From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Kieser Subject: Poor performance with qemu and bcache FUA Date: Sat, 28 Mar 2015 09:49:57 -0700 Message-ID: <5516DBB5.1070708@kieser.ca> Mime-Version: 1.0 Content-Type: multipart/signed; protocol="application/pkcs7-signature"; micalg=sha1; boundary="------------ms050301020806000701070802" Return-path: Received: from mail.pfak.org ([198.50.219.4]:46158 "EHLO mail.pfak.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751697AbbC1Q6h (ORCPT ); Sat, 28 Mar 2015 12:58:37 -0400 Received: from mail.pfak.org (localhost [127.0.0.1]) by mail.pfak.org (Postfix) with ESMTP id 8594440195DC for ; Sat, 28 Mar 2015 16:49:41 +0000 (UTC) Received: from [192.168.3.5] (unknown [192.168.3.5]) (using TLSv1 with cipher RC4-SHA (128/128 bits)) (No client certificate requested) (Authenticated sender: peter@kieser.ca) by mail.pfak.org (Postfix) with ESMTPSA id 2B15640195DB for ; Sat, 28 Mar 2015 16:49:41 +0000 (UTC) Sender: linux-bcache-owner@vger.kernel.org List-Id: linux-bcache@vger.kernel.org To: linux-bcache@vger.kernel.org This is a cryptographically signed message in MIME format. --------------ms050301020806000701070802 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: quoted-printable Hello, I finally was able to track down the commit that was causing poor=20 performance with qemu and LVM or qcow2 backed storage on virtual=20 machines since 2013 that use barriers on their guest FS (the default.)=20 This was most obvious with tools like apt-get and installing=20 linux-headers (it would take ~8 minutes on my system with 2 Intel 530=20 SSDs in RAID1): http://git.kernel.org/cgit/linux/kernel/git/stable/linux-stable.git/commi= t/?h=3Dlinux-3.10.y&id=3D3fcbc17636c83da9d85e2604de4af56c215e6e3c Looks like FUA implemented in bcache does not play nicely with qemu. I=20 reverted this commit, and now a single virtual machine is no longer able = to DoS all other virtual machines on the system. The symptoms would be=20 that the cache devices would be fully utilized with a small number of=20 writes. Are there any plans to revisit this? I don't like the idea of leaving=20 FUA unadvertised as data loss could occur if there is a power failure. 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