Re: IBMs LVM ?

[Andreas Dilger]

On Sep 11, 2001  17:26 +0200, Christoph Hellwig wrote:
> > I heard rumors about IBM porting their LVM code from AIX to Linux.
> 
> IBM has an OpenSource volume manager called evms, and although
> it does support AIX Volumes is has it's root in IBM's OS/2
> volume management system.  (I believe someone at IBM thinks of PCs
> when hearing Linux so all their ports start from OS/2...)

Actually, this isn't quite correct.  Yes, the EVMS folks started with
the LVM design from OS/2 (which was based on AIX LVM, but not compatible).
However, IBM DID GPL the AIX 4.3 LVM code, and there IS support for AIX
VGs under EVMS.

> > Will it be replaced with the one from IBM ?
> 
> ALthough the current LVM has its's issues I hope not so - the current
> EVMS is the best example on hhow to not write kernel subsystems.

Do you have a specific complaint?  The current LVM kernel code isn't all
that robust either (and the user-space code is not very pretty).

I DO think that EVMS will replace the current LVM code in the kernel.  Why?
- It already has 100% compatibility with the current LVM on-disk layout.
- It already has 100% compatibility with AIX LVM code.
- It is already possible to do LVM autoconfig of volumes within the kernel
  without needing an initrd to activate the volumes.
- It removes knowlege of partitions out of the disk drivers and makes them
  a simple form of LV.
- It is already possible to use partial Linux-LVM volumes (i.e. use LVs
  that are 100% available in a VG, even if a disk is bad/missing).  You
  can't do that with the current LVM (sometimes you can't even do it if
  ALL of the disks are there).
- The AIX LVM on-disk metadata layout is FAR more robust than the current
  LVM metadata layout.  While this is supposed to be fixed for Linux-LVM
  at some point, the previous Linux-LVM changes have been TERRIBLE with
  respect to compatibility, while EVMS is DESIGNED to support multiple
  on-disk metadata layouts.
- It is possible (not done yet) to add things like NT Logical Disk Manager
  (NT LVM-stuff) into EVMS.
- It is possible (not done yet) to add HPUX LVM/veritas/OSF volume stuff.
- It is possible to merge MD RAID support into EVMS also, to support all
  of the different RAID layouts with common code*.

All in all, instead of having things like striping/RAID-1/concatenation/
RAID-5 reimplemented in each subsystem (e.g. LDM/MD/veritas/etc) we can
take the current MD code and make it a layer in EVMS which can handle all
of these cases.

(*) While the exact on-disk layout of each RAID implementation may vary
slightly (block sizes, stripe widths, etc), I'm guessing that the majority
of it is common enough that we can re-use the existing MD code (parity
calculation, resync, etc) to handle most kinds of RAID-0/1/5 setups.

Cheers, Andreas
-- 
Andreas Dilger  \ "If a man ate a pound of pasta and a pound of antipasto,
                 \  would they cancel out, leaving him still hungry?"
http://www-mddsp.enel.ucalgary.ca/People/adilger/               -- Dogbert