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



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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