Re: [fuse-devel] FS block count, size and seek offset?

[Goswin von Brederlow <brederlo@informatik.uni-tuebingen.de>]

"David Brown" <dmlb2000@gmail.com> writes:

> I was looking at various file systems and how they return
> stat.st_blocks and stat.st_size for directories and had some questions
> on how a fuse filesystem is supposed to implement readdir with the
> seek offset when trying to union two directories together.
>
> Is the offset a byte offset into the DIR *dp? or is it the struct
> dirent size (which is relative based on the name of the file) into the
> dir pointer?

I think it is totaly your call what you store in it. Only requirement
is that you never pass 0 to the filler function.

off_t is 64bit so storing DIR* in it should be no problem. Or a
pointer to

struct UnionDIR {
  DIR *dp;
  struct UnionDIR *next;
}

> Also, if you want to be accurate when you stat a directory that's
> unioned in the fuse file system how many blocks should one return?
> Since each filesystem seems to return different values for size and
> number of blocks for directories. I know I could just say that its not
> supported with my filesystem built using fuse... but I'd like to at
> least try to be accurate.

You could add them up and round it to some common block size (if they
differ). But I don't think it maters and nothing uses that info.

What is more important is the link count. For example find uses the
link count to know how many subdirs a directory has. Once it found
that many it assumes there are no more dirs and saves on stat() calls.

> Is it accurate to assume that the size or number of blocks returned
> from a stat will be used to pass a seek offset?
>
> When does fuse use the seek offset?

Afaik never. The offset is only stored for the next readdir call but
never used inside fuse.

> These are the number of blocks and size on an empty dir.
> ext3
> size 4096 nblocks 8
> reiserfs
> size 48 nblocks 0
> jfs
> size 1 nblocks 0
> xfs
> size 6 nblocks 0
>
> Any help to figure out how to union two directories and return correct
> values would be helpful.

Why don't you use the existing fuse-unionfs?

> Thanks,
> - David Brown
>
> P.S. maybe a posix filesystem interface manual would be good?

MfG
        Goswin
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