RAID 5 questions

I'm about to embark on building a RAID 5 Linux software array with 4 disks (I've only ever built 2 disk RAID 1 arrays before).

Q1: Is it true that Linux can't/shouldn't boot off a RAID 5 array?

Q2: The four disks have identical manufacturer, model and size, but two are brand new while two have had NTFS filesystems on them. Accordingly, if I run hdparm -g on them, two disks have these specs:

geometry = 2498/255/63, sectors = 40132503, start = 0

while the other two disks have these specs:

geometry = 39813/16/63, sectors = 40132503, start = 0

Is this an issue with software RAID?

Thanks
Michael Hall

Grub certainly can't boot of a software RAID-5 array, and I don't believe that lilo can either. This is because of these tools not understanding the RAID-5 disk layout, not any other limitation.

No. Software RAID, unlike hardware raid, cares only that you have enough blocks on the disk. You don't need to match *any* other characteristic of the disks, and not even that if you don't mind wasting a few bytes.

Daniel

The PC is the LSD of the '90s.
        -- Timothy Leary, __Guardian_ (June 1, 1996)

LILO does not support booting off of RAID 5, and I don't think GRUB does either. I usually make a small RAID-1 partition to boot from (mirrored across all four drives), since LILO does handle that just fine.

..

No.

-jim

no but it'd be better/faster/easier to put 1 disk per ide cable
- its $20 or less to get a promise ide controller

- or if you're brave, to disable the onboard raid in the bios and use its connectors as regular ide

1 disk per ide cable if you can ...

c ya
alvin

Umm, I maybe writing on complitely different setup (can't check the original post), but our server with software raid 5 (3 disks) is booting with lilo just fine.

BR,
Jarmo

check your setup, but your kernel is not on a raid 5 device.

L.

Luca Berra -- [email protected]
       Communication Media & Services S.r.l.
/"\
\ /     ASCII RIBBON CAMPAIGN
 X        AGAINST HTML MAIL
/ \
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

Hmm, we have 3 SW raid5 devices (md0, md1 and md3), with kernel on md0. Lilo.conf has root and boot=/dev/md0. Lilo v22.3.1.

-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html

README.raid from lilo-22.5.9 (the latest version) states:

"Only RAID1 is supported. LILO may be used to boot a system containing other RAID level partitions, but it may not be installed on any RAID partition other than RAID level 1."

so I don't believe you. You may be confusing "booting off RAID-5" with "having a RAID-5 root".

-jim

Hmm, I'm not sure how to convince you :-)

The only possibility I can think of is that the lilos.conf's raid-extra-boot= -line affects the booting, thus it's silently booting from /dev/hdX and not from /dev/md0. I'm not sure how to test this though.

BR,
Jarmo

yep. and even that may go wrong.

try running lilo with -v -v -v

Then you might see the BIOS drive number, geometry it assumes etc. You will probably see only _one_ disk and in that case your system boots by sheer luck.

With -v -v -v I discovered why my system refused to boot from raid 1. It appears to actually boot from the first disk in the raid set, e.g:

	md1 : active raid1 hdg1[1] hda1[0]
				   ^^^^^^^

This is mounted on /boot and lilo will boot from hda1 (bios disk 0x80). I had to reassemble the raid set in opposite order to get this because hdg is unbootable (lilo assumed bios drive 0x81). But maybe my lilo is old (21.4-4).

Frank
-
To unsubscribe from this list: send the line "unsubscribe linux-raid" in
the body of a message to [email protected]
More majordomo info at  http://vger.kernel.org/majordomo-info.html

You can do this sort of thing to override lilo's idea of which linux device maps to which BIOS disk number e.g. I use this on one box:

disk=/dev/hda

       bios=0x80

boot=/dev/hda

#disk=/dev/hdf
# bios=0x80
#boot=/dev/hdf

# Specifies the device that should be mounted as root. (`/') #
root=/dev/md3

I run lilo twice (once with the hda lines uncommented, and once with the hdf lines uncommented) - so that the system will still boot with hda pulled from the machine - with hda broken, the machine still won't boot, but at least you can disconnect hda at that point, or disable it in the BIOS (so that hdf becomes 0x80)...

You can do a similar thing with grub, but with the advantage that you don't have to play around with the config file every time you upgrade the kernel (you could use two different lilo.conf's I suppose)...

I'm not sure if this has already been stated (Frank just implied it), but a simple workaround to the original problem is to make a small RAID1 /boot partition on a couple of the disks, and have the rest of the drives carved up as RAID5, and I think that if you're using lilo to load the kernel off RAID5, then you're being lucky (AFAIK).

e.g. I sometimes do this:

drive1 drive2 drive3 drive4
[ boot(raid1) ] [ swap(raid1) ]

[     more-swap (raid5)        ]
[        /root  (raid5)        ]
[        /other (raid5)        ]

Tim.

it cannot no boot loader can load the kernel from a striped (or raid 3/4/5/6) device
create a small raid 1 array spanning the 4 disks and use it for /boot

...

no, nevertheless if you wipe the partition table on the to 'used' disks linux will see them just like the 'new' ones.

Luca Berra -- [email protected]
       Communication Media & Services S.r.l.
/"\
\ /     ASCII RIBBON CAMPAIGN
 X        AGAINST HTML MAIL
/ \

From software RAID it can't.

USB storage devices (memory stick) are incredibly useful for this -- I've even booted a webserver from a digital camera ;-). The only catch may be the latency of the USB code when you don't want to use initrd:
The kernel panics (no root dev) because it takes noticeable time before the kernel sees an USB storage device. But there are several tiny patches for this, it's merely a nuisance.

Frank

Personally I use sw raid5 as my /, and as I messed up and now can't fix it to work with a /boot, I instead use a lilo'ed 3.5 inch bootdisk, which loads the kernel and mounts root. *shrugs* it works fairly well, I can't complain much.

Tim McGrath

Followup to:  <20040801102430.GC28625@janus>
By author:    Frank van Maarseveen <[email protected]>
In newsgroup: linux.dev.raid

Actually, it should be able to if the firmware can see all the drives, AND you're not running it in degraded mode. Obviously not a good solution. Use RAID-1 for /boot.

-hpa