linux-ext4 - Re: [RFC 4/5] MMC: Adjust unaligned write accesses.

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Message-Id: <7BE97618-725C-4BFA-9FE5-59C893BDA097@dilger.ca>
Date:	Tue, 22 Mar 2011 00:58:12 +0100
From:	Andreas Dilger <adilger@...ger.ca>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Andrei Warkentin <andreiw@...orola.com>, linux-mmc@...r.kernel.org,
	linux-ext4@...r.kernel.org
Subject: Re: [RFC 4/5] MMC: Adjust unaligned write accesses.

On 2011-03-21, at 8:05 PM, Arnd Bergmann wrote:
> On Monday 21 March 2011 19:03:09 Andreas Dilger wrote:
>> Note that mballoc was specifically designed to handle allocation
>> requests that are aligned on RAID stripe boundaries, so it should
>> be able to handle this for MMC as well.  What is needed is to tell
>> the filesystem what the underlying alignment is.  That can be done
>> at format time with mke2fs or afterward with tune2fs by using the
>> "-E stripe_width" option.
> 
> Ah, that sounds useful. So would I set the stripe_width to the
> erase block size, and the block group size to a multiple of that?

When you write "block group size" do you mean the ext4 block group?  Then yes it would help.  You could also consider setting the flex_bg size to a multiple of this, so that the bitmap blocks are grouped as a multiple of this size.  However, they may not be aligned correctly, which needs extra effort that isn't obvious.  

I think it would be nice to have mke2fs take the stripe_width and/or flex_bg factor into account when sizing/aligning the bitmaps, but it doesn't yet.

> Does this also work in (rare) cases where the erase block size is
> not a power of two?

It does (or is supposed to), but that isn't code that is exercised very much (most installations use a power-of-two size).

Cheers, Andreas

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