[SATLUG] LVM and software RAID

[Robert Parkhurst]

It's been a while since I've used RAID+LVM in Linux (we switched to
Solaris/ZFS for the iSCSI), but the linux RAID tools (mdadm) can grow
some RAID sets, but not all..  I believe you can grow a RAID-5 set (one
drive at a time), and I think you can grow a JBOD RAID, but not a RAID-0
set.

LVM lets you grow a RAID-0 set.  However, it won't restripe the current
data across the new drives.  It will, however, stripe all NEW data
across all the drives.  So new data will benefit from increased drives.

If anything I've said needs correction, please correct me :)


Robert




On Thu, 2007-11-29 at 10:19 -0800, Chris Lemire wrote:

> Why is LVM a better solution if only RAID0 is needed?
> 
> Robert Parkhurst <rparkhurst at scarabimaging.com> wrote: I've played with LVM + RAID (for use in an iSCSI setup).  The two are
> designed to work together.  As Don was saying the only time you'd
> probably want to use LVM only would be if you were doing a RAID-0 setup.
> 
> As far as a filesystem goes, I'd recommend XFS as the fs because of it's
> ability to grow on the fly and very quickly.  Reisferfs does this also I
> believe (my experience is with XFS though ;-)).  JFS I don't have
> experience with (or none that are good) and EXT3 can grow, but you have
> to take the filesystem off-line while you do it and growing can take a
> while (like, go watch t.v. while).
> 
> 
> 
> Robert
> 
> 
> 
> 
> 
> On Thu, 2007-11-29 at 00:42 -0600, Don Wright wrote:
> 
> > On Wed, 28 Nov 2007 21:39:23 -0800 (PST), Chris Lemire
> >  wrote:
> > 
> > >I was told that LVM does the same thing as raid 0. It does striping, and it's does it in  a much much friendlier and more flexible manner.
> > >
> > >http://www.linuxquestions.org/questions/linux-software-2/software-raid-question-595657/
> > >
> > >Also I was told that there is no purpose to use RAID 0 in conjunction with LVM2, but Steve Kolars presentaton on RAID and LVM used both, so why are they used together?
> > 
> > Short answer: The two have different functions. RAID is used for
> > reliability and LVM is used for flexibility. 
> > 
> > RAID 0 is RAID without the reliability. It takes two or more drives and
> > makes one pseudo-drive that is less reliable than either separately.
> > (Don't ask how I know.) There are applications where the tradeoffs
> > between access time, capacity, and reliability work out in favor of RAID
> > 0, but that's beyond the scope of this reply.
> > 
> > For many applications the answer is LVM on top of a RAID 1 or 5 volume.
> > In that case two or more drives in RAID provide the reliability while
> > the LVM layer allows flexibility to allocate the space to various
> > filesystems as needed. You can even replace or add physical drives
> > (hardware permitting) in the RAID while LVM is running. This works
> > because LVM only sees the RAID volume, such as /dev/md0, and is not
> > concerned with the underlying hardware.
> > 
> > There should be plenty of discussion of LVM and RAID on the Web, with
> > arguments for every possible combination. Have fun learning!  --Don
> > 
> > -- 
> > Be well - or at least have interesting symptoms!
> 
> Robert Parkhurst
> IT Engineer
> Scarab Digital Imaging
> 110 Broadway, Suite 190
> San Antonio, TX 78205
> Office:    210.527.1390
> Mobile:   210.289.2538
> -- 
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Robert Parkhurst
IT Engineer
Scarab Digital Imaging
110 Broadway, Suite 190
San Antonio, TX 78205
Office:    210.527.1390
Mobile:   210.289.2538