RE: Three drive raid set.

[Chris Bipes <chris.bipes@xxxxxxxxxxxxxxx>]

All drives in a raid set have to be the same size and in fact are.  If you
look all three drives are the same.  3 drive raid sets spread the parity
data across 2 drives.  You cannot change this.  4 drive raid sets spread the
data across all 4 drives.  Every write to the raid set puts data on at least
2 of the 3 drives.  For instance, data written to drive 53 puts the parity
on one or both of the other two drives.  Data written to either drive 54 or
55 puts the parity on the other drive.  So if 53 dies, 54 and 55 are used to
re-build the data on the replacement drive.  If 54 or 55 die, the other
drive is used to rebuild the failed drive and 53 is not.

4 and 8 drive raid sets puts parity data across all drives.  When a raid set
is built, the drive is partitioned showing only what is available for data
and not the who drive.  That is why you see difference in sizes of the
drives.

Chris Bipes

-----Original Message-----
From: midrange-l-bounces@xxxxxxxxxxxx
[mailto:midrange-l-bounces@xxxxxxxxxxxx] On Behalf Of Alexei Pytel
Sent: Friday, October 01, 2004 3:15 PM
To: Midrange Systems Technical Discussion
Cc: midrange-l@xxxxxxxxxxxx;
midrange-l-bounces+pytel=us.ibm.com@xxxxxxxxxxxx
Subject: Re: Three drive raid set.


To me it does not look like a RAID problem per se.
53  4327    70564  74.9  131.3      5.9    47.3   83.9
54  4327    35282  74.9   50.7      6.6    13.8   36.8
55  4327    35282  74.9   70.0      5.6    22.0   47.9

I think, in this configuration the problem is because there is a 
significant imbalance in unit size.
Unit 53 is twice the size of other two.  Having set aside potential hot 
spots, normally I/O load is spread proportionally to amount of data stored 
on the drive, which again is roughly proportional to disk size.
As a result, unit 53 has to handle twice the load. If I/O load is light, 
this is not an issue, but with heavy load this unit will be simply 
overloaded.
RAID configurations are always somewhat unbalanced in size, but usually 
not so much (25% on 4-unit set, 12.5% on 8-unit set).
This configuration is simply not for heavy disk workload.

You will have to frequently rebalance data to keep data allocation 
"skewed" to compensate for disk size imbalance.
Adding one drive to the RAID set will make all drives equal in size, which 
is very very importnat for disk performance under heavy load.