From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mout.kundenserver.de ([212.227.17.10]:53881 "EHLO mout.kundenserver.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751059AbbIQR4Q (ORCPT ); Thu, 17 Sep 2015 13:56:16 -0400 Received: from durkon.lan ([178.201.238.189]) by mrelayeu.kundenserver.de (mreue101) with ESMTPSA (Nemesis) id 0MTQ5X-1ZCx9F0QfH-00SNmf for ; Thu, 17 Sep 2015 19:56:14 +0200 Received: from xykon.lan (xykon.lan [192.168.2.42]) by durkon.lan (Postfix) with ESMTPSA id 2539A85EA for ; Thu, 17 Sep 2015 19:56:09 +0200 (CEST) Subject: Re: BTRFS as image store for KVM? To: linux-btrfs@vger.kernel.org References: <55F88ECC.1040604@menke.ac> From: Gert Menke Message-ID: <55FAFEB8.6030404@menke.ac> Date: Thu, 17 Sep 2015 19:56:08 +0200 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Sender: linux-btrfs-owner@vger.kernel.org List-ID: Hi, thank you for your answers! So it seems there are several suboptimal alternatives here... MD+LVM is very close to what I want, but md has no way to cope with silent data corruption. So if I'd want to use a guest filesystem that has no checksums either, I'm out of luck. I'm honestly a bit confused here - isn't checksumming one of the most obvious things to want in a software RAID setup? Is it a feature that might appear in the future? Maybe I should talk to the md guys... BTRFS looks really nice feature-wise, but is not (yet) optimized for my use-case I guess. Disabling COW would certainly help, but I don't want to lose the data checksums. Is nodatacowbutkeepdatachecksums a feature that might turn up in the future? Maybe ZFS is the best choice for my scenario. At least, it seems to work fine for Joyent - their SmartOS virtualization OS is essentially Illumos (Solaris) with ZFS, and KVM ported from Linux. Since ZFS supports "Volumes" (virtual block devices inside a ZPool), I suspect these are probably optimized to be used for VM images (i.e. do as little COW as possible). Of course, snapshots will always degrade performance to a degree. However, there are some drawbacks to ZFS: - It's less flexible, especially when it comes to reconfiguration of disk arrays. Add or remove a disk to/from a RaidZ and rebalance, that would be just awesome. It's possible in BTRFS, but not ZFS. :-( - The not-so-good integration of the fs cache, at least on Linux. I don't know if this is really an issue, though. Actually, I imagine it's more of an issue for guest systems, because it probably breaks memory ballooning. (?) So it seems there are two options for me: 1. Go with ZFS for now, until BTRFS finds a better way to handle disk images, or until md gets data checksums. 2. Buy a bunch of SSDs for VM disk images and use spinning disks for data storage only. In that case, BTRFS should probably do fine. Any comments on that? Am I missing something? Thanks! Gert