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

[Marc A Kaplan]

Kums is our performance guru, so weigh that appropriately and relative to 
my own remarks... 

Nevertheless,   I still think RAID-5or6 is a poor choice for GPFS 
metadata.  The write cache will NOT mitigate the read-modify-write problem 
of a workload that has a random or hop-scotch access pattern of small 
writes.  In the end you've still got to read and write several times more 
disk blocks than you actually set out to modify.  Same goes for any large 
amount of data that will be written in a pattern of non-sequential small 
writes.  (Define a small write as less than a full RAID stripe). 

For sure, non-volatile write caches are a good thing - but not a be all 
end all solution.

Relying on RAID-1 to protect your metadata may well be easier to 
administer, but still GPFS replication can be more robust. 

Doing both - belt and suspenders is fine -- if you can afford it. Either 
is buying 2x storage, both is 4x.

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