> in-flight I/O to go to zero? Something like that is needed for some dm targets to support barriers. (We needn't always wait for *all* in-flight I/O.) When faced with -EOPNOTSUP, do all callers fall back to a sync in the places a barrier would have been used, or are there any more sophisticated strategies attempting to optimise code without barriers?
If I didn't misunderstand the idea is that no caller will face an -EOPNOTSUPP in future. IOW every layer or driver somehow makes sure the right thing happens.
An efficient I/O barrier implementation would not normally involve flushing AFAIK: dm surely wouldn't "cause" a higher layer to assume stronger semantics than are provided.
Seems there are at least two assumptions about what the semantics exactly _are_. Based on Documentation/block/barriers.txt I understand a barrier implies ordering and flushing. But regardless of that, assume the (admittedly constructed) following case: You got a linear target that consists of two disks. One drive (a) supports barriers and the other one (b) doesn't. Device-mapper just maps the requests to the appropriate disk. Now the following sequence happens: 1. block x gets mapped to drive b 2. block y (with barrier) gets mapped to drive a Since drive a supports barrier request we don't get -EOPNOTSUPP but the request with block y might get written before block x since the disk are independent. I guess the chances of this are quite low since at some point a barrier request will also hit drive b but for the time being it might be better to indicate -EOPNOTSUPP right from device-mapper. Stefan -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
