From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4F742FDD.1010204@redhat.com> Date: Thu, 29 Mar 2012 11:48:13 +0200 From: Zdenek Kabelac 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> <4F7357C3.4060205@bmsi.com> In-Reply-To: <4F7357C3.4060205@bmsi.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" To: LVM general discussion and development Dne 28.3.2012 20:26, Stuart D Gathman napsal(a): > 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)? You may check the man page for dmsetup suspend operation - options nolockfs and noflush. For lvm creation of snapshot I guess everyone wants to get the filesystem in 'stable' condition - so all in-flight operation before creation of snapshot happened should hit the disk - and if you run fsck you should get pretty consistent results. Or thin it would be preferable that flush & lockfs part should be skipped at this moment and users would get their snapshot fs in quite broken state ? zdenek