[linux-lvm] LVM vs. md: Benefits in RAID0?

[Gonyou, Austin]

What I see in my mind is this should work the same way veritas or hardware
raid should work. Adding a new partition to a stripe set shouldn't get
concatenated. The data should get re-striped to include the new disk, and as
time goes on, data will be then evened out to include this new partition and
stripe proplerly. Doing this approach preserves the stripe, at a small cost
of CPU during the slow re-stripe. The alternative is to have quick
re-restripes occurr, and to have XX% of cpu allocated to making that happen.
That way you can say, I want to give 98% of my cpu to the re-stripe
operation, but I know that my system will be unuseable during that
time(mostly), but it will take less time than say several hours where
re-striping would occurr slowly as data or accesses occurr and bits are
remapped. This of course means that you need a small amount of freespace not
allocated at the time of stripe creation. Adaptec and AMI accomplishes this
by setting about 4MB per volume in the striped set or raid x, as unused
automatically. That way, even if you have a raid set which is full of data
and perhaps <1% left, you still have these "swap regions" for use during
dynamic reallocation/restriping. This is what I see LVM being able to do in
the future, since it is already done in software from other vendors anyway. 

-- 
Austin Gonyou
Systems Architect, CCNA
Coremetrics, Inc.
Phone: 512-796-9023
email: austin at coremetrics.com 

> -----Original Message-----
> From: Jason Tackaberry [mailto:tack at linux.com]
> Sent: Tuesday, July 24, 2001 6:33 PM
> To: linux-lvm at sistina.com
> Subject: Re: [linux-lvm] LVM vs. md: Benefits in RAID0?
> 
> 
> Hi José, thanks for your reply.  I have a few questions:
> 
> On 24 Jul 2001 22:46:56 +0000, José Luis Domingo López wrote:
> > The quick and dirty hack to get resizable RAIDed storage devices is:
> > 
> > a) Layer LVM on top of (hard/soft) RAID
> 
> The problem I see with this is that if you extend the volume group by
> adding a new partition, the new partition must be linearly 
> concatenated
> and so it won't be part of the stripe set.  So while you can 
> technically
> extend this configuration, you're not truly resizing the RAID device.
> Suppose you start off with an md stripe set with sda1 and 
> sdb1, and then
> later on you want to add sdc1, you get something like this:
> 
> <lame ASCII art>
> +----------------------------+
> |         logical_vol01      |
> +----------------------------+
> |         volume_group01     |
> +-------------------+--------+
> |     md0 RAID0     |        |
> +-------------------+  sdc1  |
> |   sda1   |  sdb1  |        |
> +----------+--------+--------+
> </lame ASCII art>
> 
> So yes, you can extend your logical volume, but you're not extending
> your stripe set.  I guess this has the advantage over linear
> concatenation in that at least for sda1 and sdb1 you'll get the
> performance benefit.
> 
> > b) Layer sotfware RAID on top of software LVM
> 
> I'm just not sure how this would work?  You can't extend md 
> devices, as
> far as I know.  So you can extend the underlying logical volume, but
> then from the RAID layer on top, it just sees one device, so it's not
> really RAID at all.  Can you clarify on this one?
> 
> Cheers,
> Jason.
> 
> _______________________________________________
> linux-lvm mailing list
> linux-lvm at sistina.com
> http://lists.sistina.com/mailman/listinfo/linux-lvm
> read the LVM HOW-TO at http://www.sistina.com/lvm/Pages/howto.html
>