From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wm0-f51.google.com ([74.125.82.51]:36541 "EHLO mail-wm0-f51.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752164AbcCIUZV (ORCPT ); Wed, 9 Mar 2016 15:25:21 -0500 Received: by mail-wm0-f51.google.com with SMTP id n186so1792945wmn.1 for ; Wed, 09 Mar 2016 12:25:20 -0800 (PST) MIME-Version: 1.0 In-Reply-To: References: Date: Wed, 9 Mar 2016 15:25:19 -0500 Message-ID: Subject: Re: dstat shows unexpected result for two disk RAID1 From: Nicholas D Steeves To: linux-btrfs@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-btrfs-owner@vger.kernel.org List-ID: grr. Gmail is terrible :-/ I understood that a btrfs RAID1 would at best grab one block from sdb and then one block from sdd in round-robin fashion, or at worse grab one chunk from sdb and then one chunk from sdd. Alternatively I thought that it might read from both simultaneously, to make sure that all data matches, while at the same time providing single-disk performance. None of these was the case. Running a single IO-intensive process reads from a single drive. Did I misunderstand the documentation and is this normal, or is this a bug? Nicholas On 9 March 2016 at 15:21, Nicholas D Steeves wrote: > Hello everyone, > > I've run into an expected behaviour for a my two disk RAID1. I mount > with UUIDs, because sometimes my USB disk gets /dev/sdc instead of > /dev/sdd. The two elements of my RAID1 are currently sdb and sdd. > > dstat -tdD total,sdb,sdc,sdd > > It seems that per process, reads come from either sdb or sdd. This > surprises me, because I understood that a btrfs RAID1