[Beowulf] Software Raid

Joe Landman landman at scalableinformatics.com
Tue Dec 13 22:22:20 EST 2005



Vincent Diepeveen wrote:

>>The remaining advantage of hardware is still hot-swapping
>>failed drives without having to shutdown the server.
> 
> 
> Those same nerds of above, they do not take into account that if 
> something complex like a raid array gets suddenly handled in 
> software instead of hardware, that even the tiniest 
> undiscovered bug in a file system, will impact you.

<scratch />
<scratch />
<huh? />

As the raid device is being created at the block device level, and the 
file system resides above this, a file system bug will be just as 
detrimental to a hardware raid system as it would a software raid system.

Of course, you could have meant a bug in the software raid block device 
driver.  Yes such things do exist.  So do bugs in the hardware raid 
controllers.  In *neither* case do you want to touch the buggy code. 
Best case is completely innocuous behavior.  Worst case, well, lets not 
get into that.

Bugs can and do occur in any software.  Whether burned into firmware, 
written as VHDL/Verilog that creates the ASICs or FPGAs on the hardware 
raid, or in the software raid block device.

[...]

> And be sure that there is bugs. So doing a hardware XOR (or whatever) in
> RAM of the raid controller instead of in the software, is a huge advantage.

RAID is *far more* than doing hardware XOR.  Most XOR implementations 
tend to be bug free given how atomic this operation is.  The code around 
it however occasionally has bugs.  Firmware and software code.

> It reduces complexity of what software has to do, so it reduces the
> chance that a bug will occur in the OS somewhere, causing you to lose
> all your files.

No.  Absolutely not.  Software raid simply does in software what the 
hardware may do in part in hardware.  Any bug, anywhere in this process 
(in either HW or SW raid) and you can have problems.  Problems and bugs 
are not just the provenance of SW raid.


-- 
Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics LLC,
email: landman at scalableinformatics.com
web  : http://www.scalableinformatics.com
phone: +1 734 786 8423
fax  : +1 734 786 8452
cell : +1 734 612 4615

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