9 Replies Latest reply: Oct 21, 2013 6:45 AM by D.BEREZENKO RSS

Consistency: NVRAM, NDU

D.BEREZENKO Cyclist
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Hi.

 

Mostly I will talking about NDU, so keep that in mind.

One of our competitor sad to a customer that in situation when in HA system one controller has fall, second will not switch in to "Write Through Mode" (which is not the best idea for the performance) and will continue "cache data" (but will have one point of failure which is Controller or NVRAM). As far as I know FAS systems not supporting "Write Through mode" and it can't be switched manually although.

They complain that after second controller will die, because of it data in NVRAM will (can) die as well. So theoretically we can have non-consistent (for a little bit, for the size of not written) data after we change (restore) the controllers. That can be very dangerous for some apps especially in SAN environment. From that point of view NetApp should not recommend "not disruptive update" (NDU) for 2 node systems, which include alternately update of each controllers.

 

>Some other vendors will switch to write-through if partner fails. This eliminates risk of lost acknowledged writes due to remaining controller failure. Stress point here being “acknowledged”.

In my opinion in this case NAS environment not so sensitive for such an event (at list we loose consistency for some files not whole FS). So I'm interesting mostly in SAN with such a failure.

 

It seams reasonable so the question is simple: how to replay for that? 

 

Thanks.

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