[gpfsug-discuss] RAID config for SSD's - potential pitfalls

[Marc A Kaplan]

As I've mentioned before, RAID choices for GPFS are not so simple.    Here 
are  a couple points to consider, I'm sure there's more.  And if I'm 
wrong, someone will please correct me - but I believe the two biggest 
pitfalls are:

Some RAID configurations (classically 5 and 6) work best with large, full 
block writes.  When the file system does a partial block write, RAID may 
have to read a full "stripe" from several devices, compute the differences 
and then write back the modified data to several devices.  This is 
certainly true with RAID that is configured over several storage devices, 
with error correcting codes.  SO, you do NOT want to put GPFS metadata 
(system pool!) on RAID configured with large stripes and error correction. 
This is the Read-Modify-Write Raid pitfall.

GPFS has built-in replication features - consider using those instead of 
RAID replication (classically Raid-1).  GPFS replication can work with 
storage devices that are in different racks, separated by significant 
physical space, and from different manufacturers.  This can be more robust 
than RAID in a single box or single rack.  Consider a fire scenario, or 
exploding power supply or similar physical disaster.  Consider that 
storage devices and controllers from the same manufacturer may have the 
same bugs, defects, failures. 




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