From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from frost.carfax.org.uk ([85.119.82.111]:44220 "EHLO frost.carfax.org.uk" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751113AbbEZO0J (ORCPT ); Tue, 26 May 2015 10:26:09 -0400 Date: Tue, 26 May 2015 14:26:07 +0000 From: Hugo Mills To: sri Cc: linux-btrfs@vger.kernel.org Subject: Re: btrfs on disk stability Message-ID: <20150526142607.GE16826@carfax.org.uk> References: <20150526085246.GC16826@carfax.org.uk> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha1; protocol="application/pgp-signature"; boundary="jKBxcB1XkHIR0Eqt" In-Reply-To: Sender: linux-btrfs-owner@vger.kernel.org List-ID: --jKBxcB1XkHIR0Eqt Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Tue, May 26, 2015 at 09:29:25AM +0000, sri wrote: > Hugo Mills carfax.org.uk> writes: > > > > > On Tue, May 26, 2015 at 08:45:06AM +0000, sri wrote: > > > Hi, > > > > > > According to btrfs wiki page, under "Stability status" it is written > that > > > > > > "The filesystem disk format is no longer unstable". > > > > > > Does this mean if there are more I/Os are going on a btrfs file > system, > > > copy of entire disk (all disk blocks) gives a stable disk? > > > > No, it means that the format isn't changing in incompatible ways > > any more. You're guaranteed that if you upgrade your kernel, the FS > > will still be readable on the new kernel. (And, if you don't enable > > any extra features with btrfstune, that the kernel will still be > > readable if you downgrade to the earlier kernel you were using). > > > > > Just to elaborate more, if btrfs file system is created on 2 disks > > > /dev/sda and /dev/sdb and if I start copying blocks of sda and sdb > to sdc > > > and sdc respectively by just opening file handlers of sda and sdb > and > > > mounting the new copy via /dev/sdc and /dev/sdd will give consistent > file > > > system?? > > > > That's always the case, with the very large caveat that you remove > > /dev/sdc and /dev/sdd from the system before you try mounting anything > > related to that FS. Making block level copies of btrfs filesystems and > > leaving them visible to the same kernel as the originals is a very bad > > idea, and can cause massive FS corruption. > > > > It's OK to make the copy, but not to try mounting the FS with both > > copies present, as the kernel will see both copies as the same > > filesystem (because they have the same UUID), and it will get very > > confused about which device(s) it's meant to be writing to. > > > > Thank you Hugo, > copies of disk will not be there once block level copy is done. Assume > that sdc and sdd (which are used for copy sda and sdb) are ISCSI disks > from different machine and as soon as disk copy sda->sdc and sdb->sdd is > done, I will remove both disks and use it on other machine. > > In this case > 1) After copy at disk level whether the btrfs on sdc and sdd will be in > consistent state? even though there are any IOs on original disks sda > and sdb? > My guess is no but I am not sure If you're trying to copy a filesystem that's in use, then no, you can't expect the copies to be consistent. Imagine: At the point where you've copied half the disk, and the FS makes two updates which depend on each other -- one in the piece you've already copied, and one in the piece you haven't copied yet. Also, by this point, you've got the situation I warned against above: multiple copies of a filesystem that's mounted, so your FS is probably already damaged beyond repair. Doing it with a read-only mount of the FS might be possible. Basically, you can only do this reliably if the filesystem is not mounted. > 2) new disks sdc and sdd can be mounted on another machine where btrfs > fs is supported ? Yes. Hugo. -- Hugo Mills | Do not meddle in the affairs of wizards, for they hugo@... carfax.org.uk | are subtle, and quick to anger. http://carfax.org.uk/ | PGP: E2AB1DE4 | J.R.R. Tolkein --jKBxcB1XkHIR0Eqt Content-Type: application/pgp-signature; name="signature.asc" Content-Description: Digital signature -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.12 (GNU/Linux) iQIcBAEBAgAGBQJVZIJ/AAoJEFheFHXiqx3kDrIP/0IJvudjGJ726GcfSa4w5mNH mVzfZ71JsprpBBXWRYfOWf9mjaXXbos71yrfiHhJjxy5Fj8i1f0u7cprkrSJ82Us fgChdK5NddGhuG52PBFYrr1o203dxbWwcoPH8vQQblWheJjE3TPKe2Gj+BH+uCrb LcIdhyckD7Eg9eInzARTsEs4qLKxLCeHN/eqTU0vIROw4Z2uo7CfIRvfzrLTvfgZ 2REk9aBS4jD/5pzFDwxt3+5M4oY3oKyoybzOHh1aT5eY00tsWVlB/Qqc4hP8/jVX WhXTWtE2AWQzVwP6Blbnfl+8CVZtARHI9yHrsomzzTu1usaQoWF7dxs9BavLM4k/ i3xRMySv43IardEn664XPgQumLIfZLN4nciYSLvcLf5hOyCmSXOnalQ7KW9tN/lP fRCSiTyLYJwUfaxIFSE9W5M8PnFEzvFMz9ebt2pr5ZbX5RBB8ixfOpkxUk/QJxux DRdGqyoP0/URnSlqv52e0zxBBvyEGR66uGNlFtW0fukPDEQ7O8FS//EGB2MkH6cN 9dm5ookN3qI6HEEaZH9y2YLRxcRNh4n9ssdSb62TARIiV0MEbX0goQKJKC9lhCXu zL2t34pIqZzURTbvG3KFmNla0uExpAzqucjFPcfULTRY3HEAFeDJTdGHga5mtuOQ vKwy/GGzGWgU/v5NWPIf =nMW7 -----END PGP SIGNATURE----- --jKBxcB1XkHIR0Eqt--