writing to RW-mounted UFS2 snapshots - confirmed.

[Q]

On 01/07/2004, at 4:22 PM, Q wrote:

> On 01/07/2004, at 12:25 PM, Sam Lawrance wrote:
>
>>> This is unexpected.  You can successfully mount the snapshot
>>> read/write and create and write to files in that snapshot.  You can
>>> also write to files that existed in the snapshot prior to mounting it
>>> read/write.
>>
>> Perhaps the writing is done from a point where the schg flag is not
>> checked or obeyed?
>
> While this may not be "expected" behavior, I am curious why this is 
> something that should be prevented, rather than verified for 
> correctness?  By "correct" I mean, that the copy on write process is 
> performed correctly and modifications made to the snapshot don't 
> modify the underlying filesystem elements also.

Ok, I decided to be self sufficient and read Marshall McKusick's paper 
on background fsck (which also covers snapshots) to answer my own 
question.

Firstly, one of the stated requirements for snapshots to function as 
outlined in the paper is that they must be read only. The reason they 
need to be read only is because the support for multiple snapshots 
assumes that they cannot change, and handles block allocation of freed 
blocks with this in mind. Supporting snapshot modification is outside 
the scope of the original implementation and would require further 
thought.

-- 
Seeya...Q

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                           _____  /  Quinton Dolan - q_dolan at yahoo.com.au
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