From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from len.romanrm.net ([195.154.117.182]:40704 "EHLO len.romanrm.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932320AbcJLTfa (ORCPT ); Wed, 12 Oct 2016 15:35:30 -0400 Date: Thu, 13 Oct 2016 00:33:31 +0500 From: Roman Mamedov To: Zygo Blaxell Cc: Chris Murphy , Hugo Mills , "linux-btrfs@vger.kernel.org" , Austin Hemmelgarn Subject: Re: RAID system with adaption to changed number of disks Message-ID: <20161013003331.5e33c006@natsu> In-Reply-To: <20161012191915.GF26140@hungrycats.org> References: <20161011160601.GI7683@carfax.org.uk> <20161012122551.27d949ee@natsu> <20161012173141.GX21290@hungrycats.org> <20161012191915.GF26140@hungrycats.org> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; boundary="Sig_/ws1AFzh8twodZzjm=+3YgZP"; protocol="application/pgp-signature" Sender: linux-btrfs-owner@vger.kernel.org List-ID: --Sig_/ws1AFzh8twodZzjm=+3YgZP Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: quoted-printable On Wed, 12 Oct 2016 15:19:16 -0400 Zygo Blaxell wrote: > I'm not even sure btrfs does this--I haven't checked precisely what > it does in dup mode. It could send both copies of metadata to the > disks with a single barrier to separate both metadata updates from > the superblock updates. That would be bad in this particular case. It would be bad in any case, including a single physical disk and no RAID, = and I don't think there's any basis to speculate that mdadm doesn't implement write barriers properly. > In degraded RAID5/6 mode, all writes temporarily corrupt data, so if there > is an interruption (system crash, a disk times out, etc) in degraded mode, Moreover, in any non-COW system writes temporarily corrupt data. So again, writing to a (degraded or not) mdadm RAID5 is not much different than writi= ng to a single physical disk. However I believe in the Btrfs case metadata is always COW, so this particular problem may be not as relevant here in the first place. --=20 With respect, Roman --Sig_/ws1AFzh8twodZzjm=+3YgZP Content-Type: application/pgp-signature Content-Description: OpenPGP digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v2 iEYEARECAAYFAlf+kA4ACgkQTLKSvz+PZwj0TwCgk+baIxumpDlwS3WfWNLgScKw LnAAn2BT8N+ka+1x89R9hCvhuvcJJPli =8J4h -----END PGP SIGNATURE----- --Sig_/ws1AFzh8twodZzjm=+3YgZP--