linux-kernel - Re: Question about REQ_FLUSH and bios with data

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Date:	Fri, 09 Aug 2013 21:18:15 +0100
From:	Steven Whitehouse <swhiteho@...hat.com>
To:	scameron@...rdog.cce.hp.com
Cc:	linux-kernel@...r.kernel.org, jens.axboe@...nel.dk, tj@...nel.org
Subject: Re: Question about REQ_FLUSH and bios with data

Hi,

On Fri, 2013-08-09 at 14:09 -0500, scameron@...rdog.cce.hp.com wrote:
> So, I'm working on a block driver using the make_request_fn
> interface, and have to handle a bio that comes in with
> (bi_rw & REQ_FLUSH) set AND data to transfer. 
> 
> According to Documentation/block/writeback_cache_control.txt:
> 
> 	The REQ_FLUSH flag can be OR ed into the r/w flags of a
> 	bio submitted from the filesystem and will make sure the
> 	volatile cache of the storage device has been flushed
> 	before the actual I/O operation is started. 
> 
> So I've written code that handles that case, along with the
> various error cases I might encounter, I think, which leads
> to my question:
> 
> How do I get such a bio with a data transfer AND the REQ_FLUSH bit
> set to come into the driver?   Just wondering how to test this case.
> 
GFS2 does this... if you look at the log flush code and the function
log_write_header() in particular, you'll see that it sets this on each
log header that gets written. You don't need a cluster to generate this
kind of i/o, just supply mkfs.gfs2 with -p lock_nolock and mount it as a
local filesystem. The combination of touch foo; sync should be enough to
generate a log flush writing a log header with this flag set,

Steve.

> Thanks,
> 
> (I cc'ed Tejun Heo just because his name is in block/blk-flush.c)
> 
> -- steve
> 
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