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Large Disk HOWTO: Problem solving Next Previous Contents

14. Problem solving

Many people think they have problems, while in fact nothing is wrong. Or, they think that the problems they have are due to disk geometry, while in fact disk geometry has nothing to do with the matter. All of the above may have sounded complicated, but disk geometry handling is extremely easy: do nothing at all, and all is fine; or perhaps give LILO the keyword lba32 if it doesn't get past `LI' when booting. Watch the kernel boot messages, and remember: the more you fiddle with geometries (specifying heads and cylinders to LILO and fdisk and on the kernel command line) the less likely it is that things will work. Roughly speaking, all is fine by default.

And remember: nowhere in Linux is disk geometry used, so no problem you have while running Linux can be caused by disk geometry. Indeed, disk geometry is used only by LILO and by fdisk. So, if LILO fails to boot the kernel, that may be a geometry problem. If different operating systems do not understand the partition table, that may be a geometry problem. Nothing else. In particular, if mount doesnt seem to work, never worry about disk geometry - the problem is elsewhere.

14.1 Problem: My IDE disk gets a bad geometry when I boot from SCSI.

It is quite possible that a disk gets the wrong geometry. The Linux kernel asks the BIOS about hd0 and hd1 (the BIOS drives numbered 80H and 81H) and assumes that this data is for hda and hdb. But on a system that boots from SCSI, the first two disks may well be SCSI disks, and thus it may happen that the fifth disk, which is the first IDE disk hda, gets assigned a geometry belonging to sda. Such things are easily solved by giving boot parameters `hda=C,H,S' for the appropriate numbers C, H and S, either at boot time or in /etc/lilo.conf.

Since Linux 2.5.51 this BIOS information is not used anymore, and the same problem occurs for all disks. See below.

14.2 Nonproblem: Identical disks have different geometry?

`I have two identical 10 GB IBM disks. However, fdisk gives different sizes for them. Look:

# fdisk -l /dev/hdb
Disk /dev/hdb: 255 heads, 63 sectors, 1232 cylinders
Units = cylinders of 16065 * 512 bytes

   Device Boot  Start      End   Blocks   Id  System
/dev/hdb1           1     1232  9896008+  83  Linux native
# fdisk -l /dev/hdd
Disk /dev/hdd: 16 heads, 63 sectors, 19650 cylinders
Units = cylinders of 1008 * 512 bytes

   Device Boot  Start      End   Blocks   Id  System
/dev/hdd1           1    19650  9903568+  83  Linux native
How come?'

What is happening here? Well, first of all these drives really are 10gig: hdb has size 255*63*1232*512 = 10133544960, and hdd has size 16*63*19650*512 = 10141286400, so, nothing is wrong and the kernel sees both as 10.1 GB. Why the difference in size? That is because the kernel gets data for the first two IDE disks from the BIOS, and the BIOS has remapped hdb to have 255 heads (and 16*19650/255=1232 cylinders). The rounding down here costs almost 8 MB.

If you would like to remap hdd in the same way, give the kernel boot parameters `hdd=1232,255,63'.

On the other hand, if the disk is not shared with DOS or so, it may be better to set hdb to Normal in the BIOS setup, instead of asking for some translation like LBA.

Since Linux 2.5.51, the IDE driver no longer uses BIOS info on the first two disks, and the different treatment of the first two disks has disappeared.

14.3 Problem: 2.4 and 2.6 report different geometries? 2.6 reports the wrong geometry? 2.6 reports no geometry at all?

Since geometry does not exist, it is not surprising that each of 2.0/2.2/2.4/2.6 reports a somewhat different disk geometry.

Some people will maintain that geometry *does* exist, and in that case do not mean a property of the disk, but mean the values reported by the BIOS. That is what several other operating systems will use. Since Linux 2.5.51, the kernel no longer uses the values reported by the BIOS - it is difficult to match BIOS device numbers with Linux disk names, maybe data is only available for two disks, maybe some disks are not present in the BIOS setup, etc. However, if one needs these values, since Linux 2.6.5 one can set CONFIG_EDD and mount sysfs, and then find the BIOS data for the various disks under /sys/firmware/edd/int13_dev*. Now the matching of BIOS numbers, represented in directory names like int13_dev82, with Linux names like sda can be done by user space software, possibly with help from the user.

This 2.5.51 change caused problems when many people using both Linux and Windows on the same disk upgraded from 2.4 to 2.6 and used as partitioning tool the program parted that had not yet been updated. I have not checked whether current parted is OK.

14.4 Nonproblem: fdisk sees much more room than df?

fdisk will tell you how many blocks there are on the disk. If you make a filesystem on the disk, say with mke2fs, then this filesystem needs some space for bookkeeping - typically something like 4% of the filesystem size, more if you ask for a lot of inodes during mke2fs. For example:

# sfdisk -s /dev/hda9
# mke2fs -i 1024 /dev/hda9
mke2fs 1.12, 9-Jul-98 for EXT2 FS 0.5b, 95/08/09
204798 blocks (5.00%) reserved for the super user
# mount /dev/hda9 /somewhere
# df /somewhere
Filesystem         1024-blocks  Used Available Capacity Mounted on
/dev/hda9            3574475      13  3369664      0%   /mnt
# df -i /somewhere
Filesystem           Inodes   IUsed   IFree  %IUsed Mounted on
/dev/hda9            4096000      11 4095989     0%  /mnt
We have a partition with 4095976 blocks, make an ext2 filesystem on it, mount it somewhere and find that it only has 3574475 blocks - 521501 blocks (12%) was lost to inodes and other bookkeeping. Note that the difference between the total 3574475 and the 3369664 available to the user are the 13 blocks in use plus the 204798 blocks reserved for root. This latter number can be changed by tune2fs. This `-i 1024' is only reasonable for news spools and the like, with lots and lots of small files. The default would be:
# mke2fs /dev/hda9
# mount /dev/hda9 /somewhere
# df /somewhere
Filesystem         1024-blocks  Used Available Capacity Mounted on
/dev/hda9            3958475      13  3753664      0%   /mnt
# df -i /somewhere
Filesystem           Inodes   IUsed   IFree  %IUsed Mounted on
/dev/hda9            1024000      11 1023989     0%  /mnt
Now only 137501 blocks (3.3%) are used for inodes, so that we have 384 MB more than before. (Apparently, each inode takes 128 bytes.) On the other hand, this filesystem can have at most 1024000 files (more than enough), against 4096000 (too much) earlier.

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