scratch/cache folder and shadow copy storage.

The scratch folder should have free space that equals at least 5-10% of the total size of the backup data. For a 9.1TB backup, you would need approximately 455GB to 910GB of free space in the scratch folder. Since you mentioned that you moved the scratch folder to a drive with 2.35TB free, this should be sufficient, but it's worth double-checking the configuration.

Ensure that the volume allocated for shadow copy storage has enough space. The MARS agent might be throwing errors if the shadow copy storage is low on space. You can try increasing the shadow copy storage size or setting it to unbounded.

If you are also backing up the system state, you will need an additional 30-35GB of free space in the volume containing the cache folder

Although you have plenty of space on the data drive and OS drive, the MARS agent might still be encountering issues if the space is not properly allocated or if there are other constraints on the system.

Try backing up smaller subsets of data incrementally. This helps isolate whether the issue is related to specific data or the overall amount of data being backed up.

Make sure you are running the latest version of the MARS agent. Sometimes, updating to the latest version can resolve unexpected issues. Find the latest version in the portal, or download from here.

If you have checked all these factors and are still encountering issues, it might be helpful to review the logs in the "C:\Program Files\Microsoft Azure Recovery Services Agent\Temp" directory for any specific error messages related to disk space.

Let us know if you need further assistance or if there's anything else I can help with!

If the response helped, do "Accept Answer" and up-vote it
Bullshevic 0 Reputation points

2024-09-09T18:23:46.9866667+00:00

Is there a way I can PM you the log file? I don't want to post it in here for everyone to see. There are some backups that are cancelled because another backup is running but those are OK. However, I do see issues with Disk I/O latency but not sure if I am looking at the right entry or not.
SadiqhAhmed-MSFT 44,336 Reputation points Microsoft Employee

2024-09-10T04:21:12.2266667+00:00

@Bullshevic I have shared the email through private message.
Bullshevic 0 Reputation points

2024-09-10T13:27:12.6166667+00:00

Thank you! I have emailed you the logs from 9/9 and 9/10 which should contain the latest error:

This time the folders selection for backup totaled to 2.4TB which is way less that what I observed before. Yet it failed again with an "Inadequate disk space"

1 answer

Bullshevic 0 Reputation points

2024-09-09T13:18:37.1066667+00:00

Hello Ahmed,

I think we have done all of the above as you mentioned. Here are some screenshots of the disks and how they look in terms of space:

Just to recap:

E: = Data being backed up by MARS

F: = MARS Scratch/cache folder relocated

Below is Shadow Copies configs for each drive:

I have also tried to select subfolders gradually to see where the error starts and looks like around the 4TB mark. If I go over that value, then MARS starts failing with the not enough space allocated.

We have also upgraded the MARS agent to the latest version (2.0.9269.0)

Server has been rebooted a few times to make sure it had no pending reboot status. It has also been patched with the latest MS updates (8/2024)

In the "C:\Program Files\Microsoft Azure Recovery Services Agent\Temp" there are a bunch of logs. Can you provide more specifics as to what log we need to look at?
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SadiqhAhmed-MSFT 44,336 Reputation points Microsoft Employee

2024-09-09T15:42:27.5866667+00:00

@Bullshevic Check for CBEngine logs and see that gives you error details.

Bullshevic 0 Reputation points

2024-09-09T20:23:17.9766667+00:00

Is there a way I can PM you the log file? I don't want to post it in here for everyone to see. There are some backups that are cancelled because another backup is running but those are OK. However, I do see issues with Disk I/O latency but not sure if I am looking at the right entry or not.

SadiqhAhmed-MSFT 44,336 Reputation points Microsoft Employee

2024-09-11T07:56:40.8+00:00

@Bullshevic Thanks for sharing the logs. Went through the logs and it appears that there are high I/O operations at the disk level which is causing issues. I suggest troubleshooting this at the disk level from your side and see if that helps resolve the issue.
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MARS Backup failed to access the shadow copy because of inadequate disk space on the volume (0x08174)

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