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internal:lilo:raid [2009/10/20 19:03] (current)
galaxy created
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 +====== LILO MBRs on RAID drives ======
  
 +FIXME This is a little bit edited copy of Solar'​s message on one of the lists where he described the result of his analysis of LILO's source code.  Eventually, we need to convert this to a Wiki article.
 +
 +<code email>
 +Date: Wed, 21 Oct 2009 03:25:51 +0400
 +From: Solar Designer
 +
 +Galaxy,
 +
 +I read up on LILO (the source code, disassembly of the MBRs,
 +values/​flags embedded into the MBRs).
 +
 +I found out that LILO's MBR (first.S in this case) is capable of using
 +the following approaches to determine the BIOS drive number to read
 +"the rest" from:
 +
 +1. In theory, it can use the drive number as passed by the BIOS in the
 +DL register upon MBR code startup. ​ However, in our case this
 +functionality is disabled. ​ The first.S code checks for FLAG_MAP_ON_BOOT,​
 +and this flag appears to not be set in our MBRs, perhaps because of this:
 +
 +/* if the state of the BIOS is DL_GOOD, always mark when boot==map
 +   if the state of the BIOS is < DL_GOOD, never mark */
 +
 +        if ( bios_boot == bios_map ​ &&
 +                (bios_passes_dl == DL_GOOD
 +                 || (do_md_install && !(extra==X_MBR_ONLY ))
 +                  ) )
 +            bsect.par_1.prompt |= FLAG_MAP_ON_BOOT;​
 +
 +It appears that DL_GOOD is never set in the current code.  The code
 +which might set it is #if 0'ed:
 +
 +#if 0
 +        if ( mask && ​ ( extra == X_NONE ||
 +                        (extra == X_AUTO ​ && ​ autocount == 0) ) )  {
 +            if (bios_passes_dl > DL_BAD) bios_passes_dl = DL_GOOD;
 +        }
 +#endif
 +
 +2. The MBRs have the BIOS drive numbers embedded in them, at offset 30
 +(decimal). ​ However, it appears that these are only used as a fallback
 +in case the search described below (#3) fails.
 +
 +Right now, the numbers are: 82, 82, 82, 83 for MBRs of sda, sdb, sdc,
 +and sdd, respectively.
 +
 +3. There'​s code to search all hard drives for the 32-bit "​VolumeID"​ or
 +"​serial no" (it is referred to differently) that is encoded in MBRs at
 +offset 440 (decimal). ​ This is documented:
 +
 +http://​en.wikipedia.org/​wiki/​Master_boot_record
 +
 +"​01B8 ​  ​0670 ​   440     ​Optional Disk signature ​        ​4"​
 +
 +LILO also stores the value it will search all MBRs for at offset 24
 +(decimal). ​ For our four hard drives, these are currently set to:
 +
 +Offset 440 (actual unique ids):
 +
 +0b6b879c
 +5d690ae1
 +09f27048
 +70a523b3
 +
 +Offset 24 (the values being searched for, with offset 440 ones being
 +checked against these):
 +
 +09f27048
 +09f27048
 +09f27048
 +70a523b3
 +
 +That is, even if sda or sdb are used for reading the MBR from, they will
 +then try to read the rest from sdc and, failing that, from sdd.
 +
 +Only if neither sdc nor sdd are found (by their VolumeIDs), then there
 +will be fallback to the drive number encoded in the MBR, which for both
 +sda and sdb is currently 0x82 (not good given the circumstances we're
 +considering here, although with the root fs on sdc+sdd only this does
 +not matter anyway).
 +
 +I haven'​t figured out the exact effect of raid-extra-boot yet (didn'​t
 +spend time on it).  I am not sure if our use of it is appropriate.
 +
 +Some of my commands were:
 +
 +dd if=/dev/sda of=sda-mbr count=1
 +objdump -D -b binary -m i8086 sda-mbr | less
 +cmp -l sda-mbr sdb-mbr
 +dd if=sda-mbr bs=1 skip=30 count=1 | od -tx1
 +
 +(and ditto for other hard drives)
 +
 +for n in a b c d; do dd if=sd$n-mbr bs=1 skip=440 count=4 2> /dev/null | od -tx4; done
 +for n in a b c d; do dd if=sd$n-mbr bs=1 skip=24 count=4 2> /dev/null | od -tx4; done
 +</​code>​
internal/lilo/raid.txt ยท Last modified: 2009/10/20 19:03 by galaxy
 
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