From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from out3-smtp.messagingengine.com ([66.111.4.27]:42367 "EHLO out3-smtp.messagingengine.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751505AbeAZOre (ORCPT ); Fri, 26 Jan 2018 09:47:34 -0500 Message-Id: <1516978054.4103196.1249114200.76EC1546@webmail.messagingengine.com> From: Christophe Yayon To: "Austin S. Hemmelgarn" , "Majordomo vger.kernel.org" MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Date: Fri, 26 Jan 2018 15:47:34 +0100 References: <1516975360.4083556.1249069832.1B287A04@webmail.messagingengine.com> <5d342036-0de0-9bf7-3e9e-4885b62d8100@gmail.com> In-Reply-To: <5d342036-0de0-9bf7-3e9e-4885b62d8100@gmail.com> Subject: Re: degraded permanent mount option Sender: linux-btrfs-owner@vger.kernel.org List-ID: Hi Austin, Thanks for your answer. It was my opinion too as the "degraded" seems to be flagged as "Mostly OK" on btrfs wiki status page. I am running Archlinux with recent kernel on all my servers (because of use of btrfs as my main filesystem, i need a recent kernel). Your idea to add a separate entry in grub.cfg with rootflags=degraded is attractive, i will do this... Just a last question, i thank that it was necessary to add "degraded" option in grub.cfg AND fstab to allow boot in degraded mode. I am not sure that only grub.cfg is sufficient... Yesterday, i have done some test and boot a a system with only 1 of 2 drive in my root raid1 array. No problem with systemd, but i added rootflags and fstab option. I didn't test with only rootflags. Thanks. -- Christophe Yayon cyayon-list@nbux.org On Fri, Jan 26, 2018, at 15:18, Austin S. Hemmelgarn wrote: > On 2018-01-26 09:02, Christophe Yayon wrote: > > Hi all, > > > > I don't know if it the right place to ask. Sorry it's not... > No, it's just fine to ask here. Questions like this are part of why the > mailing list exists. > > > > Just a little question about "degraded" mount option. Is it a good idea to add this option (permanent) in fstab and grub rootflags for raid1/10 array ? Just to allow the system to boot again if a single hdd fail. > Some people will disagree with me on this, but I would personally > suggest not doing this. I'm of the opinion that running an array > degraded for any period of time beyond the bare minimum required to fix > it is a bad idea, given that: > * It's not a widely tested configuration, so you are statistically more > likely to run into previously unknown bugs. Even aside from that, there > are probably some edge cases that people have not yet found. > * There are some issues with older kernel versions trying to access the > array after it's been mounted writable and degraded when it's only two > devices in raid1 mode. This in turn is a good example of the above > point about not being widely tested, as it took quite a while for this > problem to come up on the mailing list. > * Running degraded is liable to be slower, because the filesystem has to > account for the fact that the missing device might reappear at any > moment. This is actually true of any replication system, not just BTRFS. > * For a 2 device raid1 volume, there is no functional advantage to > running degraded with one device compared to converting to just use a > single device (this is only true of BTRFS because of the fact that it's > trivial to convert things, while for MD and LVM it is extremely > complicated to do so online). > > Additionally, adding the `degraded` mount option won't actually let you > mount the root filesystem if you're using systemd as an init system, > because systemd refuses to mount BTRFS volumes which have devices missing. > > Assuming that the systemd thing isn't an issue for you, I would suggest > instead creating a separate GRUB entry with the option set in rootflags. > This will allow you to manually boot the system if the array is > degraded, but will make sure you notice during boot (in my case, I don't > even do that, but I'm also reasonably used to tweaking kernel parameters > from GRUB prior to booting the system that it would end up just wasting > space). > > > > Of course, i have some cron jobs to check my array health. > It's good to hear that you're taking the initiative to monitor things, > however this fact doesn't really change my assessment above.