linux-ext4 - Fwd: What should be the optimum value of G (number-of-groups) when using flex

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Date:	Tue, 6 Nov 2012 10:06:19 +0530
From:	Ashish Sangwan <ashishsangwan2@...il.com>
To:	"Theodore Ts'o" <tytso@....edu>, adilger@...ger.ca
Cc:	Namjae Jeon <linkinjeon@...il.com>, linux-ext4@...r.kernel.org
Subject: Fwd: What should be the optimum value of G (number-of-groups) when
 using flex_bg?

We have a 2TB HDD which we formated using default options of mkfs.ext4(v1.42.6).
I took around ~15 seconds to complete this operation.

Than we formatted the same partition with "-G 1024" and there was
around 30% performance improvement in
the mkfs.ext4 timings.

Than, out of curiosity, we increased the G's value to 1048576 and
there was a drastic degradation in the performance of mkfs.ext4.
It took more than 5 minutes to complete the execution.

Is it the expected behavior?

AFAIK flex_bg is on by default with G's vaule as 16. Can we consider
this the optimum value?

Any suggesstions on what  we can set the Gs value for a 2TB HDD such
that both the FS and the
mkfs.ext4 gives optimum performance?
It would be really helpful if somebody can shed some light over this feature.

Thanks,
Ashish
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