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

[Tejun Heo]

Hello, Jens.

Jens Axboe wrote:
> On Mon, May 28 2007, Neil Brown wrote:
>> I think the implementation priorities here are:
>>
>> 1/ implement a zero-length BIO_RW_BARRIER option.
>> 2/ Use it (or otherwise) to make all dm and md modules handle
>>    barriers (and loop?).
>> 3/ Devise and implement appropriate fall-backs with-in the block layer
>>    so that  -EOPNOTSUP is never returned.
>> 4/ Remove unneeded cruft from filesystems (and elsewhere).
> 
> This is the start of 1/ above. It's very lightly tested, it's verified
> to DTRT here at least and not crash :-)
> 
> It gets rid of the ->issue_flush_fn() queue callback, all the driver
> knowledge resides in ->prepare_flush_fn() anyways. blkdev_issue_flush()
> then just reuses the empty-bio approach to queue an empty barrier, this
> should work equally well for stacked and non-stacked devices.
> 
> While this patch isn't complete yet, it's clearly the right direction to
> go.

Finally took a brief look. :-) I think the sequencing for zero-length
barrier can be better done by pre-setting QUEUE_ORDSEQ_BAR in
start_ordered() rather than short circuiting the request after it's
issued.  What do you think?

Thanks.

-- 
tejun