Re: Spares and partitioning huge disks

On Thursday 06 January 2005 18:31, Guy wrote:

This idea of splitting larger disks into smaller partitions, then
re-assembling them seems odd.  But it should help with the "bad block kicks
out a disk" problem.

Yes.  And I'm absolutely sure I read it on linux-raid.  Couple of months back.

However, if you are going to use LVM anyway, why not allow LVM to assemble
the disks?  I do that sort of thing all the time with HP-UX.  I create
stripped mirrors using 4 or more disks.  With HP-UX, use the -D option with
lvcreate.  No idea if Linux and LVM can strip.

I think so.  But I am more familiar with md, so I'll still use that.  In any 
case LVM's striping is akin raid-0, whereas I will definitely use raid-5.

You are making me think!  I hate that!  :)  Since your 6 RAID5 arrays are

;-)  Terrible, isn't it.

on the same 4 disks, striping them will kill performance.  The poor heads
will be going from 1 end to the other, all the time.  You should use LINEAR
is you combine them with md.  If you use LVM, make sure it does not stripe
them.  With LVM on HP-UX, the default behavior is to not stripe.

Exactly what I thought.  That they are on the same disks should not matter; 
only when one full md set (6-1)*40GB=200GB is full (or used, or whatever) 
will the "access" move on to the next set of drives.  It is indeed imperative 
to NOT have LVM striping (nor use raid-0, thanks for observing that!), as 
that would be totally counterproductive and may thus kill performance. 
(r/w head thrashing)

For all clarity, this is how it would look:

md0 : active raid5 sda1[0] sdb1[1] sdc1[2] sdd1[3]
      40000000 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]
...
...
...
md5 : active raid5 sda6[0] sdb6[1] sdc6[2] sdd6[3]
      40000000 blocks level 5, 64k chunk, algorithm 2 [4/4] [UUUU]

The lvm part is still new to me, but the goal is simply to add all PVs /dev/
md0 through /dev/md5 to one LV yielding... well, a very large volume. :-)

I was planning to do this quickly tonight, but I've overlooked one essential 
thing ;-|  The old server has already 220 GB of data built of 4 80GB disks in 
raid-5.  But I cannot connect all 8 disks at the same time, so I'll have to 
'free up' another system to define the arrays and copy the data over Gbit 
LAN.  I definitely don't want to lose the data!
What complicates this a bit is that I wanted to copy the OS verbatim (it is 
not part of that raid-5 set, just raid-1). But I suppose booting a rescue CD 
would enable me to somehow netcat the OS over to the new disks...
We'll see.

But for now I'm searching my home for a spare system with SATA onboard... :-)

Maarten

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