> The LVM approach with the "virtual block device" makes many things
> much easier. You can keep filesystem code very simple, and the LVM
> code also isnt very complex. The only thing you might take care on is
> the Block Allocation of the LVM which you might do as complex and
> intelligent as you like but a bug in there will NOT cause data to get
> lost or corrupt.
> I disagree; the block allocation issue gets very complex if you try and
> treat the LVM as a virual block device with "holes" in the device. It's
> much, much simpler if the filesystem is intimately aware of each logical
> volume, and knows what size it is.
I think you are misunderstanding the way the LVM works. If
it is anything like the AIX LVM, then you split all disks
up into 4MB chunks and allocate them to the filesystems
any way you please. That's a lot of little chunks, do
you really want to handle them in the filesystem?
When you shrink a filesystem under this system you
always shrink at the end. If the 4MB chunks you want
to free weren't at the end, then you move them off the
disks they are on, and onto a freed up 4MB chunk. This
requires some cleverness in the LVM to do online, but it
doesn't seem impossible (redirect writes to new device,
keep a bitmap of which blocks are written on new device,
move old blocks over, don't overwrite new blocks, during
the move, reads consults the bitmap and reads from the
right side).
This means the fs doesn't need to know where the underlying
block devices stop and start.
Advantages:
1) One level of block virtualisation, not two (mirror/stripe in
block device, concatenation in fs)
2) Works for other filesystems (though many of them are for
compatibility with other OSs anyway, and since the other OS
doesn't understand the LVM it won't work).
3) Nice small chunks so you can divide up between partitions
in a very flexible way. This lets us have /var and /home
on different partitions, but still allows us to change their
relative sizes without repartitioning.
4) You don't need to complicate the mount interface to specify
several partitions. It's still just one block device.
5) Filesystem still works with simple fast contiguous block
numbers internally.
-- Erik Corry- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.rutgers.edu