Stephen, I'll respond to the rest of your email in two days.
Hans
Stephen C. Tweedie wrote:
>
> Hi,
>
> On Mon, 22 Jun 1998 18:26:30 -0600 (MDT), Colin Plumb <colin@nyx.net>
> said:
>
> > Building a fake device out of bits of real devices is not that complicated.
> > The RAID code does this and the file system doesn't even need to know about
> > it.
>
> > The tricky part comes when you want to add or remove real devices, because
> > then your fake device changes size, and the file system needs to know
> > about *that*.
>
> Correct. There is in fact so much filesystem interaction required that
> I'm not at all convinced that a block-device LVM is needed or even
> useful. Virtual disks for redundancy or performance are just fine, but
> when it comes to filesystem sizing, the fs has to be actively involved
> in any change. Given that, we can actually implement the whole thing in
> the filesystem.
>
> Miguel's prototype LVM stuff works by letting you mke2fs a new partition
> and then daisy-chain that new device on to the end of the existing
> filesystem, at run time, while it is all mounted. Removing such a
> partition from the middle of a logical volume set is harder but
> certainly feasible in theory. Is there really any overwhelming
> justification for needing extra device-level support for this
> functionality? Given that we _need_ filesystem support, my own reaction
> is that splitting the support between fs and block device just
> complicates the matter; it's better just to do it in one place.
>
> --Stephen
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