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    SubjectRe: [PATCH 00/32] VFS based Union Mount (V3)
    On Tue, May 19, Miklos Szeredi wrote:

    > On Tue, 19 May 2009, Jan Blunck wrote:
    > > The directory in the topmost filesystem is created during
    > > lookup. The contents of the directory isn't copied up presistently
    > > at that point in time. Therefore you have an empty directory in the
    > > topmost filesystem after the lookup. This was necessary to get rid
    > > of the union_relookup_topmost() calls during create, mknod, mkdir
    > > etc.
    > >
    > > When readdir is called, the topmost directory is filed up with
    > > fallthru entries which are persistently stored. This is only
    > > necessary to get readdir right wrt POSIX. During lookup the fallthru
    > > dentry, which is in fact a special negative dentry, is ignored and
    > > therefore the lookup continues on the lower filesystem.
    > So this means that the topmost branch always needs to be writable,
    > right? It isn't possible to make a union of two iso9660 filesystems,
    > for example?

    Exactly. Although, you can do that with the help of tmpfs on top of the two
    iso9660 filesystems. Or by adding fake write support to iso9660 ...

    I know that this seems to be suboptimal but it is the cost that the POSIX
    correct readdir comes with.

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      Last update: 2009-05-19 12:41    [W:0.019 / U:0.620 seconds]
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