From: Nitin Gupta Subject: Re: [PATCH V2 0/7] Cleancache (was Transcendent Memory): overview Date: Fri, 04 Jun 2010 15:06:49 +0530 Message-ID: <4C08C931.3080306@vflare.org> References: <20100528173510.GA12166%ca-server1.us.oracle.comAANLkTilV-4_QaNq5O0WSplDx1Oq7JvkgVrEiR1rgf1up@mail.gmail.com> <489aa002-6d42-4dd5-bb66-81c665f8cdd1@default> <4C07179F.5080106@vflare.org> <3721BEE2-DF2D-452A-8F01-E690E32C6B33@oracle.com 4C074ACE.9020704@vflare.org> <6e97a82a-c754-493e-bbf5-58f0bb6a18b5@default> Reply-To: ngupta@vflare.org Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Cc: andreas.dilger@oracle.com, Minchan Kim , chris.mason@oracle.com, viro@zeniv.linux.org.uk, akpm@linux-foundation.org, adilger@sun.com, tytso@mit.edu, mfasheh@suse.com, joel.becker@oracle.com, matthew@wil.cx, linux-btrfs@vger.kernel.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, ocfs2-devel@oss.oracle.com, linux-mm@kvack.org, jeremy@goop.org, JBeulich@novell.com, kurt.hackel@oracle.com, npiggin@suse.de, dave.mccracken@oracle.com, riel@redhat.com, avi@redhat.com, konrad.wilk@oracle.com To: Dan Magenheimer Return-path: Received: from mail-pv0-f174.google.com ([74.125.83.174]:51575 "EHLO mail-pv0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754469Ab0FDJgu (ORCPT ); Fri, 4 Jun 2010 05:36:50 -0400 In-Reply-To: <6e97a82a-c754-493e-bbf5-58f0bb6a18b5@default> Sender: linux-ext4-owner@vger.kernel.org List-ID: On 06/03/2010 09:13 PM, Dan Magenheimer wrote: >> On 06/03/2010 10:23 AM, Andreas Dilger wrote: >>> On 2010-06-02, at 20:46, Nitin Gupta wrote: >> >>> I was thinking it would be quite clever to do compression in, say, >>> 64kB or 128kB chunks in a mapping (to get decent compression) and >>> then write these compressed chunks directly from the page cache >>> to disk in btrfs and/or a revived compressed ext4. >> >> Batching of pages to get good compression ratio seems doable. > > Is there evidence that batching a set of random individual 4K > pages will have a significantly better compression ratio than > compressing the pages separately? I certainly understand that > if the pages are from the same file, compression is likely to > be better, but pages evicted from the page cache (which is > the source for all cleancache_puts) are likely to be quite a > bit more random than that, aren't they? > Batching of pages from random files may not be so effective but it would be interesting to collect some data for this. Still, per-inode batching of pages seems doable and this should help us get over this problem. Thanks, Nitin