Guys,
I'm trying to better understand the unplug-watermark parameter settings, and honestly I don't find much info about a rational way to do it. I understand that it strongly relies on the general specs of the storage subsystem in use, but except that. I've seen a discussion about the CCISS driver stating that it prefers high values, but that's nearly all. If I use its follow-up HPSA driver is it the same ? If I use a QLogic FC HBA is it the same ? Is there any sort of "mathematical" way to size it ? Reading the doc, I understand that the cache size of the adapter probably plays a role. I suspect the size of the generated IOs is involved too. From the software prospective, would the driver queue length also play a role? More generally, is there kind of a method to size it properly (even if a bit approximative) based on various criterias, and if so which ones ? Because I have the fair impression that 90% of the DRBD users set it up without clearly knowing why, am I wrong ? Best regards, Pascal.
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