linux-kernel - Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Tue, 15 Jan 2008 20:24:27 -0500
From:	"Daniel Phillips" <phillips@...gle.com>
To:	"Alan Cox" <alan@...rguk.ukuu.org.uk>
Cc:	"Pavel Machek" <pavel@....cz>, "David Chinner" <dgc@....com>,
	"Theodore Tso" <tytso@....edu>, "Al Boldi" <a1426z@...ab.com>,
	"Valerie Henson" <val.henson@...il.com>,
	"Rik van Riel" <riel@...hat.com>, linux-fsdevel@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [Patch] document ext3 requirements (was Re: [RFD] Incremental fsck)

On Jan 15, 2008 7:15 PM, Alan Cox <alan@...rguk.ukuu.org.uk> wrote:
> > Writeback cache on disk in iteself is not bad, it only gets bad if the
> > disk is not engineered to save all its dirty cache on power loss,
> > using the disk motor as a generator or alternatively a small battery.
> > It would be awfully nice to know which brands fail here, if any,
> > because writeback cache is a big performance booster.
>
> AFAIK no drive saves the cache. The worst case cache flush for drives is
> several seconds with no retries and a couple of minutes if something
> really bad happens.
>
> This is why the kernel has some knowledge of barriers and uses them to
> issue flushes when needed.

Indeed, you are right, which is supported by actual measurements:

    http://sr5tech.com/write_back_cache_experiments.htm

Sorry for implying that anybody has engineered a drive that can do
such a nice thing with writeback cache.

The "disk motor as a generator" tale may not be purely folklore.  When
an IDE drive is not in writeback mode, something special needs to done
to ensure the last write to media is not a scribble.

A small UPS can make writeback mode actually reliable, provided the
system is smart enough to take the drives out of writeback mode when
the line power is off.

Regards,

Daniel
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/