On Wed, Aug 01, 2007 at 09:40:54PM +0930, Tim wrote: > Tim: > >> I'm curious about two things: Wouldn't resizing LVM involve fragmenting > >> the drive, in another way? > > Ewan Mac Mahon: > > Only physically; if I allocate space to one filesystem, then create > > another, then extend the first one then the physical storage for the > > first one will be in two chunks with the second fs sitting between them. > > The point of LVM is that I don't need to care about it since it appears > > as a single logical space. > > Isn't that the situation with fragmentation of any sort, though? It depends what you mean by 'fragmentation'; if you have several non-LVM disks in a system each with some free space on them a user has to decide which disk to use, or, if they've got a large chunk of data they may find that no single disk has enough free space so they have to split it up. It all gets very messy. In the LVM case the free space may be physically fragmented over just as many drives, but the user (and the admin :-) ) don't know or care; they just see simple logical divisions. My killer use case is the one where a user comes and asks for more disk space - say an extra 200G: With LVM I need 200G free, and then I extend their filesystem, and the jobs done. Without it I'd either have to give them a new separate 200G chunk of space, or find enough room for all their existing data+200G; allocate it, copy the existing data across, and reclaim the old space. > The heads having to skate about more, and only the drive really knows > where all the bits are (pun intended). In this particular case there's really no point trying to second guess what the drive heads are doing since the 'drives' are actually hardware RAID arrays. I'm not sure it makes much difference for most single drive systems either; modern drives play all sorts of interesting games with the layout, so even on a laptop only the drive really knows where all the bits are anyway. > Does LVM really manage that more efficiently? > It's more efficient for the user since the process of figuring out who needs how much space, and which physical resources to use to provide that space are taken off your hands and given to the computer. Which is /good/ at boring fiddly stuff. It's not more efficient in terms of raw disk speed - there will be an overhead. Ewan
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