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RE: MOVEs across file systems

From: Jason Crawford <nn683849@smallcue.com>
Date: Tue, 11 Mar 2003 14:54:43 -0500
To: "Lisa Dusseault" <lisa@xythos.com>
Cc: "'WebDAV'" <w3c-dist-auth@w3.org>
Message-ID: <OFF79B160F.7238439C-ON85256CE6.005B8168@us.ibm.com>




Warning... this note is more interesting to server designers than
spec designers....  :-)

On Tuesday, 03/11/2003 at 08:27 PST, "Lisa Dusseault"
<nnlisa___at___xythos.com@smallcue.com> wrote:
> > Anyway... the thought I just wanted to point out is that if you have a
> > resource on one file system with multiple bindings to it, and you MOVE
> > that resource via one of those bindings "to another file system"...
> > you might actually want to NOT "copy/delete" it since you
> > still also have
> > several bindings to it from the original file system.  You might
> > *need* to keep a cross file system binding and not necessarily
> > move implementations around during MOVE/BIND and other
> > namespace operations.  And if you're forced to support cross
> > file system bindings, then perhaps you never really need to take the
> > COPY/DELETE approach?
>
> A couple counter-reasons:
>  - Quota within the system (top level directories or lower directories
> limiting space) this may depend on how the quota is counted - the
> system, through its acknowledged limitations, may need to actually move
> the resources (not leave them and rebind them) in order to calculate
> quota.
>
>  - Disk space outside the system - different parts of the namespace are
> frequently literally stored on different disks. Users/Administrators may
> move content from one part of the namespace to another *in order* to
> transfer to a less-used disk.

The system *might* not want to move the implementation to the other file
system because it has reason to keep it on the current file system also.
But as you say, perhaps it might want to move it for reasons of it's own.
Either way, the server has to support cross file system bindings.  And the
choice of which file system it will chose to keep it on becomes arbitrary
from the WebDAV perspective.   I would expect that it would actually end up
keeping it where it was and just get the operation done quickly.

You are probably talking about a situation where it's the last binding, so
it's probably best to move it to the same file system used by its final
parent.  Again, it probably could get away not moving it, but as you say,
users might make assumptions about certain parts of the URI space being
stored in certain file stores.   One complication with this approach is
that in order to try to maintain that relationship, the operation will try
to move the reources to file store that might not have enough quota from
one that already has enough.  That might make the operation abort despite
the fact that the user has enough quota overall.  Another is that if there
is a global quota, the safe copy approach might cause the global quota to
be exceeded (because for some period there will be two copies of one or
more resources) and possibly abort the operation despite there being plenty
of quota overall.  Either way, the half finished MOVE can be a real pain
for clients.   How bad this is depends on how the COPY/DELETE operations
are ordered and when the abort occurs.  (I know I find it a real mess when
I've encountered this situation.  It's tedious to fix if it's even fixable
and the user knows how to fix it.)

Interestingly enough... even if the root of a subtree ends up changing file
systems because it has only one parent binding, many of it's children might
not migrate if there are links from the original file system to them.

So in the presence of multiple bindings, this association between URI and
the file store used for the implementation is weak and sometimes
problematic.  For this reason it doesn't seem well advised to be
compromising WebDAV operations to maintain this mapping  that it can't
really guarantee anyway.  It does seem like there needs to be some other
way for the user to move resources between various storage systems in order
to manage the consumption of his quotas.  --  I repeat though... this is
only in the presence of multiple bindings.  In the absense of multiple
bindings, if we ignore the issue of aborts due to secondary issues, and
ignoring the unfortunate possibility that resource-id's might change as
implementations migrate, the server should be able to maintain the URL to
file store mappings and that can be attractive from a UI perspective.



> Jason, you've clearly got the right instincts for designing a WebDAV
> server -- thinking carefully about how to do the best possible job.
> It's just a question of can the *protocol* enforce a good job on server
> implementors?  Sometimes it can, but sometimes it can't.

Agreed.  In this thread I'm just pointing out possibilities that I, for
one, hadn't thought about or noticed mentioned before.  Thank you for
responding and cleaning up weaknesses in the message that started this
thread.  :-)

> We wouldn't
> want to force atomic DELETEs and have an implementation choose to reject
> a great number of large DELETE requests just because it can't guarantee
> them all atomic. In the extreme, that would be an even worse tradeoff.


Yes, for DELETE, there is a possibility that a client doesn't care if it's
atomic or not.  He just "wants to free resources eating up [his] quota...
and if [he] can't get all of the subtree freed, that's okay for now."   I
will point out weakly though, that if that's the user's thinking, even in
an atomic server, if the error message provided is informative enough,
there is a significant possibility that user will be able to free up the
portion of the subtree that isn't resisting the DELETE.  Obviously a server
can do that more efficiently though than the client can if the server
understands what the user/client wants.

Speaking of the server knowing what the user wants, that does bring up
another point... is the best effort DELETE operation allowed to
stop the first time it encounters a resource that prevents the unbinding of
the request-URI binding (for example to keep it reachable)?   Or must it
continue to find as many resources that it's free to delete as possible?
Or in the case of request URI is "protected" by a lock in the subtree, the
server will almost certainly be aware of the "protection" issue right away,
so is the server required to not remove any bindings at all? --  Anyway, if
we really think clients will *want* best effort DELETE available, we'll
have to specify what the prefered/required behavior is for that.  (Maybe
later if we don't see the demand yet.)

J.

------------------------------------------
Phone: 914-784-7569,   ccjason@us.ibm.com
I do not check nn621779@smallcue.com
Received on Tuesday, 11 March 2003 14:56:10 GMT

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