Stefan Bader wrote:
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.
The device mapper needs to ensure that ALL underlying devices get a barrier request when one comes down from above, even if it has to construct zero length barriers to send to most of them.
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