linux-ext4 - Re: [PATCH, RFC] Use WRITE_SYNC in __block_write_full_page() if WBC_SYNC

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Date:	Mon, 5 Jan 2009 20:38:20 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Theodore Tso <tytso@....edu>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-ext4@...r.kernel.org, Arjan van de Ven <arjan@...radead.org>
Subject: Re: [PATCH, RFC] Use WRITE_SYNC in __block_write_full_page() if WBC_SYNC_ALL

On Mon, Jan 05 2009, Theodore Tso wrote:
> So long-term, I suspect the hueristic which makes sense is that in the
> case where there is an fsync() in progress, any writes which take
> place as a result of that fsync (which includes the journal records as
> well as ordered writes that are being forced out as a result of
> data=ordered and which block the fsync from returning), should get a
> hint which propagates down to the block layer that these writes *are*
> synchronous in that someone is waiting for them to complete.  They

If someone is waiting for them, they are by definition sync!

> shouldn't necessarily be prioritized ahead of other reads (unless they
> are readahead operations that couldn't be combined with reads that
> *are* synchronous that someone is waiting for completion), but they
> should be prioritized ahead of asynchronous writes.

And that is *exactly* what flagging the write as sync will do...

-- 
Jens Axboe

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