Note: "permalinks" may not be as permanent as we would like,
direct links of old sources may well be a few messages off.
/ 2007-03-13 07:39:33 +0200 \ Siim Vahtre: > > Hello, > > I haven't really used DRBD, but I've read all the documentation I was able to find on the internet. However, > there was one question that I didn't get good answer to: how (and if) DRBD is able to handle storage with > 'write cache' turned on? sure. but not how you aparently wish it would. so to not confuse you: no. > What happens if one node dies, and the data that disc reported as 'written' isn't actually written? > When the node is restarted, does it somehow know that it actually > needs full sync (because the data in the disc's cache may not be > actually written)? no. > Or is it actually able to do partial sync thanks to the internal log > so it knows what was really written? no. because there is no such thing as an "internal log". we have something we call "activity log". this does not however store any application data. it keeps a log (a list of numbers) of what areas on disk may be currently written to. > AFAIK for the latter, you need to be sure that the data, even though > maybe not actually written to disc, would be written in correct order > (so that the counters and consistency flags are updated AFTER the > other writes to the disc). Is my assumptions correct? If so, not at all. > is DRBD somehow able to enforce ordered writes? of course. > E.g short question: may I use write cache on both nodes and not worry > about data integrity (that is - until two of them die at the same > time...) no. when a Primary dies, then comes up again, drbd _might_ "do the right thing" anyways, though for different reasons. when a Secondary dies, then comes back, it will not know that something is wrong, and any resync that might happen is likely to miss some of the blocks that your disk previously reported as on stable storage. sorry. if your hardware pretends to have done something which it actually did not do, there is no drbd magic to cover for you. -- : Lars Ellenberg Tel +43-1-8178292-0 : : LINBIT Information Technologies GmbH Fax +43-1-8178292-82 : : Vivenotgasse 48, A-1120 Vienna/Europe http://www.linbit.com : __ please use the "List-Reply" function of your email client.