linux-kernel - Re: [PATCH] LogFS take three

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Date:	Thu, 17 May 2007 14:25:17 +0800
From:	David Woodhouse <dwmw2@...radead.org>
To:	Dongjun Shin <djshin90@...il.com>
Cc:	Jörn Engel <joern@...ybastard.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mtd@...ts.infradead.org, Albert Cahalan <acahalan@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Jan Engelhardt <jengelh@...ux01.gwdg.de>,
	Evgeniy Polyakov <johnpol@....mipt.ru>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Greg KH <greg@...ah.com>, Ingo Oeser <ioe-lkml@...eria.de>
Subject: Re: [PATCH] LogFS take three

On Thu, 2007-05-17 at 15:12 +0900, Dongjun Shin wrote:
> The current trend of flash-based device is to hide the flash-specific details
> from the host OS. The flash memory is encapsulated in a package
> which contains a dedicated controller where a small piece of software (F/W or FTL)
> runs and makes the storage shown as a block device to the host.

Yes. These things are almost always implemented _very_ badly by the same
kind of crack-smoking hobo they drag in off the streets to write BIOSen.

It's bog-roll technology; if you fancy a laugh try doing some real
reliability tests on them time some. Powerfail testing is a good one.

This kind of thing is OK for disposable storage such as in digital
cameras, where it doesn't matter that it's no more reliable than a
floppy disc, but for real long-term storage it's really a bad idea.

> IMHO, for a flash-optimized filesystem to be useful and widely-used, it would be better
> to run on a block device and to be designed to run efficiently on top of the FTL.
> (ex. log-structured filesystem on general block device)

There's little point in optimising a file system _specifically_ for
devices which in often aren't reliable enough to keep your data anyway.
You might as well use ramfs.

It's unfortunate really -- there's no _fundamental_ reason why FTL has
to be done so badly; it's just that it almost always is. Direct access
to the flash from Linux is _always_ going to be better in practice --
and that way you avoid the problems with dual journalling, along with
the problems with the underlying FTL continuing to keep (and copy around
during GC) sectors which the top-level filesystem has actually
deallocated, etc.

-- 
dwmw2

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