Friday Jan 02, 2009

ZFS on external disks: Why free disk space might not increase when deleting files...

Interesting: After copying about 30,000 files from an internal disk (OpenSolaris 2008.11) to a new directory of an external USB disk and later removing a similar number of files, the free disk space as reported by zpool status or in the column "Available" of df -k did not increase! And the df -k output showed that the number of total blocks of the only file system on that disk had decreased! What had happened?

Here's the explanation:

For backup purposes, I wanted to use an external 2.5" USB disk with a capacity of about 186GB. On that USB disk, I had created a zpool, using:
$ zpool create -f dpool-2 c5t0d0

For a first backup, I copied the mentioned 30,000 files with a total size of about 75GB from one system to a USB disk connected to another server (running OpenSolaris 2008.11 as well) via the network. I used a simple scp -pr which preserves modes and times but not uids and gids. One possible procedude for preserving file ownership as well, as mentioned here, for example, would be:
cd source_dir; tar -cf - .|ssh user@targethost "cd target_dir; tar -xvf -".

Anyway, I thought it would be a good idea to export that USB disk and import it on another server (also on OpenSolaris 2008.11) to check if there were any problems. I also wanted to run a
$ zpool scrub dpool-2
to verify the data integrity. Everything went smoothly.

OK. So I exported the zpool again, connected the disk to the server on which the original data is located, imported the zpool and started copying the same data which I had copied before (via the network) locally, using
$ tar -cf - | ( cd /dpool-2; tar -xpf - )

As this finished without errors as well, and a comparison using /usr/bin/diff or the default /usr/gnu/bin/diff only showed a problem with one of the files that had an interesting character in its file name (the problem went away after changing LANG from en_US.UTF-8 to C), I decided to remove the old 30,000 files (the ones which I had copied via the network). I had removed files on ZFS before, using rm -rf, and it always was amazingly fast, compared to PCFS or UFS. But this time, removing those files took quite a while, with some disk activity.

After all the "old" files were removed, I discovered the following:

  • Free space as in df -k or zpool status dropped from 112GB to 36GB!
  • Used space as in zpool status increased from 75GB to 150GB!
  • df -k reported a total size of the only file system on that zpool of just 108GB, compared to 183GB as it was reported before!

The solution was to execute the following command:
$ zfs list -t snapshot
The output showed several snapshots, taken during the time the disk was connected to the second server (on which I had set up time slider for creating automated snapshots). When I connected the disk to the final server, the snapshots were still there, and when removing files, the snapshots were still valid so it would have been possible to create the deleted files again. So the easy solution for recreating the free space again was just to remove all those snapshots, using zfs destroy, as in the following example:
$ zfs destroy dpool-2@zfs-auto-snap:frequent-2009-01-02-18:45
After the final snapshot for dpool-2 was removed, things showed up in df -k and zpool status as expected. "Problem" solved!

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