[DRBD-user] unplug-watermark settings

Pascal BERTON pascal.berton3 at free.fr
Fri Mar 16 19:58:40 CET 2012

Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.



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,






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