Get the amount used up in a snapshot (OnlineJFS)

[Daniel Robert]

If I use du, it obviously does not give me the actual amount of disk space used-up in the snapshot volume because it is reporting the size of the volume that I am cloning and the size of the snapshot volume is defined as much smaller then that (unless I am not using the right option).

How can I get a (fairly) accurate account of how much disk space is being used in the snapshot volume? I want something to monitor the disk space being used as the snapshot is in progress so that I can get an idea as to the size it really needs to be (increase/decrease/leave it alone).

[Matti_Kurkela · Accepted solution]

The size required by the snapshot = the size of data added or modified during the lifetime of the snapshot.

If your filesystem contains mostly small files, you might get a good (conservatively high) estimate by calculating the sum of the sizes of all files whose modification time is newer than the snapshot creation time. If your filesystem contains mostly large files, this obviously won't give good results, since only some parts of the large files will have been changed.

At least back in 2002 or so, HP Support used to have a tool called "snapbdf". It would report the usage level of the snapshot volume, both in kilobytes and as a percentage. It seems to be exactly what you need. If you have a support agreement with HP, open a call and request to have it. (It was not available by default, apparently because it was a somewhat experimental tool.)

MK