Ben Hutchings writes ("Re: Bug#584881: Lockups under heavy disk IO; md (RAID)
resync/check implicated"):
> I/O barriers are block I/O operations (not specific to md) that inhibit
> reordering of read and write operations. They certainly should not be
> blocking operations. Also, device-mapper did not support barriers until
> after 2.6.26 so md will not be using them in the configuration you are
> using.
No, I think there are two meanings of the word "barrier". AFAICT md
has its own thing which it confusingly calls a "barrier"; it can be
"raised" and "lowered".
Ian.
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