snapshot question

[fuvasido]

I am new to setting up a FreeNas system. Now that things are working with sharing and my users have access to files, I am ready to run snapshots then store them offsite.

Here is what I don't understand. I did a manual snapshot of a dataset with 236mb of data. This was the first snapshot and it was 1.42 mb. I took two snapshots from other datasets manually. One dataset was 550 Gb and the snapshot is 504 kb. The last one was 1.1 tb and the snapshot was 2.4 mb. I get this data from the snapshot tab of the web interface. Am I not understanding what the interface is showing or did I do something wrong? Shouldn't the first snapshot be about the size of the dataset?

Mark

 

[dakotta]

Hello fuvasido,

I am new, also, and my explanation is very simplistic. If I write something that is wrong, hopefully someone will correct me.

For more detailed information, see:
How Do Snapshots Work?
Why Do Snapshots Increase in Size?

ZFS is a copy-on-write file system. That means: when you save a file you are working on, it is written to a new location on your hard drive, rather than overwriting the old file. If the write action is successful, then the original file is deleted.

A snapshot is like a timestamp. When you create a snapshot, all the files that are older than the snapshot become a part of the 'snapshot'. (Well, the metadata does, not the actual data, which is why your snapshots are so small.) If you edit and save a file after the snapshot is taken, then ZFS writes the new data to disk, but does not delete the old data.

So, each time you edit a file that is protected by a snapshot, more of your disk space is allocated. If you keep editing files, the space on your hard drive protected by the old snapshots keeps increasing and can become significant.

Cheers,