linux-ext4 - Re: [PATCH 5/6] libext2fs/e2fsck: provide routines to read-ahead metadata

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Date:	Mon, 11 Aug 2014 14:32:58 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	"Darrick J. Wong" <darrick.wong@...cle.com>
Cc:	linux-ext4@...r.kernel.org
Subject: Re: [PATCH 5/6] libext2fs/e2fsck: provide routines to read-ahead
 metadata

On Mon, Aug 11, 2014 at 11:05:09AM -0700, Darrick J. Wong wrote:
> 
> Using the bitmap turns out to be pretty quick (~130us to start RA for 4 groups
> vs. ~70us per group if I issue the RA directly).  Each fadvise call seems to
> cost us ~1ms, so I'll keep using the bitmap to minimize the number of fadvise
> calls, since it's also a lot less code.

4 groups?  Since the default flex_bg size is 16 block groups, I would
have expected that you would want to start RA every 16 groups.

(And BTW, I've been wondering whether we should increase the flex_bg
size for bigger file systems.  By the time we get to 4TB disks, Having
a flex_bg every 2GB seems a little small.)

						- Ted
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