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Date: Tue, 06 Jan 2015 23:31:44 -0600
From: Eric Sandeen <sandeen@sandeen.net>
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Subject: Re: What is a recommended XFS sector size for hybrid (512e) advanced
	format hard drives?
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To: Chris Murphy <lists@colorremedies.com>
Cc: "xfs@oss.sgi.com" <xfs@oss.sgi.com>

On 1/6/15 1:55 PM, Chris Murphy wrote:
> What's sectsz= used for? Historically this would be sectsz=512,
> bsize=4096 before AF hard drives appeared. So is sectsz used for
> optimizing sub blocksize changes? e.g. if only 50 bytes needs
> changing, the fs doesn't need to read modify and write the entire 4096
> block, just change the affected sector?

Nope, filesystem data blocks are always fully written, but the sector
size is i.e. the minimum _log_ IO size.

+/*
+ * The xfs_buftarg contains 2 notions of "sector size" -
+ *
+ * 1) The metadata sector size, which is the minimum unit and
+ *    alignment of IO which will be performed by metadata operations.
+ * 2) The device logical sector size
+ *
+ * The first is specified at mkfs time, and is stored on-disk in the
+ * superblock's sb_sectsize.
+ *
+ * The latter is derived from the underlying device, and controls direct IO
+ * alignment constraints.

-Eric

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