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Re: [SOLVED] Re: Partitioning And Formatting A Large Disk (2086.09GB)

On Fri, Feb 02, 2007 at 12:10:33PM +0100, Pim Bliek wrote:
> On 2/1/07, Douglas Allan Tutty <dtutty@porchlight.ca> wrote:
> >On Thu, Feb 01, 2007 at 01:06:54PM -0500, Michael S. Peek wrote:
> >> I.e. Right now I've got two large arrays.  Should I maybe break that
> >> down unto just a bunch of disks and then use LVM to group them together
> >> (not use hardware RAID at all), or should I break the disks into each
> >> bundles of three and make as many small raid5 arrays as I can and then
> >> group them under LVM?
> >>
> >> What's the general consensus on actually using LVM with hardware RAIDs?
> >
> >Separate the two concepts:
> >        raid protects you from drive failure
> >
> >        LVM allows you to move partitions from one block device (drive,
> >        raid array, whatever) to another and to resize those partitions.
> >
> >So do both.
> >
> >If you're using hardware raid and one disk starts failing, I would think
> >that you would want one port free on your hardware controller where you
> >can add a new disk (or leave a hot spare) to allow swapping out a
> >failing (but not totally failed) drive without degrading the array by
> >pulling the failing drive first.
> >
> >Since I've never had a hardware raid card, I could be wrong on this.
> >
> >If I recall correctly, the drive-space efficiency goes up the more
> >drives you have in a raid5 array.  If you broke it up into 3-drive
> >arrays, you'd only have 66% efficiency vs whatever you have now.
> >
> >I'm assuming that your two large arrays aren't one huge array because of
> >using two hardware raid controllers.
> >
> >Personally, I'd use both arrays as PVs for LVM.  Then the LVs can be
> >made stripe for better performance (since raid protects you from drive
> >failures).
> >
> >FYI, remember that /boot can't be on lvm.  I have mine on a raid1.
> >
> >I have no idea how to set this up after install if you're wanting / on
> >LVM.  I did mine during Etch install.
> >
> >Since you're going to all this trouble, choose a good FS like XFS or
> >JFS.
> - I have 4 300GB disks in a RAID5 array. On a hardware RAID
> controller. This gives me 900 GB space. Secure. If a drive fails, the
> array is degraded but my data is safe. Just replace the drive, have
> the array rebuilt and you're done.
> - Second layer is LVM. LVM is not there to protect your data. Anyone
> who makes you think that, ignore. LVM is there to give you
> flexibility.
> - I use this 900GB array as one big pool. I can create LV's on it,
> resize them, etc. Also, what I find very nice about LVM: I can give
> them logical names. Reads a lot easier than /dev/sda7.
> - If you happen to have a *lot* of disks in multiple hardware RAID5
> arrays, you can use LVM to shuffle around LV's. You can move and LV
> from one array to another, as long as they are both part of your VG.
> Anyway, do some reading on LVM in the LVM-Howto. But don't confuse LVM
> with RAID. My Advise: keep your RAID array in place, put LVM on top.
> Do some thinking on the lay-out of your volumegroups and logical
> volumes.

Hi Pim,

I think we're in basic agreement.  The only difference that I can see
between our approaches is that I suggested having a spare port on the
hardware raid card to add the new drive before removing a
partially-failed drive.  IIRC, normal raid5 can only tolerate one failed
drive.  What happens if a second drive fails totally while you're
swapping out a failing one?


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