Disk partitions and LVM limits

[Nigel Wade]

Peter Blajev wrote:
> Hi,
> 
> I've got a DAS DELL MD1000 with a bunch of SATA drives in RAID 5 configuration 
> with total space of 5.4TB. This box is attached to a CentOS5 system (kernel 
> 2.6.18-53.1.6.el5).
> 
> Any idea how to make this space usable?

Is the RAID visible to the host? The method for doing this depends on 
the RAID controller and how it's attached to the host.

For example, I've just added some disks into an external FC RAID. Within 
the RAID I created a logical drive of size 3000000MB and mapped this to 
LUN 2 on the SCSI controller. On the host I issued the command
  # echo "scsi add-single-device 4 0 0 2" > /proc/scsi/scsi
to get the SCSI sub-system to rescan scsi4, id 0 LUN 2 which is where 
the RAID is attached.

In the system log I can see that the LUN has been found:

kernel: sdd : very big device. try to use READ CAPACITY(16).
kernel: SCSI device sdd: 6144000000 512-byte hdwr sectors (3145728 MB)
kernel: SCSI device sdd: drive cache: write back
kernel: sdd : very big device. try to use READ CAPACITY(16).
kernel: SCSI device sdd: 6144000000 512-byte hdwr sectors (3145728 MB)
kernel: SCSI device sdd: drive cache: write back
kernel:  sdd: unknown partition table
kernel: Attached scsi disk sdd at scsi4, channel 0, id 0, lun 2
kernel: Attached scsi generic sg11 at scsi4, channel 0, id 0, lun 2,  type 0
scsi.agent[1364]: disk at 
/devices/pci0000:80/0000:80:0c.0/0000:82:00.0/host4/target4:0:0/4:0:0:2

I can now go ahead and create a filesystem on /dev/sdd, or partition it 
into smaller partitions if I want.

> Is there a limit how big a partition can be? 

Yes, but it's bigger than your RAID.

What is the work around?

None is required, as there is nothing to work around.

> Is there a limit how big a file system ca be?

Yes, but it's bigger than your RAID.

> 
> I've tried to partition it but no matter how bug partition I create fdisk 
> spits out these messages on the console:
> ---
> sdb: very big device. try to use READ CAPACITY(16).
> SCSI device sdb: 10248519680 512-byte hdwr sectors (5247242 MB)
> sdb: Write Protect is off

That's ok. It's just a warning. It got the right size didn't it?

> ---
> 
> I decided to not partition the drive and use LVM but the physical volume 
> stopped at 2TB.

IIRC there's a limit to the size of a physical volume in LVM, but not to 
the size of a logical volume. I don't remember the nitty gritty details 
of LVM offhand. But you don't need to use LVM.

> 
> So, right now I can't use LVM because of this 2TB limit and I'm not sure if I 
> partition the drive how good these partitions are because of the the message 
> from fdisk.

It's only an informative message. Go ahead and use it.


-- 
Nigel Wade, System Administrator, Space Plasma Physics Group,
             University of Leicester, Leicester, LE1 7RH, UK
E-mail :    nmw at ion.le.ac.uk
Phone :     +44 (0)116 2523548, Fax : +44 (0)116 2523555