[Beowulf] hardware RAID versus mdadm versus LVM-striping

Joe Landman landman at scalableinformatics.com
Tue Jan 19 06:21:10 PST 2010

Richard Chang wrote:
> Joe Landman wrote:
>> Its a software RAID implementation pretending to be a hardware RAID 
>> implementation.  They are rarely if ever as good as MD.  Many of them 
>> in Linux will invoke dm (the "other" RAID engine) as dm has "support" 
>> for fake-raid.  Note that we have lost data (multiple times) with 
>> dm+fake-raid in testing, so we don't recommend its use in important 
>> machines (ones which you can't afford to lose).  This could be due to 
>> bad drivers for the chips in question, but we aren't taking chances.
> Hello Joe,
> I would like to know specifically what models of LSI boxes are software 
> RAID implementation pretending to be a hardware RAID implementation. I 

Hi Richard:

   This I cannot tell you, as I don't have a comprehensive list of what 
uses what driver.  I'd suggest looking at what drivers it loads for 
disks when it comes up.  If dmraid comes up *and* enumerates devices, 
you have a strong probability that it is a fake-raid.  This is not to 
say dmraid is bad.  Again, its the underlying driver or chipset that we 
often run into problems with.

> have a few LSI boxes where I work, and your post made me think if they 
> really are Hardware RAID implementation.
> I have the LSI 2822(old), LSI 4900 & also LSI 7900 controllers based 
> storage.
> How do we differentiate between the software and hardware RAID 
> implementations. ANY visual difference?, are they identifiable?.

Rarely.  Fake raid will generally not have any RAM cache or battery 
backup capability.

In some instances, fake raid is *ok* for OS drives (RAID1 only), if the 
bios is smart enough to use it correctly, the underlying fake raid 
driver is relatively stable, and you have reasonable disks.  Otherwise, 
mdadm works great, though you have to patch Redhat/Centos, as they, by 
default, use dmraid for the moment.  Later model Fedora appear to have 
switched to MD raid (after 9 from what I saw, last time I played with it).

> Richard.

Joseph Landman, Ph.D
Founder and CEO
Scalable Informatics Inc.
email: landman at scalableinformatics.com
web  : http://scalableinformatics.com
phone: +1 734 786 8423 x121
fax  : +1 866 888 3112
cell : +1 734 612 4615

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