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SubjectRe: [RFC] shared subtrees
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Ram wrote:
> On Tue, 2005-02-01 at 15:21, J. Bruce Fields wrote:
>
>>On Tue, Jan 25, 2005 at 01:07:12PM -0800, Ram wrote:
>>
>>>If there exists a private subtree in a larger shared subtree, what
>>>happens when the larger shared subtree is rbound to some other place?
>>>Is a new private subtree created in the new larger shared subtree? or
>>>will that be pruned out in the new larger subtree?
>>
>>"mount --rbind" will always do at least all the mounts that it did
>>before the introduction of shared subtrees--so certainly it will copy
>>private subtrees along with shared ones. (Since subtrees are private by
>>default, anything else would make --rbind do nothing by default.) My
>>understanding of Viro's RFC is that the new subtree will have no
>>connection with the preexisting private subtree (we want private
>>subtrees to stay private), but that the new copy will end up with
>>whatever propagation the target of the "mount --rbind" had. (So the
>>addition of the copy of the private subtree to the target vfsmount will
>>be replicated on any vfsmount that the target vfsmount propogates to,
>>and those copies will propagate among themselves in the same way that
>>the copies of the target vfsmount propagate to each other.)
>
>
> ok. that makes sense. As you said the private subtree shall get copied
> to the new location, however propogations wont be set in either
> directions. However I have a rather unusual requirement which forces
> multiple rbind of a shared subtree within the same shared subtree.
>
> I did the calculation and found that the tree simply explodes with
> vfsstructs. If I mark a subtree within the larger shared tree as
> private, then the number of vfsstructs grows linearly O(n). However if
> there was a way of marking a subtree within the larger shared tree as
> unclonable than the increase in number of vfsstruct is constant.
>
> What I am essentially driving at is, can we add another feature which
> allows me to mark a subtree as unclonable?
>
>
> Read below to see how the tree explodes:
>
> to run you through an example:
>
> (In case the tree pictures below gets garbled, it can also be seen at
> http://www.sudhaa.com/~ram/readahead/sharedsubtree/subtree )
>
> step 1:
> lets say the root tree has just two directories with one vfsstruct.
> root
> / \
> tmp usr
> All I want is to be able to see the entire root tree
> (but not anything under /root/tmp) to be viewable under /root/tmp/m*
>
> step2:
> mount --make-shared /root
>
> mkdir -p /tmp/m1
>
> mount --rbind /root /tmp/m1
>
> the new tree now looks like this:
>
> root
> / \
> tmp usr
> /
> m1
> / \
> tmp usr
> /
> m1
>
> it has two vfsstructs
>
> step3:
> mkdir -p /tmp/m2
> mount --rbind /root /tmp/m2

At this step, you probably shouldn't be using --rbind, but --bind
instead to only bind a copy of the root vfsmount, so it now looks like:

> root
> / \
> tmp usr
> / \
> m1 m2
> / \ / \
> tmp usr tmp usr
> / \ / \
> m1 m2 m1 m2

- --
Mike Waychison
Sun Microsystems, Inc.
1 (650) 352-5299 voice
1 (416) 202-8336 voice

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NOTICE: The opinions expressed in this email are held by me,
and may not represent the views of Sun Microsystems, Inc.
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