Re: Three drive raid set.

[keith@xxxxxxxxx]

Rob,

I have always been instructed to keep drives in a RAID set as close to the 
same size as possible to avoid the types of problem you are encountering.

Again, Alexei has stated what I have always understood.  The configuration 
in question will always have a problem, no matter in what type of system 
it is installed.  Many other systems would not even let you create a RAID 
configuration of this type, as a number of RAID controller cards will 
require identical size disks to create a RAID 5 set, if they will let you 
configure mismatched drives, it would only be as a RAID 0 (JBOD) set, 
which does not have parity protection.

Seems to me the only way to improve your situation is to modify you RAID 
set, either add additional drives to lessen the I/O load, or preferably, 
replace the odd sized drive with one or more drives to balance the RAID 
set.


Good Luck,


Keith Blazek
MIS Coordinator




Alexei Pytel <pytel@xxxxxxxxxx> wrote on 10/01/2004 06:14:53 PM:

> 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.
> 
>     Alexei Pytel
> always speaking for myself only
> 
> Where is the beginning of the end, which ends the beginning?
> 
> 
> 
> rob@xxxxxxxxx 
> Sent by: midrange-l-bounces+pytel=us.ibm.com@xxxxxxxxxxxx
> 10/01/2004 04:10 PM
> Please respond to
> Midrange Systems Technical Discussion
> 
> 
> To
> midrange-l@xxxxxxxxxxxx
> cc
> 
> Subject
> Three drive raid set.
> 
> 
> 
> 
> 
> 
> Anyone else out there have serious performance issues with a three drive 

> raid set?  We are.  And we even ran STRASPBAL.  IBM tried iDoctor, etc. 
> Nothing out of the ordinary.  So, their suggestion is a STRASPBAL 
*ENDALC 
> and then add them back in with STRASPBAL *CAPACITY.
> 
> Our BP says he knows of another customer that had the same issue but 
just 
> ordered another drive and it was solved.  Never opened pmr with IBM.
> 
> Trying to find a few "me too's", hopefully with pmr #'s, to encourage 
IBM 
> to look harder.
> 
> We will buy more drives.  We were hoping to put it off awhile.  Boss 
does 
> think that getting them in before year end is a good thing though, 
> budgetwise.
> 
> System ASP . . . . . . . :     3316 G
> % system ASP used  . . . :    74.9333
> 
> WRKDSKSTS
>                Size    %     I/O   Request   Read  Write
> Unit  Type      (M)  Used    Rqs  Size (K)    Rqs   Rqs 
> ...
> 50  4327    52923  74.9   15.3      8.6     7.0    8.2
> 51  4327    70564  74.9   25.3      7.9    13.7   11.6
> 52  4327    61744  74.9   29.4      6.4    17.7   11.6
> 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
> 
> The last three are the raid set in question.
> 
> Rob Berendt
> -- 
> Group Dekko Services, LLC
> Dept 01.073
> PO Box 2000
> Dock 108
> 6928N 400E
> Kendallville, IN 46755
> http://www.dekko.com
> 
> --