On a Dell PowerEdge 1900, I was noticing in the OpenManage Alert log that the disk cache policy kept “bouncing” between Write Through and Write Back.
The key events are Storage Service 2188 and 2189:
2188 The controller write policy has been changed to Write Through.
2189 The controller write policy has been changed to Write Back.
Looking back, I could see this had been going on for months. I actually called Dell support about this back in March and was told this is normal during a reboot. Well now I was sure it was happening even when I wasn’t rebooting. I called Dell again. This time the tech had me export the PERC log:
The log is saved to the C:\Windows folder on the server, and named lsi_mmdd.log.
Absolute State of Charge
Once we opened that log, he had me look for the Absolute State of Charge statistic. Sure enough, the PERC’s charge was wavering from 22% to 25% at least since March (the oldest log entry). According to the tech, 25% is the absolute threshold; I assume when it goes below 25%, the PERC puts itself in write-through mode because the battery might not last long enough. That would go along with Dell’s description of the cause of Storage Service message 2188: “The controller battery is unable to maintain cached data for the required period of time.”
The tech said they recommend replacing the battery whenever this statistic falls below 30%, so he sent me a new battery under warranty. A few hours after installing it, I exported the PERC log again. It showed that the battery had finished charging in about 2.5 hours, but the only Absolute State of Charge in the file was 0%, the value immediately after installation. Turns out I needed to reboot the server to force the LSI firmware to reload, which re-generates the statistics. This time, the last Absolute State of Charge in the exported log is a comforting 96%:
T30: *** BATTERY CAPACITY INFORMATION *** T30: _____________________________________________ T30: Relative State of Charge : 100 % T30: Absolute State of Charge : 96 % T30: Remaining Capacity : 1715 mAh T30: Full Charge Capacity : 1715 mAh T30: Time to Empty : 65535 Min T30: Average Time to Empty : 65535 Min T30: Average Time to Full : 65535 Min T30: Cycle Count : 1 T30: Max Error : 2 % T30: _____________________________________________
Events after Battery Replacement
For the record, following are the events on boot-up immediately following the battery replacement. I thought it was interesting that the server knew the battery had been replaced and automatically started a charge cycle: