Robin's Blog

Note: Mounting over an existing directory ‘hides’ the files

This is really just a quick note for me in the future, and for anyone else who might find this useful.

I have been involved in doing some administration of a Linux server recently – although I haven’t had full control over the server as administrators from the company that own the server have been doing the ‘low-level’ administration, and we need to get permission from them to do various administration tasks.

Anyway, recently they installed a new hard drive as we needed more space on the server. Prior to the installation we had one disk mounted on /, and a lot of data stored in /data. When they mounted the new hard disk they mounted it on /data. Now, we then logged on to the server and found that there was nothing in /data…all of our data had vanished!

Now, although we thought we knew what the cause was, we wanted to tread carefully so we didn’t accidentally do anything would actually lose us any data. So, we did a bit of investigation.

The output of df -h showed that the data that had gone ‘missing’ was still on the disk, as we only had about 50Gb free (hence why a new hard disk had been installed). However, running du -h --max-depth=1 / showed no space was taken up by /data, and the total given at the bottom of the du output didn’t match the total disk usage according to df.

This all confirmed our suspicion that the data was there, but that it was hidden by the new hard disk being mounted ‘over’ it. We simply ran umount /data, and all of our data appeared again, and df and du now agreed.

So, we resolved this long term by:

1. Mounting the new disk as /newdata
2. Copying everything from /data to /newdata
3. Deleting everything inside /data (but not the folder itself)
4. Remounting /newdata as /data

So, overall it was quite simple, but it was one of those occasions in which we really needed to stop and think, just so we didn’t do anything stupid and lose the valuable data that was on the server.

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This post originally appeared on Robin's Blog.

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Categorised as: Computing, Linux