Friday, October 21, 2005

logical volume problem

logical volume problem

Hi all,
I am using veritas VM 3.5. When I want to create a raid 5 volume by the
following command,

bash-2.05# vxassist -g diskgroup make vol-1 10m layout=raid5
vxvm:vxassist: ERROR: Too few disks for striping; at least 4 disks are
needed
bash-2.05# vxdisk -g diskgroup list
DEVICE TYPE DISK GROUP STATUS
c2t1d0s2 sliced diskgro02 diskgroup online
c2t2d0s2 sliced diskgro03 diskgroup online
c2t3d0s2 sliced diskgro01 diskgroup online

I get the error "ERROR: Too few disks for striping; at least 4 disks".
Raid 5 only need 3 disk. Why?

Reply

Because there are some interesting failure modes where a crash can occur
in the middle of a write, leaving you not knowing if parity is right or
wrong. Combined with a disk error, you can have problems.

To get around that failure mode, the default for a raid5 construction
with vxassist is to create an additional log device which is not on any
disk shared with the raid5 data. It's small, but must be on a separate
disk. There might be a way of using the same disk but replicating it.

If you don't want the log disk, you can specify 'nolog' or 'noraid5log',
then it will only need 3 columns.

vxassist -g diskgroup make vol-1 10m layout=raid5,nolog

log, nolog
Creates (or does not create) dirty region logs
(for mirrored volumes) or log plexes (for RAID-5
volumes) when creating a new volume. This attri-
bute can be specified independently for mirrored
and RAID-5 volumes with the raid5log and regionlog
layout specifications. The current implementation
does not support the creation of DCM logs in the
layout specification.

raid5log, noraid5log
Creates (default) or does not create log plexes
for RAID-5 volumes.

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