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Re: [PATCH RFC] xfs: set block device logical sector size on xfs_buftarg

To: Eric Sandeen <sandeen@xxxxxxxxxx>
Subject: Re: [PATCH RFC] xfs: set block device logical sector size on xfs_buftarg
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 13 Nov 2013 10:56:45 -0800
Cc: xfs-oss <xfs@xxxxxxxxxxx>
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <5283C41D.7070503@xxxxxxxxxx>
References: <5283C41D.7070503@xxxxxxxxxx>
User-agent: Mutt/1.5.21 (2010-09-15)
On Wed, Nov 13, 2013 at 12:25:33PM -0600, Eric Sandeen wrote:
> Pure RFC; this might be crazy.  Here's the problem I'm trying to solve:
> 
> Today, mkfs.xfs will select a 4k sector size for a 4k physical / 512 logical
> drive.  (that change was done by me).  The thought was that it'd be an
> efficiency gain to not make the drive do the (possible) RMW cycles on
> 512-byte log IO, primarily.
> 
> However, now this restricts all DIO to 4k alignment, not the otherwise-
> possible 512.
> 
> This came up when qemu-kvm, in cache=none mode, tries to boot off an
> image hosted on such a filesystem, and its bios wants to do a 512 byte
> direct IO read off the disk - it fails.
> 
> But I'm wondering - the buftarg's bt_sshift and bt_smask are only used
> in a few places.  

No need to mess with kernel code IFF we want to change that, just keep
the sector size at 512 bytes and set a log stripe unit at mkfs time.

I have to admit that I'm not really sure if that's what we really want,
through.  A drive that has a larger physical block size will need
read-modify-write cycles internally, which we try to avoid.

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