[Beowulf] hardware RAID versus mdadm versus LVM-striping

Joe Landman landman at scalableinformatics.com
Tue Jan 19 06:43:11 PST 2010

Joe Landman wrote:

>   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.

I should also point out that the presence of dmraid and device 
enumeration is still not sufficient for determining whether something is 
or is not a fake-raid.  To wit

root at crunch:~# df -h /data
Filesystem            Size  Used Avail Use% Mounted on
                       4.6T  2.4T  2.2T  53% /data

and /data is on a most assuredly a hardware accelerated RAID.  There 
were some additional tools I installed with the distribution which also 
installed device-mapper.  I could turn it off, but then I have some 
other bits to work around.  It's easier to leave it on.

FWIW:  I am no fan of device mapper (the dm part of dmraid).  It has 
caused us some serious grief in the past (serious grief == data 
lossage).  Its not in a league with things like rieserfs, ext2, NTFS, 
and whatnot ...

When device mapper works correctly (as above) it works fine.  A 
tautology/Yogi-Berra-ism for sure.

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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