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    SubjectRe: scsi naming (was: devfs patch v3)
    James Mastros writes:
    > On Sun, 11 Jan 1998, Richard Gooch wrote:
    > [...]
    > > However, this raises an issue that's been troubling me: how deep a
    > > directory structure should be implemented for SCSI discs? If you
    > > imagine a system with 8 SCSI hosts each with 8 SCSI channels (buses)
    > > with 8 drives per channel and 8 partitions per drive (let's leave the
    > > number of LUNs to a measly 1), that's 4096 directory entries in
    > > /dev/sd, which is painful to <ls>. Note that is a rather large system
    > > (someone with a huge disc farm), but the point is valid.
    > > So perhaps it would be better to have:
    > > /dev/sd/H/CciIlLpP
    > >
    > > But that's still 512 entries in each /dev/sd/*, so instead we could
    > > have:
    > > /dev/sd/H/C/iIlLpL
    > >
    > > which brings it down to 64. But then you can argue that you may have
    > > only 1 host with 8 channels, 8 targets per channel and 8 LUNs per
    > > target, each with 8 partitions. And on it goes.
    > >
    > > So, before I make any changes towards subdirectories in /dev, I want
    > > to get some kind of consensus (at least from those not implacably
    > > opposed to devfs) as to how deep these directory structures should go.
    >
    > I should think that we want them as deep as we can go. After-all, there is
    > an overhead of one dentry and one inode in memory for each one, but we only
    > have that overhead for directories that we acatually have... So my
    > suggested directory structure looks like:
    >
    > SCSI/
    > c/
    > controller
    > b/
    > bus
    > id/
    > id
    > lun/
    > lun
    > partition
    >
    > (where the direcories c, b, id, and lun (and the node partition) are
    > numbers, and the nodes controller, bus, id, and lun are nodes for ioctls
    > that effect the entire controller, bus, id, or lun). Indeed, perhaps the
    > partitions should be directories that mirror the partition structure on-disk
    > (so exteneded dos partitions would be directories.) Note that my scheme
    > dosn't diferincate between hard-drives, tape-drives, cdrom-drives, and
    > generics -- I think that they should be merged: does anybody know why they
    > weren't in the first place? (I don't have any SCSI devices, so I don't
    > really know.)

    Firstly, I think the entire SCSI disc should be a file: reading and
    writing directories seems not to be the done thing. I note that ext2
    disallows reading of directories, and the VFS disallows opening of
    directories in write mode.

    Secondly, take drives don't have partitions. I dunno about CD-ROMS.

    Thirdly, I think you *want* to differentiate between hard discs and
    tape drives, at a fairly high level. "What tape drives do I have?"
    Looking for one tape driver amongst 100 SCSI discs is like looking for
    a needle in a haystack.

    Regards,

    Richard....

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