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Re: unionfs: ULFS information storage issues

From: olafBuddenhagen
Subject: Re: unionfs: ULFS information storage issues
Date: Thu, 29 Oct 2009 07:30:41 +0100
User-agent: Mutt/1.5.19 (2009-01-05)


On Mon, Aug 17, 2009 at 01:03:30PM +0300, Sergiu Ivanov wrote:

> If one would like to keep *both* the information about the filesystems
> *and* the ports to their root nodes in *a single* place, one would
> have two choices: either add something like a ``port'' field to each
> entry in the ulfs_chain, thus duplicating the port list in the
> netfs_root_node and still leaving the necessity of synchronizing
> things explicitly; or keep the information about the filesystems (i.e.
> the information stored in ulfs_chain) in *every* node, thus
> duplicating the same bits of information across a (potentially) large
> number of locations.

Obviously we can't keep the information for all the nodes and for the
constituent filesystmes in one place, considering that there are several
nodes per filesystem. All we want is to make sure that the information
in the list of any particular node and in the list of filesystems can be
easily associated.

I can see at least three ways to do it:

- Make sure that the per-node lists can also easily be addressed by a
  numeric index, so we can read both the filesystem list and the
  per-node lists with a simple loop counter.

- Make sure that it's easy to figure out the numeric index of any given
  per-node list entry, so when iterating the per-node list, we can
  directly look up the corresponding entry in the filesystem list
  without having to keep an extra counter.

- Let the per-node entries keep pointers to the entries in the
  filesystem list, so we can access them directly when iterating the
  per-node lists.


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