From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jan Kara Subject: Re: recursive mtime patches Date: Thu, 14 Apr 2011 11:49:30 +0200 Message-ID: <20110414094930.GF5054@quack.suse.cz> References: <20110412154830.GF5246@quack.suse.cz> <20110413213937.GB4648@quack.suse.cz> <20110414092117.GB5054@quack.suse.cz> Mime-Version: 1.0 Content-Type: text/plain; charset=iso-8859-1 Content-Transfer-Encoding: QUOTED-PRINTABLE Cc: Jan Kara , Ext4 Developers List To: Amir Goldstein Return-path: Received: from cantor2.suse.de ([195.135.220.15]:34442 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758233Ab1DNJtb (ORCPT ); Thu, 14 Apr 2011 05:49:31 -0400 Content-Disposition: inline In-Reply-To: Sender: linux-ext4-owner@vger.kernel.org List-ID: On Thu 14-04-11 12:36:40, Amir Goldstein wrote: > On Thu, Apr 14, 2011 at 12:21 PM, Jan Kara wrote: > > On Thu 14-04-11 10:12:26, Amir Goldstein wrote: > >> On Thu, Apr 14, 2011 at 12:39 AM, Jan Kara wrote: > >> > On Wed 13-04-11 21:16:40, Amir Goldstein wrote: > >> >> On Tue, Apr 12, 2011 at 5:48 PM, Jan Kara wrote: > >> >> > modification stamps have possibly larger race windows but I h= aven't really > >> >> > tried how much (I just know that even mtime races are not tha= t hard to > >> >> > trigger if you try). So it really depends on how big reliabil= ity do you > >> >> > expect and I personally don't find much value in just rescann= ing and > >> >> > checking for mtime after a crash. Reading all the data and do= ing checksum > >> >> > certainly has more value but at a high cost. > >> >> > > >> >> > >> >> What do you thing about the approach to store recursively modif= ied dir inodes in > >> >> a journal "modified inode descriptor block" and update the recu= rsive mtime of > >> >> those dirs on journal recovery? > >> > =A0The trouble is you don't know the number of directories that = may need > >> > to have timestamp updated - you find that out only as you travel= upwards. > >> > So it's hard to reserve any fixed space for this. > >> > > >> > >> True, but you can save *so* many inode numbers in just one descrip= tor > >> block and in case of an overflow, we can just pass a hint to the t= op > >> level application to do a full directory scan, so I hardly see tha= t as a > >> big problem. > > =A0Well, about 1000 but you can still have about 8000 inodes modifi= ed in a > > transaction for a standard 128 MB journal. You can notify the users= pace > > when an overflow happens but the interface gets kind of ugly... Als= o it > > would be only specific to ext3/4 while I'd prefer to get a wider fs > > support. >=20 > Well, the persistent inode notification (by the way a feature provide= d by NTFS), > can be specific to ext4, but it can work together with a generic recu= rsive mtime > code. > ext4 will simply touch directories during journal recovery. > other fs will only have the generic runtime recursive mtime. But then applications cannot rely on the behavior and cannot take muc= h benefit from it. Well, they could still ignore the risks and ext4 would= be nicer to them. But I'm not really sure what are you aiming at... > >> >> I would also consider to use a mount option rec_mtime and then = just > >> >> store recursive > >> >> mtime in the directory's inode mtime instead of an extended att= ribute. > >> >> That doesn't break any contract with user space, it's just a re= -interpretation > >> >> of the dir modification notion. > >> > =A0It breaks POSIX specification - POSIX pretty much specifies w= hen mtime is > >> > supposed to be changed - so I'm not sure we really want to do th= at... > >> > >> I disagree, POSIX doesn't forbid a user space daemon from touching= directory > >> inodes and updating their mtime. The rec_mtime feature should be t= reated as > >> a little kernel "daemon" which propagates information to user spac= e by touching > >> recursively modified directories. > > =A0OK, if you look at it this way it makes some sense. You loose th= e > > distinction whether something has been created / deleted in the dir= ectory > > or whether only something happened in its subdirectory or file but = that > > does not seem too important for any use case I can think of. >=20 > Personally, whenever I look at a dir mtime I would much rather I see > recursive mtime (I would much rather see recursive size as well but t= hat > is too much to ask). rsync can be easily modified to skipped entire > directories if their (recursive) mtime hasn't changed. I would like = to > view dir (recursive) mtime using existing tools (from ls to folder > manager) and not use specialized tools that look at extended attribut= es, > but hey, that's just me :-) That would be neat but note that even my patches don't provide comple= te recursive mtime behavior. They just update the time stamp once and then stop updating it until you ask about the update again. This makes the w= hole framework really efficient for often modified directories but less usef= ul for cases like "I want to see time when something has changed in this subtree". But still I kind of like your idea of hijacking directory mtime/ctime f= or these purposes because it would make several things simpler. Honza --=20 Jan Kara SUSE Labs, CR -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" i= n the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html