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SubjectRe: Volume management on Linux with the ext2fs.
   Date: Wed, 23 Apr 1997 13:27:48 -0500
From: Miguel de Icaza <miguel@nuclecu.unam.mx>

That is exactly what I would like to change. From a one-night reading
of the ext2 file system, it seems possible to implement the
extensions. We just need to get a routine that maps from a given
global block number to the device/local-to-the-device-block-number.

It's not nearly so simple. Right now the ext2 filesystem relies on the
generic filesystem's mmap, and generic file read routines. Those
routines break in the face of breaking the single device to single
filesystem. Swapping to a file on an ext2 filesystem would also break,
unless you implemented some special case code. All doable, but it adds
quite a bit of complexity and kernel bloat.

That is right. Your scheme looks nicer, but then again, it does not
allow a system to continue operation and still let you add a new disk
to the system. This is something at least the IRIX XFS/XVM lets you
do. I would love to have this functionality in Linux.

I see this as something that's nice, but I don't share your urgency
about this feature. Most of the time you need to powerdown the system
to cable the new disk into the SCSI chain anyway.....

I would like to follow what the IRIX guys did: you can unplug a device
from a volume by running some program that: 1. sanity checks if it is
possible to do this; 2. disables all of the ongoing i/o to that file
system while moving the information and 3. moves the information away
from the disk you want to remove and 4. re-enables disk I/O.

Again, not so simple.

The moment you allow arbitrary devices to be removed from a logical
volume, now you have gaps in the associated block numbering space. How
do you handle the resulting block numbering name space fragmentation.
Suppose we take out of service a 1 gig disk, and add a disk which is 800
megs; where do we put the disk in the block number space? What if the
new disk is 1.2 gigs?

Given that disks will be inserted at arbitrary places in the global
block numbering name space, how do you record this information? Because
if the system shuts down and comes back up, the information about where
in the multi-disk volume is very important.

- Ted

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