ZFS hangs on single filesystem mount

[stuart]

I'm using release/9 (zfs v5, zpool v28) with a single zpool with three filesystems. I can manually mount two of them with no problem, but trying to mount the third one just causes the system to hang (ctrl+t responds though).

I can't snapshot the third filesystem, nor can I add L2ARC or ZIL devices to the pool (because it just hangs). If I run zpool add pool cache label/ssd1, it just seems to hang.

I've run zdb pool/filesystem3 and it completes, but zdb pool never seems to complete - although there is disk activity on gstat.

The only thing I can think of is that I did (for a short time) have dedup turned on on that filesystem, but turned it off because I was concerned about the memory usage. The pool has been fine for months, but this happened yesterday.

Can anyone suggest how I can recover the data/gain access to that filesystem? I am sure the data is still physically there, its just that I can't seem to access it.

Any advice/pointers greatly appreciated. I'm not really sure how to use zdb and don't know enough about zfs internals.

 

[t1066]

You're maybe in the same situation as in https://forums.freebsd.org/showthread.php?t=31820.

Try to run # zpool import -o readonly=on [i]yourpool[/i] and then mount your filesystem.

 

[stuart]

Thanks for that, it did seem to mount correctly. I suppose the only way forward is to create a new pool and copy the data across to that.

How can I (e.g. by using [CMD="zdb"][/CMD] or another command) work out what is happening with this pool? I'd like to understand the problem so that I can avoid it happening in the future as I don't really know what's caused it.

I use ZFS exclusively now, and it's been absolutely fine but it has made me wonder how to debug problems like this!