linux-ext4 - Re: suspiciously good fsck times?

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Message-ID: <20080710175354.GA3447@mit.edu>
Date:	Thu, 10 Jul 2008 13:53:55 -0400
From:	Theodore Tso <tytso@....edu>
To:	linux-ext4@...r.kernel.org
Subject: Re: suspiciously good fsck times?

Based on the graphs which Eric posted, One interesting thing I think
you'll find if you repeat the ext3 experiment with e2fsck -t -t is
that pass2 will be about seven times longer than pass1.  (Which is
backwards from most e2fsck runs, where pass2 is about half pass 1's
run time --- although obviously that depends on how many directory
blocks you have.)

Yes, some kind of reservation windows would help on ext3 --- but the
question is whether such a change would be too-specific for this
benchmark or not.  Most of the time directories don't grow to such a
huge size.  So if you use a smallish (around 8 blocks, say) for many
directories this might lead to more filesystem fragmentation that in
the long run would cause the filesystem not to age well; it also
wouldn't help much when you have over 11 million files in the
directory, and a directory with over 100,000 blocks.

I don't think delayed allocation is what's helping here either,
because the journal will force the directory blocks to be placed as
soon as we commit a transaction.  I think what's saving us here is
that flex_bg and mballoc is separating the directory blocks from the
data blocks, allowng the directory blocks to be closely packed
together.

					- Ted
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