Stupid Question?

Hi,

I feel like this is a stupid question. But I actually don't know the answer to it. If I'm going to make a Software RAID array with a bunch of identical disks, do the disks have to have at least one partition on them? Or can I use disks with NO partitions?

Similarly, if I have made a hardware RAID array (say, with a 3ware 8506 card), do I have to create at least a single partition on it before I put a file system on it?

If partitions aren't necessary, is there any advantage or disadvantage to having a single partition on a disk versus having none? Is having no partitions faster?

Andy Liebman

You can use any block device as part of a Linux software RAID device, so full disks with no partitions inside should be OK. I seem to remember that this is not the case with LVM, wher you have (should?) to create a partition even if you want to use the full disk, but is late and maybe I and mixing things.

I don't think so. You can "format" any block device, like a disk partition, a logical volume, and even a file on disk through the loop device, so the requirement for a partition table doesn't seem to exist. I think the only thing needed is a couple of (major,minor) trhough which to access the underlying block device.

A disk with no partition table is a contiguous block device from sector zero to the end of the device. Maybe you should follow LVM's advice about using full disks with no partition table. From pvcreate(8):

DESCRIPTION
pvcreate initializes PhysicalVolume for later use by the Logical Volume Manager (LVM). Each PhysicalVolume can be a disk partition, whole disk, meta device, or loopback file. For DOS disk partitions, the par- tition id should be set to 0x8e using fdisk(8), cfdisk(8), or a equiva-
lent. For whole disk devices only the partition table must be erased,
which will effectively destroy all data on that disk. This can be done by zeroing the first sector with:

dd if=/dev/zero of=PhysicalVolume bs=512 count=1

Greetings.

Jose Luis Domingo Lopez
Linux Registered User #189436     Debian Linux Sid (Linux 2.6.6)

Yes, the disks have to have partitions. When you specify "make a raid out of X, Y, Z", the X, Y, and Z are partition names, like /dev/hda1, /dev/hdb1. You can't make a raid out of /dev/hda and /dev/hdb - whole devices.

Hardware raid is usually device oriented, so you do say "make a raid pair out of drive 0 and drive 1, equivalent to /dev/hda and /dev/hdb for example. This drive pair is then presented to Linux as a single hardware devices, like /dev/hdi. You always need to make partitions with fdisk before initializing the partition with a filesystem using mkfs.

Why not? I do it all the time.

You cannot get the kernel to auto-detect the raid if it is made from whole devices, but I don't care much about that. It works for me.

NeilBrown

Hi,

Whats the benefit of doing an Array over the whole devices? Are there any problems, for example that you maybe cannot boot from that array?

Thanks,

Greetings,
Stephan

No particular benefit. But if I want to RAID together a number of whole drives, there seems little point partitioning them first. It would certainly work either way. I choose the way that makes most sense in my situation.

I often RAID1 a pair of whole devices, partition them into root/swap/rest and boot off them (this is using md partitioning code which isn't in 2.4, but is in 2.6). There is a problem here that lilo thinks it knows all about md arrays. However I figured out how to trick it into not realising they are md arrays and it works fine (I create a symlink from /dev/MD* -> /dev/md* and only tell lilo about /dev/MD*).

NeilBrown

Neil,

Would you care to elaborate?

I'm about to build a box with 6 x 250GB SATA disks and am currently designing the partitioning scheme.

Could I potentially put in a small IDE disk onto which to install just enough system to boot and mount a RAID5 array built from the 6 whole disks?

Presumably you then partition the RAID array, create the filesystems and mount the partitions as you would a single disk?

Thanks,

R.

In this configuration I wouldn't raid whole drives. I would partition each drive into
6 gig root
2 gig swap
N-8 gig extra

I would raid1 2 or 3 root partitions together and add the rest as hot spares.
I would raid1 2 or 3 swap partitions together and add the rest as hot spares.
I would raid5 the extra partitions together and put the bulk storage there.

If you want a single large root and no extra partitions, I would then make a 1 gig boot and an N-1 gig root on each device, raid1 the boot devices together and put kernels there. raid5 the root devices together and put everything there. swap to a file on the root filesystem.

The two situations where I raid together whole devices are:

1/ computers with only two drives - then I mirror and partition. 2/ fileservers where the main storage drives are physically separate in some way, either in a different cage, or in the same cage but a different size (2 smaller drives for boot/root/etc, several larger drives for storage). Then I raid5 all the storage devices together.

In between these two (just two drives, and lots of drives) is the several-drives-in-one-case situation, where partitioning and then RAIDing works well.

NeilBrown