From mboxrd@z Thu Jan 1 00:00:00 1970 From: Valerie Aurora Subject: Re: [PATCH 00/32] VFS based Union Mount (V3) Date: Mon, 8 Jun 2009 12:44:04 -0700 Message-ID: <20090608194404.GC4363@fsbox> References: <1242662968-11684-1-git-send-email-jblunck@suse.de> <20090519172327.GB722@shell> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: jblunck@suse.de, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, viro@zeniv.linux.org.uk, bharata@in.ibm.com, dwmw2@infradead.org, mszeredi@suse.cz To: Miklos Szeredi Return-path: Received: from mx2.redhat.com ([66.187.237.31]:50284 "EHLO mx2.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753726AbZFHToa (ORCPT ); Mon, 8 Jun 2009 15:44:30 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-fsdevel-owner@vger.kernel.org List-ID: On Wed, May 20, 2009 at 11:05:27AM +0200, Miklos Szeredi wrote: > On Tue, 19 May 2009, Valerie Aurora wrote: > > As Jan said, readdir() of read-only unioned file systems works with a > > tmpfs top layer. If you think about it, this is the exact equivalent > > of the version of union mounts which used the in-kernel caching > > approach - except that it's better, because it reuses existing code > > and caches between readdir() calls. Cool, huh? > > Yeah... OTOH tmpfs is probably a way too heavyweight solution for > cases where memory is short, and union mounts would typically be used > on such systems. (Sorry for the delay - I've been on vacation.) Hm, my intuition is that a tmpfs mount would be fairly lightweight in terms of memory - the main overhead over the barebones solution would be one superblock and vfsmount struct per mount. What am I missing? > The big reason why kernel impementation of readdir is hard is that > unswappable kernel memory needs to be used for caching directory > contents while the directory is open. Well, tmpfs does the same, > dentries and inodes are _not_ swappable, and they gobble up memory. That's a good point. It seemed to me that it wouldn't be too difficult to make those entries evictable - drop a reference count and set the ->d_release to mark the directory as needing rebuilding. What do you think? > So where's the advantage over implementing a thin deduplicating and > caching layer for union mounts? > > Thanks, > Miklos -VAL