On Tue, 2010-03-16 at 11:30 +0900, Tejun Heo wrote: > Hello, > > On 03/10/2010 06:14 PM, Denys Vlasenko wrote: > > 63s/255h is more or less "standard" now. > > > > Alignment issues can be solved by picking a good multiple of > > _heads_ or _cylinders_: > > I've got a couple of comments stating that picking a good geometry > parameters can resolve the whole issue but I simply fail to see how it > could. We can pick any parameter we wish, but there is no reliable > way to communicate the custom geometry parameters to others. > > Geometry is determined by two parameters sec/trk and heads/cyl. You > can punch in those numbers if the BIOS has a menu for it (many don't > these days). Or hope that BIOS can somehow figure it out from the > partition table which some BIOSs actually try to do. The problem is > that to determine the two parameters you need to equations matching > CHSs and LBAs and that's available iff the first partition ends before > CHS addressing limit according to the custom geometry, which usually > is not the case. > > So, custom geometry is only useful to trick partitioners which align > using cylinders into using better alignments but doesn't help anything > for compatibility as no one can determine the used geometry reliably > after the partitioning is complete. With compatibility benefit gone, > there simply is no reason to stick to the cylinder abstraction at all. > > Am I missing something? Sort of. As you say, C/H/S doesn't exist for any modern disk. However, the msdos label, for reasons lost in the mists of time, uses cylinders as the units of partition boundaries, so we have to invent a bogus C/H/S geometry for that partition label. Because of the problems with picking C/H/S, most boot loaders take care to ensure that BIOS never cares about it either (by using the block offset I/O routines), so for most linux bootloaders, the BIOS problems with C/H/S is a red herring. So, it is true to say that picking a certain H/S geometry (which is entirely withing the gift of the partitioner) will align msdos label partitions, but will be don't care for all other labels: all other partition labels (like gpt) use block as offset and don't have any truck with the fictitious C/H/S stuff. The big problem is that 99% of the x86 systems out there still use the ancient msdos label for their boot disks, so aligning H/S going forwards will give us a nice "just works" for x86 boxes. James -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
