linux-kernel - Re: [RFC][PATCH 5/7] writeback: use 64MB MAX_WRITEBACK

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Date:	Thu, 10 Sep 2009 06:53:18 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Theodore Tso <tytso@....edu>
Cc:	Wu Fengguang <fengguang.wu@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	Dave Chinner <david@...morbit.com>,
	Chris Mason <chris.mason@...cle.com>,
	Christoph Hellwig <hch@...radead.org>, jack@...e.cz,
	Artem Bityutskiy <dedekind1@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	linux-fsdevel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/7] writeback: use 64MB MAX_WRITEBACK_PAGES

On Wed, 2009-09-09 at 19:29 -0400, Theodore Tso wrote:
> On Wed, Sep 09, 2009 at 10:51:46PM +0800, Wu Fengguang wrote:
> > + * The maximum number of pages to writeout in a single periodic/background
> > + * writeback operation. 64MB means I_SYNC may be hold for up to 1 second.
> > + * This is not a big problem since we normally do kind of trylock on I_SYNC
> > + * for non-data-integrity writes.  Userspace tasks doing throttled writeback
> > + * do not use this value.
> 
> What's your justification for using 64MB?  Where are you getting 1
> second from?  On a fast RAID array 64MB can be written in much less
> than 1 second.

Worse, on my 5mb/s usb stick writing out 64m will take forever.

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