linux-kernel - Re: UBIFS vs Logfs (was [RFC PATCH] UBIFS

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Date:	Tue, 01 Apr 2008 12:57:46 +0300
From:	Artem Bityutskiy <dedekind@...dex.ru>
To:	Tomasz Chmielewski <mangoo@...g.org>
CC:	LKML <linux-kernel@...r.kernel.org>, penberg@...helsinki.fi,
	Jörn Engel <joern@...fs.org>,
	ext-adrian.hunter@...ia.com, jwboyer@...il.com
Subject: Re: UBIFS vs Logfs (was [RFC PATCH] UBIFS - new flash file system)

Tomasz Chmielewski wrote:
> Such as?

ext3 for example.

> Flash (also on block devices) is slow and expensive (when compared to 
> modern hard disks) and therefore compression is *very* useful here.
Well, if you are ready to trade performance to compression, then well,
go ahead :-) May be I used too strong wording, but I wanted to say then
use raw flash then. But I'd also consider implementing compression support
for a block based FS. Reiser4 claimed to have it for example.

> Do you mean using hacks like block2mtd? It's hacky, and pretty hard to 
> boot a system this way (need to build own initramfs, with a static 
> block2mtd or loads of libraries - not something an average user would 
> like to do; no distro supports it; updating a kernel would be a pain etc.).
Well, ok, it still sounds strange for me, but you may use JFFS2 and UBIFS
with block2mtd as well if you really want to.

> True.
> Unfortunately, there is no way to access flash directly on flash-based 
> block devices (USB-sticks, IDE-flash disks, SSD disks etc.).
Yeah, that's a pity :-(

> Unfortunately, traditional filesystems were rather designed for rotating 
> media / cheap disks (no transparent compression; tend to accumulate 
> writes in one area of the disk - more on that - below).
Sure.

> Performance is only one factor in the equation. Other factors are: cost 
> and reliability.
> 
> I speak from experience: flash-based block devices tend to have poor 
> wear-levelling (at least Transcend IDE-flash disks).
> To reproduce:
> - format a 2 GB Transcend IDE-flash disk with ext3
> - write a small file (50-100 kB)
> - update that file ~several hundred thousand times - as you finish, 
> IDE-flash disk will have 200-300 badblocks
Yeah, that's bad. But if you have a bad FTL, surely there is not guarantee
a flash FS will help? Isn't it better to use better hardware?

We did some experiments with MMC cards and we were unable to wear them
out with re-writing the same sectors again and again. This suggests there
_is_ better FTL hardware then that USB stick you was using.

Anyway, your original mail said Logfs can work with block devices. My answer -
UBIFS too, but this is very strange to do this IMO. But OK, it might is not
senseless, sorry for the wording. :-)

-- 
Best Regards,
Artem Bityutskiy (Артём Битюцкий)
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