From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 28 Feb 2001 11:15:10 +0000 From: "Heinz J. Mauelshagen" Subject: Re: [linux-lvm] LVM and striping, defragmentation Message-ID: <20010228111510.U4361@srv.sistina.com> References: <20010226203358.A2504@66bassett.freeserve.co.uk> Mime-Version: 1.0 In-Reply-To: <20010226203358.A2504@66bassett.freeserve.co.uk>; from joe@66bassett.freeserve.co.uk on Mon, Feb 26, 2001 at 08:33:58PM +0000 Sender: linux-lvm-admin@sistina.com Errors-To: linux-lvm-admin@sistina.com Reply-To: linux-lvm@sistina.com List-Help: List-Post: List-Subscribe: , List-Unsubscribe: , List-Archive: List-Id: Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: linux-lvm@sistina.com On Mon, Feb 26, 2001 at 08:33:58PM +0000, Joe Thornber wrote: > On Mon, Feb 26, 2001 at 05:04:41PM +0100, Urs Thuermann wrote: > > I'd like to experiment with LVM and striping. The HOWTO at > > www.linuxdoc.org says in section 8.3 LVM native striping > > > > Performance notices > > > > The performance 'gain' may well be very negative if you stripe over 2 > > partitions of the same disk - take care to prevent that. > > > > > > How can I take care of it? I've seen no option in lvcreate to specify > > where the PE should be taken from. If I have a VG with only two PVs on > > two different disks, will LVM automatically choose PEs from the two > > PVs instead of one PV? And what if I have a VG with several PVs on > > disk A and several PVs on disk B. How can I "take care to prevent" > > that LVM does not choose two PEs from the same disk for striping? Having more than one PV mapped to a physical device is *not* the recommended production configuration. The only reason why we build in that option is flexibility in test configurations. > > You have highlighted an area where we want to do more work. > Specifically we want to write a tool that allows you more control over > the LE->PE mapping. The striping allocation will try and allocate > LE's from two different PV's in the VG. When you create the striped > LV just try not to have two PV's on the same disk. You can always > disable allocation on an individual PV basis. Check the lvcreate man page, please. You can add a list of PV device specials to allocate the PEs from to the lvcreate command line or use pvchange in order to prohibit allocation on that PV before using lvcreate. > > > When I resize my LVs in a VG multiple times, I can get some > > fragmentation, i.e. LVs in the VG are not contiguous. Is there a tool > > to defragment LVs, i.e. to swap PEs so that all LVs in the VG are > > contiguous again. I read in the man page that the allocation policy > > can be set to "contiguous", but how can I make LVs contiguous after > > some creating, removing, and resizing of LVs? > > At the moment it's not easy, you might want to look at the pvmove > documentation. > > > How much performance loss is to be expected in case of non-contiguous LVs? > > It shouldn't be large, the LE's are normally 4 meg so the seeks to > different parts of the disk won't be that frequent. To contrast with > stripes where the chunk size defaults to 64K, which can cause a lot of > thrashing if the stripes are on the same disk. > > - Joe > _______________________________________________ > linux-lvm mailing list > linux-lvm@sistina.com > http://lists.sistina.com/mailman/listinfo/linux-lvm -- Regards, Heinz -- The LVM Guy -- *** Software bugs are stupid. Nevertheless it needs not so stupid people to solve them *** =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- Heinz Mauelshagen Sistina Software Inc. Senior Consultant/Developer Am Sonnenhang 11 56242 Marienrachdorf Germany Mauelshagen@Sistina.com +49 2626 141200 FAX 924446 =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-