# ZFS hangs on single filesystem mount



## stuart (May 7, 2012)

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.


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## t1066 (May 8, 2012)

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.


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## stuart (May 9, 2012)

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!


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