linux-ext4 - Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation)

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Message-ID: <Pine.LNX.4.64.0704280708500.9055@artax.karlin.mff.cuni.cz>
Date:	Sat, 28 Apr 2007 07:37:17 +0200 (CEST)
From:	Mikulas Patocka <mikulas@...ax.karlin.mff.cuni.cz>
To:	Bill Huey <billh@...ppy.monkey.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andreas Dilger <adilger@...sterfs.com>,
	Marat Buharov <marat.buharov@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jens Axboe <jens.axboe@...cle.com>,
	"linux-ext4@...r.kernel.org" <linux-ext4@...r.kernel.org>,
	Alex Tomas <alex@...sterfs.com>
Subject: Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS
 is under heavy write load (massive starvation)

On Fri, 27 Apr 2007, Bill Huey wrote:

> On Fri, Apr 27, 2007 at 12:50:34PM -0700, Linus Torvalds wrote:
>> Oh, well.. Journalling sucks.
>>
>> I was actually _really_ hoping that somebody would come along and tell
>> everybody that this whole journal-logging is stupid, and that it's just
>> better to not ever re-write blocks on disk, but instead write to new
>> blocks with version numbers (and not re-use old blocks until new versions
>> are stable on disk).
>>
>> There was even somebody who did something like that for a PhD thesis, I
>> forget the details (and it apparently died when the thesis was presumably
>> accepted ;).
>
> That sounds a whole lot like NetApp's WAFL file system and is heavily 
> patented.
>
> bill

Hi

SpadFS doesn't write to unallocated parts like log filesystems (LFS) or 
phase tree filesystems (TUX2); it writes inside normal used structures, 
but it marks each structure with generation tags --- when it updates 
global table of tags, it atomically makes several structures valid. I 
don't know about this idea being used elsewhere.

It's fsync is slow too (needs to write all (meta)data too), but it at 
least doesn't livelock --- fsync is basically:
* write all buffers and wait for completion
* take lock preventing metadata updates
* write all buffers again (those that were updated while previous write 
was in progress) and wait for completion
* update global generation count table
* release the lock

Maybe Suse will be paying me from this autumn to make more features to it 
--- so far it works, doesn't eat data, but isn't much known :)

Mikulas
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