Re: [bug #48885] zfs_mount fails with `org.zfsonlinux:large_dnode` enabled: Unsupported features in pool

[Toomas Soome]

> On 25. aug 2016, at 0:04, Andrei Borzenkov <address@hidden> wrote:
> 
> 24.08.2016 23:10, Brian Behlendorf пишет:
>> Thomas,
>> 
>> You should be able to simply add 'org.zfsonlinux:large_dnode' to the
>> list of supported features.  As long as the dataset property dnodesize
>> is set to legacy there won't be any on-disk format changes.  The dnodes
>> on disk will still be 512 bytes in size and it will just leverage some
>> previously unused pad space in the dnode_phys_t.
>> 
>> Since we were concerned about grub support the zfs command now prohibits
>> users from setting the dnodesize property to a non-legacy value for
>> datasets with the bootfs property set.
>> 
> 
> We have no way to restrict which filesystems users will access at boot
> time. Can we check value of this property in grub?

yes, this is exactly why the features for read list exist - if the pool is 
adding entry there and boot loader does not have matching entry in its 
implementation, you deny the access to pool. Which is exactly what did happen 
for this user.

> 
>> It shouldn't be a huge amount of work to fully support large dnode in
>> grub but simply allowing the feature flag and checking the property
>> should be enough for the vast majority of use cases.
>> 
> 
> It sounds like simply allowing this feature may access filesystem with
> incompatible on-disk format. Either we need additional checks for legacy
> format or we need full support for new format.

sounds reasonable. 

rgds,
toomas