linux-kernel - Re: Btrfs: broken file system design

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Message-ID: <87hbkrealw.fsf@basil.nowhere.org>
Date:	Fri, 25 Jun 2010 11:15:55 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	"Daniel Taylor" <Daniel.Taylor@....com>
Cc:	"Mike Fedyk" <mfedyk@...efedyk.com>,
	"Daniel J Blueman" <daniel.blueman@...il.com>,
	"Mat" <jackdachef@...il.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>,
	"Chris Mason" <chris.mason@...cle.com>,
	"Ric Wheeler" <rwheeler@...hat.com>,
	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Linus Torvalds" <torvalds@...ux-foundation.org>,
	"The development of BTRFS" <linux-btrfs@...r.kernel.org>
Subject: Re: Btrfs: broken file system design

"Daniel Taylor" <Daniel.Taylor@....com> writes:
>
> As long as no object smaller than the disk block size is ever
> flushed to media, and all flushed objects are aligned to the disk
> blocks, there should be no real performance hit from that.

The question is just how large such a block needs to be.
Traditionally some RAID controllers (and possibly some SSDs now) 
needed very large blocks upto MBs.

>
> Otherwise we end up with the damage for the ext[234] family, where
> the file blocks can be aligned, but the 1K inode updates cause
> the read-modify-write (RMW) cycles and and cost >10% performance
> hit for creation/update of large numbers of files.

Fixing that doesn't require a new file system layout, just some effort
to read/write inodes in batches of multiple of them. XFS did similar
things for a long time, I believe there were some efforts for this
for ext4 too.

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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