linux-kernel - Re: Performance Comparison among EnhanceIO, bcache and dm-cache.

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Date:	Mon, 17 Jun 2013 11:29:44 +0100
From:	Joe Thornber <thornber@...hat.com>
To:	OS Engineering <osengineering@...c-inc.com>
Cc:	"koverstreet@...gle.com" <koverstreet@...gle.com>,
	"linux-bcache@...r.kernel.org" <linux-bcache@...r.kernel.org>,
	"dm-devel@...hat.com" <dm-devel@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>,
	Padmini Balasubramaniyan <padminib@...c-inc.com>,
	Amit Phansalkar <aphansalkar@...c-inc.com>
Subject: Re: Performance Comparison among EnhanceIO, bcache and dm-cache.

On Tue, Jun 11, 2013 at 03:05:07PM +0000, OS Engineering wrote:

...

> Dm-cache commits on-disk metadata every time a REQ_SYNC or REQ_FUA
> bio is written. If no such requests are made then it commits
> metadata once every second. If power is lost, it may lose some
> recent writes.

Not true (though it is true for thinp, which may be where you got this
idea?).  For caching we have to commit whenever data is moved about,
otherwise a crash could result in us reading data that is not just out
of date (acceptable for some), but used to belong to a totally
different part of the device (always unacceptable).

- Joe
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