PERC Battery relearn cycles and controlling write caching settings

we have run into this issue with the db server's io performance taking a deep dive when the raid controller's cache battery backup unit goes through its learning cycle. after putting a monitor on when the learning cycle is going to hit we tried to delay the event - using Megacli it seemed like it allows for a 7day delay; after setting that it still did go ahead and start the discharge/recharge cycle.

from watching the controller log it disables cache when the battery charge level is under 25% ; and that period is about an hour.

next we tried changing the default cache policy to set "write cache with Bad BBU" - expecting that the caching will never go to WriteThrough mode; but it still does. hoping to get some guidance here on how to accomplish this.

goal: always leave Write cache enabled ; BBU charge state notwithstanging.

Using Megacli here's what was done.

# MegaCli64 -ldsetprop -cachedbadbbu -L0 -a0   ## set Default policy

and resulted in,

Default Cache Policy: WriteBack, ReadAheadNone, Direct, Write Cache OK if Bad BBU
Current Cache Policy: WriteBack, ReadAheadNone, Direct, Write Cache OK if Bad BBU

thanks,

 

-prasana