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Message-ID: <20120920084507.GB5697@localhost>
Date: Thu, 20 Sep 2012 16:45:07 +0800
From: Fengguang Wu <fengguang.wu@...el.com>
To: Namjae Jeon <linkinjeon@...il.com>
Cc: Jan Kara <jack@...e.cz>, Dave Chinner <david@...morbit.com>,
linux-kernel@...r.kernel.org,
Namjae Jeon <namjae.jeon@...sung.com>,
Vivek Trivedi <t.vivek@...sung.com>,
Linux Memory Management List <linux-mm@...ck.org>,
linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH v3 2/2] Documentation: add description of
dirty_background_centisecs
[ add CC ]
On Sun, Sep 16, 2012 at 08:26:04AM -0400, Namjae Jeon wrote:
> From: Namjae Jeon <namjae.jeon@...sung.com>
>
> This commit adds dirty_background_centisecs description in bdi sysfs
> documentation.
>
> Signed-off-by: Namjae Jeon <namjae.jeon@...sung.com>
> Signed-off-by: Vivek Trivedi <t.vivek@...sung.com>
> ---
> Documentation/ABI/testing/sysfs-class-bdi | 25 +++++++++++++++++++++++++
> 1 file changed, 25 insertions(+)
>
> diff --git a/Documentation/ABI/testing/sysfs-class-bdi b/Documentation/ABI/testing/sysfs-class-bdi
> index 5f50097..6869736 100644
> --- a/Documentation/ABI/testing/sysfs-class-bdi
> +++ b/Documentation/ABI/testing/sysfs-class-bdi
> @@ -48,3 +48,28 @@ max_ratio (read-write)
> most of the write-back cache. For example in case of an NFS
> mount that is prone to get stuck, or a FUSE mount which cannot
> be trusted to play fair.
> +
> +dirty_background_centisecs (read-write)
> +
> + It is used to start early writeback of given bdi once bdi dirty
> + data exceeds product of average write bandwidth and
> + dirty_background_centisecs. It works in parallel of
> + dirty_writeback_centsecs and dirty_expire_interval based periodic
> + flushing mechanism.
> +
> + It is mainly useful for tuning writeback speed at 'NFS Server'
> + sothat NFS client could see better write speed.
> + A good use case is setting it to around 100 (1 second) in the NFS
> + server for improving NFS write performance. Note that it's not
> + recommended to set it to a too small value, which might lead to
> + unnecessary flushing for small IO size.
> + Setting it to 0 disables the feature.
> +
> + However, sometimes it may not match user expectations as it is based
> + on bdi write bandwidth estimation. The users should not expect this
> + threshold to work accurately.
> + Write bandwidth estimation is a best effort to estimate bdi write
> + speed bandwidth. But it can be wildly wrong in certain situations
> + such as sudden change of workload (including the workload startup
> + stage), or if there are no heavy writes since boot, in which case
> + there is no reasonable estimation yet.
> --
> 1.7.9.5
>
> --
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