linux-kernel - Re: Btrfs v0.16 released

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Message-ID: <20080815013934.GE19125@one.firstfloor.org>
Date:	Fri, 15 Aug 2008 03:39:34 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Chris Mason <chris.mason@...cle.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Btrfs v0.16 released

> The async worker threads should be spreading the load across CPUs pretty
> well, and even a single CPU could keep up with 100MB/s checksumming.
> But, the async worker threads do randomize the IO somewhat because the
> IO goes from pdflush -> one worker thread per CPU -> submit_bio.  So,
> maybe that 3rd thread is more than the drive can handle?

You have more threads with duplication? 

> 
> btrfsck tells me the total size of the btree is only 20MB larger with
> checksumming on.
> 
> > > Btrfs no duplication            76.83 MB/s
> > > Btrfs no dup no csum no inline  76.85 MB/s
> > 
> > But without duplication they are basically free here at least
> > in IO rate. Seems odd?
> > 
> > Does it compute them twice in the duplication case perhaps?
> > 
> 
> The duplication happens lower down in the stack, they only get done
> once.

Ok was just speculation. The big difference still seems odd.

-Andi
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