Re: Questions about LARGE, RAID storage under Linux

[Ragnar Kjørstad]

On Tue, May 20, 2003 at 12:15:39PM -0400, Michael S. Peek wrote:
> I am looking at using a hardware RAID device that appears to it's SCSI host as
> a single large drive.  I am well aware that I can break this device down into
> multiple LUNS, but if possible I need to keep it as one big drive.  I
> understand from the Linux Info Sheet, last updated in 1998 according to it's
> text, that Linux is able to handle partitions up to 4TB in size.

What is the Linux Info Sheet?

AFAIK linux has never had a partition-limit of 4 TB.
There is a limit in any block-device of 2^32 sectors, typically 2 TB.
Some drivers may have sign-issues, leaving you with only 2^31 sectors (1
TB).

> What I want to know is:
> 
> (a) Is 4TB still the maximum limitation on a single partition size?  If not,
> what is the current maximum?
> 
> (b) Would this maximum partition size still apply when using the software RAID
> tools to combine two or more of these devices together?

I believe the 2.4 kernel is still limited to 2 TB for both scsi-devices
and virtual block-devices such as md or lvm.

I think 2.5 has an option added for large block devices. This should
allow you to use both scsi-devices and md virtual devices larger than
2TB. It may depend on changes in the actual scsi-drivers, so possible it
only works for a subset of scsi-adapters. Last time I checked it didn't
enable larger LVM-volumes, so you're limited to md for virtual devices.

> What I am looking for is the ability to mount and format an external SCSI
> device that's 3.5TB in size.  (It's a Promise UltraTrak RM1500 w/ 15 x 250GB
> drives).  I want to be able to upgrade the hard drives at a later date and
> know that the Linux box to which it is attached will still be able to handle
> them.  Ideally, I would like to purchase a second (or even a third) device
> later down the road and use software RAID to concatenate them together.

I think you would want to use LVM to concatenate them together, and
AFAIK that is not yet possible. There is work in progress though.

I think your best chances of making this work in the future without
having to reformat your system is to use LVM2 with a large physical
extentsize from the start.

My information may be out of date, so take this with a grain of salt.

-- 
Ragnar Kjørstad
Zet.no