[dm-devel] dm flakey: document corrupt_bio_byte and optional feature precedence

[Mike Snitzer]

On Wed, Jun 29 2011 at 10:41am -0400,
Mike Snitzer <snitzer at redhat.com> wrote:

> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>  Documentation/device-mapper/dm-flakey.txt |   15 ++++++++++++++-
>  1 files changed, 14 insertions(+), 1 deletions(-)
> 
> diff --git a/Documentation/device-mapper/dm-flakey.txt b/Documentation/device-mapper/dm-flakey.txt
> index 6b40f79..6b4df49 100644
> --- a/Documentation/device-mapper/dm-flakey.txt
> +++ b/Documentation/device-mapper/dm-flakey.txt
> @@ -27,7 +27,20 @@ Mandatory parameters:
>  
>  Optional feature parameters:
>    If no feature parameters are present, during the periods of
> -  unreliability, all I/O returns errors.
> +  unreliability, all I/O returns errors.  Optional features may be
> +  combined (e.g. corrupt_bio_byte on READ and drop_writes).  Conflicting
> +  features (e.g. corrupt_bio_byte on WRITE and drop_writes) are resolved
> +  according to the following feature precedence (descending order):
> +
> +  corrupt_bio_byte <Nth byte> <bio_flags> <value>:
> +	Corrupt bios that have specific bi_rw flag(s), e.g.: READ=0 or
> +	REQ_WRITE=1|REQ_META=32, by writing <value> to the <Nth byte> of
> +	bios that have data.

Gah, we should also add:

<value> may be 0 or 1.  Bios that don't match <bio_flags> fall through
to the other optional feature(s) or the default of returning I/O error.

> +	Examples:
> +	1) corrupt 32nd byte in READ bios with the value 1:
> +	   corrupt_bio_byte 32 0 1
> +	2) corrupt 224th byte in WRITE|REQ_META bios with the value 0:
> +	   corrupt_bio_byte 224 33 0
>  
>    drop_writes:
>  	All write I/O is silently ignored.
> 
> --
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