Snapshots that won't delete [was: Re: ZFS on root booting...]

[Willem Jan Withagen]

On 12-9-2014 10:19, Steven Hartland wrote:
> Is there a hold on the snapshot?
> 
> ----- Original Message ----- From: "Shane Ambler" <FreeBSD at ShaneWare.Biz>
> 
> 
>> On 11/09/2014 16:42, Ben Morrow wrote:
>>> Quoth Adam McDougall <mcdouga9 at egr.msu.edu>:
>>>>
>>>> Were you running a newer kernel with an older format zpool?  I heard
>>>> ixsystems had customers doing that and ran into corruption when they
>>>> tried to modify the zpool in some way (expand? I don't remember).
>>>> http://www.bsdnow.tv/episodes/2014_07_09-zfs_war_stories
>>>
>>> Oh! Might this be what's causing a problem I've been meaning to ask
>>> about?
>>>
>>> My desktop at home is running (a patched, but not anywhere to do with
>>> ZFS) 10-STABLE from a while ago, with a zpool that was created under
>>> 8.2-R and is still at version 15. I have been deliberately not upgrading
>>> it, because I saw no reason to and it seemed safer to leave things as
>>> they were.
>>>
>>> Recently, though, my dump script has started having occasional problems
>>> with snapshots that won't delete. Pending further investigation I have
>>> been renaming them to allow the recursive delete to succeed, and (so
>>> far) rebooting has always made it possible to get rid of them. 
>>
>> I have seen that issue with 9.2 and at least one other person mentioned
>> it as well. I currently have a snapshot that I accessed at least 3 weeks
>> ago and renamed to keep rotations working and have not accessed since,
>> it still won't delete as it is busy. I can only delete these snapshots
>> after a reboot.
>>
>> The only cause I know is accessing the snapshot.
>> I can simply ls .zfs/snapshot/daily.01/somefolder to prevent it being
>> deleted. With a manual zfs destroy I get "dataset is busy" and have not
>> found a way to find any process that has hold of it.
>>
>> It seems that some aging of the snapshot needs to happen. Testing a
>> rotate, access, rotate keeps working ok, but access last nights
>> snapshot and then rotate and it blocks.
>>
>> I'm fairly sure that I was running 9.1 when I created this zpool and
>> then later upgraded to 9.2. zpool upgrade says "This system supports
>> ZFS pool feature flags" and "Pool 'zrpleader' already has all supported
>> features enabled".

I've had more or less the same happen to me in a backup-with-snapshot
script...
The solution that worked from me uptill now is:
	zfs unmount -f <pool>

For me that is easy to do, since this pool holds a remote backup, and is
only written at nighttime. So at other times there is no harm if I
unmount.... But if you normally "live" in the pool, that might be
somewhat harder.

--WjW