Re: HP9000+LVM+Online

> Hewlett Packard have now taken to shipping HP9000s with their LVM > (Logical Volume Manager) product. Online normally operates on raw disc, > that is unmounted disc unknown to unix. Does anyone know what the > benefits/problems are from allocating disc to Online from LVM? > Do you loose performance by having LVM 'between' Online and its raw disc? > With HPUX it is said that you 'have' to use LVM with v10...is this true? > ..does it matter?....does LVM cause Online any problems? > You are quite right that OnLine works best with raw disk. Furthermore, planning a good physical disk placement strategy (ie. spreading the workload over the maximum number of drives) is important for performance of the engine. In my experience it's not the additional software layer of LVMs that slows down Online, but the tendency of any "clever" LVM configuration (ie. so that your OnLine space is fragmented) to foul up your OnLine disk placement strategy - things that OnLine thinks are physically contiguous, aren't. If you inadvertently fragment the logical logs, for example, or end up having them on the same disk as a very busy table, you could have the heads flying about all over the place. A "benefit" of LVMs is supposed to be that you can add space and so allow files to grow. This is irrelevant to OnLine because you create chunks with a fixed size. At best, LVMs are 'not harmful' to OnLine, but really only if you map one lvol to one disk. The real HP nasty to avoid in my experience is byte-striping RAID architectures, where I once saw a (very low-volume) checkpoint take 18 seconds. They got it down to well under a second by reverting to raw disk and throwing away the RAID stuff. akent@cix.compulink.co.uk (Andy Kent) -------------------------------------