Re: Which raid card to buy for Sarge

[Terrence Martin <tmartin@physics.ucsd.edu>]

me@heyjay.com wrote:

>----- Original Message ----- 
>From: "Jeff Garzik" <jgarzik@pobox.com>
>To: <me@heyjay.com>
>Cc: <linux-raid@vger.kernel.org>
>Sent: Wednesday, March 31, 2004 11:58 PM
>Subject: Re: Which raid card to buy for Sarge
>
>
>  
>
>>98% cpu doing just raid 1?  That sounds highly strange, even on an older
>>CPU.
>>
>>Typically RAID1 doesn't stress the cpu as much as PCI bus bandwidth and
>>the drives...
>>
>>    
>>
>
>Sorry, I was unclear.  Currently (without raid) my process maxes out my cpu.
>If I move to raid won't I have performance problems?  Maybe all the work
>happens at the PCI bus
>
>Jay
>
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AFAIK there is no additional calculations on RAID1 so little or no CPU, 
as long as you make sure your drivers are not in an IDE master/slave 
relationship you will get equivalent performance to a single drive 
system, but with redundancy.

This assumes that the RAID array is not recovering at the time of course 
or doing an integrity check (after an unclean shutdown).

In general though I would not expect Linux software RAID1  to have any 
additional CPU cost over a single drive.

As an aside alone the 99% CPU utilization is perhaps not a good measure 
of your system load or capacity. You should also look at how much IO 
your process produces and also the total load on the system (how much 
processes are waiting to execute).  Most processes if they do any 
significant IO are bound by that, not the CPU. If your process does 
little or no IO I would not expect any RAID config to have any impact at 
all, even RAID5.

see vmstat(8) and uptime(1)

Terrence