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

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Message-ID: <20140811201030.GH6553@thunk.org>
Date:	Mon, 11 Aug 2014 16:10:30 -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 11:55:32AM -0700, Darrick J. Wong wrote:
> I was expecting 16 groups (32M readahead) to win, but as the observations in my
> spreadsheet show, 2MB tends to win.  I _think_ the reason is that if we
> encounter indirect map blocks or ETB blocks, they tend to be fairly close to
> the file blocks in the block group, and if we're trying to do a large readahead
> at the same time, we end up with a largeish seek penalty (half the flexbg on
> average) for every ETB/map block.

Hmm, that might be an argument for not trying to increase the flex_bg
size, since we want to keep seek distances within a flex_bg to be
dominated by settling time, and not by the track-to-track
accelleration/coasting/deaccelleration time.

> I figured out what was going on with the 1TB SSD -- it has a huge RAM cache big
> enough to store most of the metadata.  At that point, reads are essentially
> free, but readahead costs us ~1ms per fadvise call. 

Do we understand why fadvise() takes 1ms?   Is that something we can fix?

And readahead(2) was even worse, right?

							- Ted
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