linux-kernel - Re: wishful thinking about atomic, multi-sector or full MD stripe width, writes in storage

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Date:	Mon, 7 Sep 2009 13:45:34 +0200
From:	Pavel Machek <pavel@....cz>
To:	Ric Wheeler <rwheeler@...hat.com>
Cc:	Krzysztof Halasa <khc@...waw.pl>,
	Christoph Hellwig <hch@...radead.org>, Mark Lord <lkml@....ca>,
	Michael Tokarev <mjt@....msk.ru>, david@...g.hm,
	Theodore Tso <tytso@....edu>, NeilBrown <neilb@...e.de>,
	Rob Landley <rob@...dley.net>, Florian Weimer <fweimer@....de>,
	Goswin von Brederlow <goswin-v-b@....de>,
	kernel list <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...l.org>, mtk.manpages@...il.com,
	rdunlap@...otime.net, linux-doc@...r.kernel.org,
	linux-ext4@...r.kernel.org, corbet@....net
Subject: Re: wishful thinking about atomic, multi-sector or full MD stripe
	width, writes in storage

Hi!

> Note that even without MD raid, the file system issues IO's in file 
> system block size (4096 bytes normally) and most commodity storage 
> devices use a 512  byte sector size which means that we have to update 8 
> 512b sectors.
>
> Drives can (and do) have multiple platters and surfaces and it is 
> perfectly normal to have contiguous logical ranges of sectors map to 
> non-contiguous sectors physically. Imagine a 4KB write stripe that 
> straddles two adjacent tracks on one platter (requiring a seek) or mapped 
> across two surfaces (requiring a head switch). Also, a remapped sector 
> can require more or less a full surface seek from where ever you are to 
> the remapped sector area of the drive.

Yes, but ext3 was designed to handle the partial write  (according to
tytso).

> These are all examples that can after a power loss,  even a local 
> (non-MD) device,  do a partial update of that 4KB write range of
> sectors. 

Yes, but ext3 journal protects metadata integrity in that case.

> In other words, this is not just an MD issue, it is entirely possible 
> even with non-MD devices.
>
> Also, when you enable the write cache (MD or not) you are buffering 
> multiple MB's of data that can go away on power loss. Far greater (10x) 
> the exposure that the partial RAID rewrite case worries about.

Yes, that's what barriers are for. Except that they are not there on
MD0/MD5/MD6. They actually work on local sata drives...

								Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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