linux-ext4 - Re: page fault scalability (ext3, ext4, xfs)

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Date:	Wed, 14 Aug 2013 23:18:01 -0700 (PDT)
From:	David Lang <david@...g.hm>
To:	Andy Lutomirski <luto@...capital.net>
cc:	Dave Chinner <david@...morbit.com>,
	"Theodore Ts'o" <tytso@....edu>,
	Dave Hansen <dave.hansen@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	xfs@....sgi.com,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Jan Kara <jack@...e.cz>, LKML <linux-kernel@...r.kernel.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: page fault scalability (ext3, ext4, xfs)

On Wed, 14 Aug 2013, Andy Lutomirski wrote:

>> The big problem with this approach is that not doing the
>> timestamp update on page faults is going to break the inode change
>> version counting because for ext4, btrfs and XFS it takes a
>> transaction to bump that counter. NFS needs to know the moment a
>> file is changed in memory, not when it is written to disk. Also, NFS
>> requires the change to the counter to be persistent over server
>> failures, so it needs to be changed as part of a transaction....
>
> NFS can do whatever it wants, although I suspect that even NFS can get
> away with deferring cmtime updates.

NFS already has to do syncs to make sure the data is safe on disk, have a flag 
that NFS can use to make the ctime safe, everyone else can get the performance 
improvement and NFS can have it's slow-but-safe approach.

David Lang
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