On 11/13/13, 4:10 PM, Dave Chinner wrote: ... > Yet all modern bios implementations you find in hardware can boot 4k > sector devices just fine. hm can they really? Most drives have 512 emulation. > So, what bios does qemu use? > > $ man qemu > ..... > QEMU uses the PC BIOS from the Bochs project and the Plex86/Bochs > LGPL VGA BIOS. > ..... > > So what we have here is an *open source bios* that doesn't handle > drives 4k sector sizes. There's the problem that needs to be fixed.... And if it wants to boot a guest OS that doesn't handle 4k sectors? <snip> >> it's our checks in XFS that fail. > > No they don't - they are working just fine. We've told XFS that the > sector size is X, and therefore we don't allow IO in smaller units, > data or metadata. That's the whole point of the filesystem having a > configurable sector size - we can *enforce* a larger minimum IO > requirement than the underlying hardware supports. Semantics. Yes, they work just fine, by failing the call. > We've done this for years - e.g. long time ago MD devices had a > massive performance penalty for sub-page sized IOs, so mkfs set the > sector size to 4k to avoid that problem, even though we could have > done 512 byte IOs to the underlying devices. > > Lets fix the problem at the source - the bios that doesn't support > 4k sector devices - like we've done for all the other utilities that > need to be aware of disk sector sizes.... I don't disagree with that, but by looking at a 4k/512 drive and deciding to make 4k sectors, we now present guests with something that barely exists in the real world: a hard 4k drive w/ no 512 logical fallback. Hacking up sector sizes in fs/xfs is probably the wrong way to go, but I'm not sure that essentially forcing hard 4k sectors on every qemu guest hosted on xfs is a great path either. Sure, the bios should support 4k - I can ask about that. But I think the concern above still stands: in effect we present a device which is less flexible than the real hardware beneath it; we've removed a compatibility layer that plenty of software still depends on. I'm not sure that's the best idea; at best it's unexpected. -Eric > Cheers, > > Dave. > _______________________________________________ xfs mailing list xfs@xxxxxxxxxxx http://oss.sgi.com/mailman/listinfo/xfs
