ATA 4 KiB sector issues.
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From: Tejun Heo on 16 Mar 2010 10:00 Hello, James. On 03/16/2010 10:24 PM, James Bottomley wrote: > For msdos labels, it's embedded in the label ... for all other labels, > it's made up on the spot. Where in the label? Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: James Bottomley on 16 Mar 2010 10:30 On Tue, 2010-03-16 at 22:56 +0900, Tejun Heo wrote: > Hello, James. > > On 03/16/2010 10:24 PM, James Bottomley wrote: > > For msdos labels, it's embedded in the label ... for all other labels, > > it's made up on the spot. > > Where in the label? No idea ... I only know you can use fdisk expert mode to change the C/H/S layout and the change is preserved across reboots. James -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Arnd Bergmann on 16 Mar 2010 10:30 On Tuesday 16 March 2010, James Bottomley wrote: > On Tue, 2010-03-16 at 22:56 +0900, Tejun Heo wrote: > > Hello, James. > > > > On 03/16/2010 10:24 PM, James Bottomley wrote: > > > For msdos labels, it's embedded in the label ... for all other labels, > > > it's made up on the spot. > > > > Where in the label? > > No idea ... I only know you can use fdisk expert mode to change the > C/H/S layout and the change is preserved across reboots. IIRC, the layout is guessed from the partition end locations, in the assumption that each partition is aligned to full cylinders. That gives you the heads/sectors number, while the cylinder number can be calculated from the total number of sectors using these numbers. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Denys Vlasenko on 16 Mar 2010 10:40 On Tuesday 16 March 2010 14:56, Tejun Heo wrote: > Hello, James. > > On 03/16/2010 10:24 PM, James Bottomley wrote: > > For msdos labels, it's embedded in the label ... for all other labels, > > it's made up on the spot. > > Where in the label? The "end of partition" is expected to be at the last head and sector. Of course this heuristic fails if there are more than one primary partition and they have differing last head and sector. But on most "sanely" partitioned disks they are the same: Disk /dev/sda: 255 heads, 63 sectors, 36481 cylinders Nr AF Hd Sec Cyl Hd Sec Cyl Start Size ID 1 00 1 1 0 254 63 850 63 13671252 0b 2 80 0 1 851 254 63 1023 13671315 572395950 05 3 00 0 0 0 0 0 0 0 0 00 4 00 0 0 0 0 0 0 0 0 00 5 00 1 1 851 254 63 972 63 1959867 83 6 00 1 1 973 254 63 1023 63 31246362 83 7 00 254 63 1023 254 63 1023 63 195318207 83 8 00 254 63 1023 254 63 1023 63 343871262 83 ^^^ ^^ Which suggests another idea how to align a partition: since there is no requirement on the partition *start*, we don't have to start at head1,sector1 or head0,sector1 In the example above, 1st partition might be modified to start at head1,sector2, IOW, LBA 64, thus making it 32k aligned. As long as partition *ends* adhere to the convention of being exactly at last_head,last_sector, nothing should break. -- vda -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
From: Tejun Heo on 16 Mar 2010 10:50
Hello, On 03/16/2010 11:21 PM, James Bottomley wrote: >>> For msdos labels, it's embedded in the label ... for all other labels, >>> it's made up on the spot. >> >> Where in the label? > > No idea ... I only know you can use fdisk expert mode to change the > C/H/S layout and the change is preserved across reboots. The CHS addresses are stored alongside with the LBA addresses. The problem is that the geometry parameters (sectors/track and heads/cyl) are not stored anywhere and CHS addresses don't make any sense without the two parameters. The only way to figure out the geometry parameters is to solve two equations involving CHS addresses and LBA addresses. e.g. If the first partition begins at CHS 0/32/33 and ends at 12/233/19 and the corresponding LBA addresses are 2048 and 206848, you can solve the equation and determine that the parameters gotta be 63 secs/trk and 255 heads/cyl to make those two pairs of addresses match each other and in fact some BIOSs try to do this depending on configuration (and sometimes falls into infinite loop or causes other boot related problems if the parameters are too uncommon). This method can't work reliably even at theoretical level because it requires at least two pairs of CHS/LBA addresses to match (two unknown parameters to solve for) and there is only single pair available if the first partition goes over the CHS limit which at maximum is 8GiB. So, CHS *values* are preserved if it falls below the CHS limit of the geometry used during partitioning but the geometry information is lost making the CHS values completely meaningless, so the only sane thing to do is to stick to whatever geometry parameters provided by the BIOS which usually is 255/63 these days. Otherwise, the results are... * If the first partition ends before the CHS limit and BIOS is configured to calculate back the parameters, BIOS may be able to report the geometry correctly. * If the first partition goes over the CHS limit, * BIOS can use 255/63 or whatever default parameters and CHS and LBA addresses won't match each other which won't be a problem for modern OSes as they don't look at the CHS addresses at all but older operating systems which consider both CHS and LBA addresses may get confused. * BIOS can set up arbitrary parameters such that the CHS and LBA for the start of the first partition match and maybe also try to cylinder align further LBA addresses but there is no guarantee these parameters match the original parameters used during partitioning and this seems to cause more compatibility problems than it solves. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo(a)vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ |