CERC Performance Question

[Neil Jones]

Grant ---

These numbers seem right.  I have the same system (4 drives, 500 Gb  
each) on a dual processor PE1800.

I get slightly worse performance under RAID 5, actually. And the  
kicker is that the write blocksize doesn't really affect performance  
so there doesn't seem to be any case where RAID5 works really well.   
Load avg jumping to 18 is also about right, and when that happens two  
things can occur:
   - it's almost impossible to use the system, even from a console
   - over prolonged periods (> 1 day), the aacraid driver starts  
falling into an error mode and emits messages to syslog---and the  
system needs to be rebooted

This happens with RHEL 4 and 3, and I do not believe it is specific  
to RH --- it seems like a hardware "feature".  If someone has the  
right kung-fu to suddenly make this configuration speedy, I'm all ears.

My solution was to split the data into "stuff I really need to be  
safe" and "stuff I can back up periodically".  Then I made two raid  
arrays: one RAID 0 and another RAID 1; I put the stuff I didn't need  
recoverable on the RAID 0 and the important stuff on the RAID 1.  Now  
the system is only somewhat slow compared to incredibly slow.   
(Writes are slow, but load only jumps to 2-3 and the system can be  
used concurrently.  Reads are tolerably fast but no stellar.)

Truthfully, this all seems quite slow for an SATA system but it's  
usable.  Dell, if you're listening, I think you should either  
*clearly mark* the CERC controller as a stop-gap measure for  
academics and flailing internet startups, or not offer it at all.   
Without a doubt, you need to remove the "supports RAID 5" from the  
website; while the controller might literally support the RAID5  
storage scheme, it is too slow for even casual single user usage.  I  
was thrilled to be able to get 1.5 Tb of RAIDed space on a system  
good for database serving for about $5k, but it was deceptively  
inexpensive.

..Neil

On Jul 25, 2006, at 7:41 AM, Sturgis, Grant wrote:

> Can anyone comment if these numbers look normal or not?
>
> Thanks.
>
> Sturgis, Grant wrote:
>> Greetings List,
>>
>> I am experiencing very poor performance with a CERC SATA 1.5/6ch RAID
>> card wtih firmware v4.1-0.  Four disks are configured in RAID 5  
>> and the
>> OS is RHEL ES 4.0.  I understand that this is not a high performance
>> RAID card or RAID configuration (write penalty associated with  
>> RAID 5),
>> but this just seems ridiculous.
>>
>> Created a 5GB file with the command:
>>
>> dd if=/dev/zero of=big_file bs=1024 count=5120000
>>
>> and then timed a move operation from an NFS mount to the local RAID
>> array connected at 1000Mbps end-to-end:
>>
>> time mv /hosts/server/test/big_file local_test_folder
>>
>> and the results were:
>>
>> 0.623u 34.342s 9:44.65 5.9%     0+0k 0+0io 3pf+0w
>>
>> This is over 10 minutes to move that much data over a gigabit
>> connection.  What's even worse is that the load average on the system
>> exceeded 18 resulting in an unusable system for all other users.   
>> There
>> was no response to any commands or login attempts.
>>
>> Does this seem reasonable to you?  What can I do to improve this
>> performance (short of doing away with RAID 5)?
>>
>> Any comments and suggestions are very much appreciated.
>>
>> Thanks,
>>
>> Grant
>> ----------------
>>
>>
>>
>>
>> Pardon this rubbish:
>>
>
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