From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mx1.redhat.com (ext-mx15.extmail.prod.ext.phx2.redhat.com [10.5.110.20]) by int-mx11.intmail.prod.int.phx2.redhat.com (8.14.4/8.14.4) with ESMTP id q2SIQDaJ025446 for ; Wed, 28 Mar 2012 14:26:13 -0400 Received: from mail.bmsi.com (www.bmsi.com [24.248.44.156]) by mx1.redhat.com (8.14.4/8.14.4) with ESMTP id q2SIQCMa024729 for ; Wed, 28 Mar 2012 14:26:13 -0400 Received: from sdg.bmsi.com (sdg.bmsi.com [192.168.9.34] (may be forged)) (authenticated bits=0) by mail.bmsi.com (8.14.3/8.14.3) with ESMTP id q2SIPHkM007555 (version=TLSv1/SSLv3 cipher=DHE-RSA-AES256-SHA bits=256 verify=NO) for ; Wed, 28 Mar 2012 14:25:18 -0400 Message-ID: <4F7357C3.4060205@bmsi.com> Date: Wed, 28 Mar 2012 14:26:11 -0400 From: Stuart D Gathman MIME-Version: 1.0 References: <4F6ECF9B.40907@nuclearwinter.com> <20120326155540.19c85fe9@bettercgi.com> <4F7100EC.6070406@nuclearwinter.com> <4F71CFFF.6090909@redhat.com> <4F722FFF.4010703@nuclearwinter.com> <4F72C38E.2080806@redhat.com> In-Reply-To: <4F72C38E.2080806@redhat.com> Content-Transfer-Encoding: 7bit Subject: Re: [linux-lvm] LVM commands extremely slow during raid check/resync Reply-To: LVM general discussion and development List-Id: LVM general discussion and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , List-Id: Content-Type: text/plain; charset="us-ascii"; format="flowed" To: linux-lvm@redhat.com Long ago, Nostradamus foresaw that on 03/28/2012 03:53 AM, Zdenek Kabelac would write: > It seems that your cfq scheduler should be tuned better for raid > arrays - I assume you allow the system to create very large queues of > buffers and your mdraid isn't fast enough to store dirty pages on disk > - I'd probably suggest to significantly lower the maximum amount of > dirty pages - as creation of snapshot requires fs sync operation it > will need to wait till all buffers before the operation are in place. A question (or minor nit): how could lvm possibly require a fs sync to create a snapshot? I could see this for Xen, where guest OS has to support a com channel to host. But for full virtualization, LVM doesn't know in general what OS is running, or how to suggest an FS sync. Or is this something an admin does, run a script that tells guest to sync before creating shapshot through lvm (to maximize the amount of useful data in the snapshot)?