From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from www.humilis.net ([82.95.169.224]:54357 "EHLO panda.humilis.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753442Ab3DPIQ4 (ORCPT ); Tue, 16 Apr 2013 04:16:56 -0400 Date: Tue, 16 Apr 2013 10:10:23 +0200 From: Sander To: Liu Bo Cc: Ochi , linux-btrfs@vger.kernel.org Subject: Re: Activating space_cache after read-only snapshots without space_cache have been taken Message-ID: <20130416081023.GA24920@panda> Reply-To: sander@humilis.net References: <516C9B43.7040605@arcor.de> <20130416031141.GB22531@liubo.jp.oracle.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20130416031141.GB22531@liubo.jp.oracle.com> Sender: linux-btrfs-owner@vger.kernel.org List-ID: Liu Bo wrote (ao): > On Tue, Apr 16, 2013 at 02:28:51AM +0200, Ochi wrote: > > The situation is the following: I have created a backup-volume to > > which I regularly rsync a backup of my system into a subvolume. > > After rsync'ing, I take a _read-only_ snapshot of that subvolume > > with a timestamp added to its name. > > > > Now at the time I started using this backup volume, I was _not_ > > using the space_cache mount option and two read-only snapshots were > > taken during this time. Then I started using the space_cache option > > and continued doing snapshots. > > > > A bit later, I started having very long lags when unmounting the > > backup volume (both during shutdown and when unmounting manually). I > > scrubbed and fsck'd the volume but this didn't show any errors. > > Defragmenting the root and subvolumes took a long time but didn't > > improve the situation much. > > So are you using '-o nospace_cache' when creating two RO snapshots? No, he first created two ro snapshots, then (some time later) mounted with nospace_cache, and then continued to take ro snapshots. > > Now I started having the suspicion that maybe the space cache > > possibly couldn't be written to disk for the readonly > > subvolumes/snapshots that were created during the time when I wasn't > > using the space_cache option, forcing the cache to be rebuilt every > > time. > > > > Clearing the cache didn't help. But when I deleted the two snapshots > > that I think were taken during the time without the mount option, > > the unmounting time seems to have improved considerably. > > I don't know why this happens, but maybe you can observe the umount > process's very slow behaviour by using 'cat /proc/{umount-pid}/stack' > or 'perf top'. AFAIUI the problem is not there anymore, but this is a good tip for the future. Sander