[dm-devel] Re: [RFD] BIO_RW_BARRIER - what it means for devices, filesystems, and dm/md.

[Phillip Susi]

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.