Stupid Question?

Stupid Question?

[AndyLiebman]

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

Re: Stupid Question?

[Jose Luis Domingo Lopez]

On Sunday, 23 May 2004, at 17:52:48 -0400,
AndyLiebman@aol.com wrote:

> 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? 
> 
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.

> 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? 
> 
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.

> 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? 
> 
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)

Re: Stupid Question?

[jim]

> 
> 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? 

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.

> 
> 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? 

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.

> 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
> -
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Re: Stupid Question?

[Neil Brown]

On Sunday May 23, jim@rubylane.com wrote:
> > 
> > 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? 
> 
> 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.

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

Re: Stupid Question?

[AndyLiebman]

>You always need to make partitions
>with fdisk before initializing the partition with a filesystem using
>mkfs.

After creating a Hardware RAID-5 with a 3ware 8506 card -- which comes up as 
/dev/sda on my machine (my system drive is a single IDE drive, /dev/hda) -- I 
have found that I AM able to put my filesystem on the RAID without making a 
partition. 

I ask this question because I have seen some discussions about some issues 
with "blockdev" and "block sizes" and how maybe you can get better throughput on 
devices if they are "higher" up the chain. I.E., /dev/sda is higher than 
/dev/sda1 and /dev/sda2. 

I didn't see any problems "formating" or "putting a file system" on a whole 
block device like /dev/sda. I ran Bonnie++ on it without issue. (I got the same 
results as with /dev/sda1 -- but I'm now looking into what I've been reading 
about blockdev and doing some tuning). But maybe I'm missing some. I'm NOT 
booting from the RAID. It's just for data storage. 

By the way, for those with 3ware 8506 cards, at the suggestion of 3ware I 
tuned the readahead value on my RAID with 'blockdev --setra 16384 /dev/sdX' 
(actually they had recommended this for their forthcoming 9500 series) and my read 
results with Bonnie++ jumped from 100 to 200 MB/sec on a 8-drive SATA RAID-5 
array. I don't yet know how the tuning will affect real world results. 

Might similar tuning make any difference for Software RAID-5 (I already get 
175 MB/sec using the same 3ware card in its JBOD mode). 

Andy Liebman