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Message-ID: <20090907114534.GP23450@elf.ucw.cz>
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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