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List:       linux-raid
Subject:    RAID0 versus Linear
From:       "Shaw, Marco" <marco.shaw () nbtel ! nb ! ca>
Date:       2002-09-24 0:39:24
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RedHat Advanced Server 2.1AS
Raidtools-1.00.2-1.2

Couldn't seem to find anything relevant in the linux-raid archive...

Had an internal discussion regarding the choice of linear versus raid0.  We'd like to \
standardize on a usage within our company.

(If this makes a difference: We have a SAN which will present a host with a disk \
using SCSI over fibre.  (That's the extent of my SAN-technical jargon/knowledge.).  \
The virtual disk presented to the host is a 0+1 RAID volume on the other end which is \
supposed to be transparent to the user/host system.)

Disks will not necessarily always be of the same size, but we do want to concatenate \
them, so I guess linear would be the way to go.  Problem is I built a RAID0 array \
                during testing, and was now trying to convert it to linear, by \
                simply:
*editing the /etc/raidtab file appropriately: raid-level 0-->raid-level linear
*umount /raid_dir
*raidstop /dev/md0
*mkraid -R /dev/md0

I would assume that this is the appropriate procedure, but I get a nasty kernel \
crash, of which I don't have anything able to capture it.

(Tried different combinations of enable/disabling the persistent-superblock.  I've \
not been able to reproduce it regularly, but was getting errors that \
"persistent-superblock was not supported by linear or RAID0" at various times during \
the boot process, but once I redid the md device with a mkraid, and built a new fs, \
it seemed to be OK.  The logs complained about an invalid superblock on the device 0 \
everytime.)

I'd also like to confirm that RAID0 can only be used for disks of aproximately the \
same size.  If they aren't, does the extra space become unusable?

Marco
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