linux-kernel - Re: ATA 4 KiB sector issues.

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Message-Id: <201003111927.24829.knikanth@suse.de>
Date:	Thu, 11 Mar 2010 19:27:24 +0530
From:	Nikanth Karthikesan <knikanth@...e.de>
To:	Theodore Tso <tytso@....edu>
Cc:	Damian Lukowski <damian@....rwth-aachen.de>,
	"linux-ide@...r.kernel.org" <linux-ide@...r.kernel.org>,
	Jeff Garzik <jeff@...zik.org>, Matthew Wilcox <matthew@....cx>,
	"Martin K. Petersen" <martin.petersen@...cle.com>,
	James Bottomley <James.Bottomley@...e.de>,
	Tejun Heo <tj@...nel.org>, lkml <linux-kernel@...r.kernel.org>,
	Daniel Taylor <Daniel.Taylor@....com>,
	Mark Lord <kernel@...savvy.com>,
	"H. Peter Anvin" <hpa@...or.com>, hirofumi@...l.parknet.co.jp,
	Andrew Morton <akpm@...ux-foundation.org>,
	Alan Cox <alan@...rguk.ukuu.org.uk>, irtiger@...il.com,
	aschnell@...e.de, jdelvare@...e.de
Subject: Re: ATA 4 KiB sector issues.

On Thursday 11 March 2010 18:34:56 Theodore Tso wrote:
> On Mar 10, 2010, at 11:19 AM, Damian Lukowski wrote:
> > I have practically no knowledge of Linux' block device drivers,
> > but is this really a partitioning issue? I think the problem is
> > with the filesystems when clustering multiple blocks without
> > knowledge of the sector alignment and sector size of the underlying
> > block device. Maybe it is a better solution to adapt the filesystem
> > buffer routine which reads/writes data from/to the block device?
> 
> No, it's really a partitioning issue.   If the paging subsystem wants a 4k
>  block to fill a particular page, we need to read that 4k block into
>  memory.  If we need to swap out that 4k block, we need to write that 4k
>  block to swap space, or to the memory segment's backing store.   If the
>  partition is misaligned by 512 bytes, this is simply not possible.   The
>  file system has to do what is requested of it by its users, and the
>  reality is that we need to do 4k aligned reads and writes with respect to
>  the beginning of the partition, far more often than not.
> 
> Hence, if we want the best performance on 4k sector drives, the partition
>  needs to be aligned with respect to what is most desirable for the device
>  in question.
> 

I guess, what he meant was, to keep filesystem blocks aligned, even if the 
partition is not. Say if the partition is mis-aligned by 512-bytes, let the 
filesystem waste 4k-512bytes and keep it's blocks aligned. But it might be a 
case of over-engineering, possibly requiring disk format change.

Thanks
Nikanth
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