From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-it0-f66.google.com ([209.85.214.66]:45371 "EHLO mail-it0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752873AbdKOMnd (ORCPT ); Wed, 15 Nov 2017 07:43:33 -0500 Received: by mail-it0-f66.google.com with SMTP id l196so1566582itl.4 for ; Wed, 15 Nov 2017 04:43:33 -0800 (PST) Subject: Re: Tiered storage? To: Marat Khalili , waxhead@dirtcellar.net, linux-btrfs@vger.kernel.org Cc: Roy Sigurd Karlsbakk References: <2093578773.146320.1510707691482.JavaMail.zimbra@karlsbakk.net> <08cbefb3-43eb-8d76-1dd6-191e2709bdc7@dirtcellar.net> <8d8b22c1-3e20-64ab-f79f-f9dc3a4060ac@rqc.ru> From: "Austin S. Hemmelgarn" Message-ID: Date: Wed, 15 Nov 2017 07:43:29 -0500 MIME-Version: 1.0 In-Reply-To: <8d8b22c1-3e20-64ab-f79f-f9dc3a4060ac@rqc.ru> Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: On 2017-11-15 04:26, Marat Khalili wrote: > > On 15/11/17 10:11, waxhead wrote: >> hint: you need more than two for raid1 if you want to stay safe > Huh? Two is not enough? Having three or more makes a difference? (Or, > you mean hot spare?) They're probably referring to an issue where a two device array configured for raid1 which had lost a device and was mounted degraded and writable would generate single profile chunks on the remaining device instead of a half-complete raid1 chunk. This, when combined with the fact that older kernels only check the filesystem as a whole for normal/degraded/irreparable instead of checking individual chunks would refuse to mount the resultant filesystem, meant that you only had one chance to fix such an array. If instead you have more than two devices, regular complete raid1 profile chunks are generated, and it becomes a non-issue. The second issue (checking degraded status at the chunk level instead of volume level) has been fixed in the most recent kernels. The first issue has not been fixed yet, but I'm pretty sure there are patches pending.