linux-kernel - Re: [PATCH] Documentation: Remove mentioning of block barriers

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Date:	Sat, 14 Mar 2015 07:23:56 +0100
From:	Jan Kara <jack@...e.cz>
To:	"Leonid V. Fedorenchik" <leonidsbox@...il.com>
Cc:	Jonathan Corbet <corbet@....net>,
	Mike Christie <michaelc@...wisc.edu>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	Jens Axboe <axboe@...com>, Hannes Reinecke <hare@...e.de>,
	Jan Kara <jack@...e.cz>, Christoph Hellwig <hch@....de>,
	linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Documentation: Remove mentioning of block barriers

On Fri 13-03-15 23:53:22, Leonid V. Fedorenchik wrote:
> Remove mentioning of block barriers since they were removed.
> 
> Signed-off-by: Leonid V. Fedorenchik <leonidsbox@...il.com>
  The change looks fine. It would be nice to at least reference
Documentation/block/writeback_cache_control.txt for description of
REQ_FLUSH and REQ_FUA flags.

								Honza
> ---
>  Documentation/block/biodoc.txt | 36 +++++++++---------------------------
>  1 file changed, 9 insertions(+), 27 deletions(-)
> 
> diff --git a/Documentation/block/biodoc.txt b/Documentation/block/biodoc.txt
> index 5aabc08..fd12c0d 100644
> --- a/Documentation/block/biodoc.txt
> +++ b/Documentation/block/biodoc.txt
> @@ -48,8 +48,7 @@ Description of Contents:
>  	- Highmem I/O support
>  	- I/O scheduler modularization
>    1.2 Tuning based on high level requirements/capabilities
> -	1.2.1 I/O Barriers
> -	1.2.2 Request Priority/Latency
> +	1.2.1 Request Priority/Latency
>    1.3 Direct access/bypass to lower layers for diagnostics and special
>        device operations
>  	1.3.1 Pre-built commands
> @@ -255,29 +254,12 @@ some control over i/o ordering.
>  What kind of support exists at the generic block layer for this ?
>  
>  The flags and rw fields in the bio structure can be used for some tuning
> -from above e.g indicating that an i/o is just a readahead request, or for
> -marking  barrier requests (discussed next), or priority settings (currently
> -unused). As far as user applications are concerned they would need an
> -additional mechanism either via open flags or ioctls, or some other upper
> -level mechanism to communicate such settings to block.
> -
> -1.2.1 I/O Barriers
> -
> -There is a way to enforce strict ordering for i/os through barriers.
> -All requests before a barrier point must be serviced before the barrier
> -request and any other requests arriving after the barrier will not be
> -serviced until after the barrier has completed. This is useful for higher
> -level control on write ordering, e.g flushing a log of committed updates
> -to disk before the corresponding updates themselves.
> -
> -A flag in the bio structure, BIO_BARRIER is used to identify a barrier i/o.
> -The generic i/o scheduler would make sure that it places the barrier request and
> -all other requests coming after it after all the previous requests in the
> -queue. Barriers may be implemented in different ways depending on the
> -driver. For more details regarding I/O barriers, please read barrier.txt
> -in this directory.
> -
> -1.2.2 Request Priority/Latency
> +from above e.g indicating that an i/o is just a readahead request, or priority
> +settings (currently unused). As far as user applications are concerned they
> +would need an additional mechanism either via open flags or ioctls, or some
> +other upper level mechanism to communicate such settings to block.
> +
> +1.2.1 Request Priority/Latency
>  
>  Todo/Under discussion:
>  Arjan's proposed request priority scheme allows higher levels some broad
> @@ -906,8 +888,8 @@ queue and specific I/O schedulers.  Unless stated otherwise, elevator is used
>  to refer to both parts and I/O scheduler to specific I/O schedulers.
>  
>  Block layer implements generic dispatch queue in block/*.c.
> -The generic dispatch queue is responsible for properly ordering barrier
> -requests, requeueing, handling non-fs requests and all other subtleties.
> +The generic dispatch queue is responsible for requeueing, handling non-fs
> +requests and all other subtleties.
>  
>  Specific I/O schedulers are responsible for ordering normal filesystem
>  requests.  They can also choose to delay certain requests to improve
> -- 
> 2.2.1
> 
-- 
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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