From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from plane.gmane.org ([80.91.229.3]:48490 "EHLO plane.gmane.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752039AbaIKJ43 (ORCPT ); Thu, 11 Sep 2014 05:56:29 -0400 Received: from list by plane.gmane.org with local (Exim 4.69) (envelope-from ) id 1XS175-0000zR-36 for linux-btrfs@vger.kernel.org; Thu, 11 Sep 2014 11:56:27 +0200 Received: from host86-174-242-234.range86-174.btcentralplus.com ([86.174.242.234]) by main.gmane.org with esmtp (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 11 Sep 2014 11:56:27 +0200 Received: from linux by host86-174-242-234.range86-174.btcentralplus.com with local (Gmexim 0.1 (Debian)) id 1AlnuQ-0007hv-00 for ; Thu, 11 Sep 2014 11:56:27 +0200 To: linux-btrfs@vger.kernel.org From: Bob Williams Subject: Re: Is it necessary to balance a btrfs raid1 array? Date: Thu, 11 Sep 2014 10:56:14 +0100 Message-ID: References: <541043B8.7010601@barrowhillfarm.org.uk> <54109BCD.5000106@inwind.it> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 In-Reply-To: <54109BCD.5000106@inwind.it> Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 10/09/14 19:43, Goffredo Baroncelli wrote: > On 09/10/2014 02:27 PM, Bob Williams wrote: >> I have two 2TB disks formatted as a btrfs raid1 array, mirroring both >> data and metadata. Last night I started >> >> # btrfs filesystem balance > > > May be that I am missing something obvious, however I have to ask which > would be the purpose to balance a two disks RAID1 system. > The balance command should move the data between the disks in order to > avoid some disk full and other empty; but this assume that there is a > not symmetrical uses of the disks. Which is not the case for a RAID1/two > disks system. > > If the disk were more than two the situation would be completely different. > But Bob reports that the system is compose by two disks only. > >> >> and it is still running 18 hours later. This suggests that most stuff >> only gets written to one physical device, which in turn suggests that >> there is a risk of lost data if one physical device fails. Or is >> there something clever about btrfs raid that I've missed? I've used >> linux software raid (mdraid) before, and it appeared to write to both >> devices simultaneously. >> So if a RAID1/two disk system uses the disks symmetrically, why did my balance command take 22 hours? That's what puzzles me, as my understanding of RAID1 is that the disk use *is* symmetrical. Bob