linux-kernel - Re: [sqlite] light weight write barriers

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Message-ID: <20121102123359.2479a7dc@pyramind.ukuu.org.uk>
Date:	Fri, 2 Nov 2012 12:33:59 +0000
From:	Alan Cox <alan@...rguk.ukuu.org.uk>
To:	Howard Chu <hyc@...as.com>
Cc:	General Discussion of SQLite Database <sqlite-users@...ite.org>,
	Vladislav Bolkhovitin <vst@...b.net>,
	"Theodore Ts'o" <tytso@....edu>, drh@...ci.com,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: [sqlite] light weight write barriers

> Isn't any type of kernel-side ordering an exercise in futility, since
>    a) the kernel has no knowledge of the disk's actual geometry
>    b) most drives will internally re-order requests anyway

They will but only as permitted by the commands queued, so you have some
control depending upon the interface capabilities.

>    c) cheap drives won't support barriers

Barriers are pretty much universal as you need them for power off !

> Even assuming the drives honored all your requests without lying, how would 
> you really want this behavior exposed? From the userland perspective, there 
> are very few apps that care. Probably only transactional databases, really.

And file systems internally sometimes. A file system is after all a
transactional database of sorts.

Alan
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