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Re: 4K drives, sectsz=512, bsize=4096

To: Michael Monnerie <michael.monnerie@xxxxxxxxxxxxxxxxxxx>
Subject: Re: 4K drives, sectsz=512, bsize=4096
From: Christoph Hellwig <hch@xxxxxxxxxxxxx>
Date: Wed, 1 Sep 2010 06:24:12 -0400
Cc: xfs@xxxxxxxxxxx
In-reply-to: <201008300956.21264@xxxxxx>
References: <201008300956.21264@xxxxxx>
User-agent: Mutt/1.5.20 (2009-08-17)
On Mon, Aug 30, 2010 at 09:56:16AM +0200, Michael Monnerie wrote:
> I have 2x 2TB 4K sector drives, joined with LVM to RAID-0. On top of 
> that, I created an XFS, but that has sectsz=512.
> Will there be any real difference if I re-format with sectsz=4096? 
> AFAIK, XFS will do I/O based on block size, so the sector size doesn't 
> do any harm. Is that correct?

As nathan mentioned XFS issues log I/O based on the sector size, and
we do allow direct I/O down to the sector size.  That's two reasons
why or why you don't want a 4096 byte sector size.  Depending on the rmv
implementation of the drive log I/O might be really slow if you set the
512 byte sector size.  On the other hand there's lots of applications
that have the 512 byte alignment for direct I/O hardcoded, which will
break if you have a larger sector size.  That's one of the fun things
4k sector size disks will bring up, especially the real 4k SAS disks
that do not accept any I/O smaller than that.

> A question for LVM: Is there anything I need to tell to LVM to let it 
> know that those are 4K sector drives and I/O should be aligned to that? 
> Drives are reported as 512b sectors, but really are 4K. There seems to 
> be no way to instruct the kernel to see those drives as 4K drives.

If you have a recent enough LVM all metdata is aligned on 1MB boundaries
which will do just fine.

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