Using kfed/dd to repair ASM disk header corruption

Overview

 

  • Note – Don’t run these commands on your production system as you can easily corrupt your data
  • In ASM versions 11.1.0.7 and later, the ASM disk header block is backed up in the second last ASM metadata block in the allocation unit 1.

Kfed parameters

  • aun - Allocation Unit (AU) number to read from. Default is AU0, or the very beginning of the ASM disk.
  • aus – AU size. Default is 1048576 (1MB). Specify the aus when reading from a disk group with non-default AU size.
  • blkn – block number to read. Default is block 0, or the very first block of the AU.
  • dev – ASM disk or device name. Note that the keyword dev can be omitted, but the ASM disk name is mandatory.

Understanding ASM disk layout

Read ASM disk header block from  AU[0]
[root@grac41 Desktop]# kfed read  /dev/asm_test_1G_disk1 | egrep 'name|size|type'
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD   <-- ASM disk header
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9          <-- ASM disk name
kfdhdb.grpname:                    TEST ; 0x048: length=4          <-- ASM DG name
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9          <-- ASM Failgroup
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200            <-- Disk sector size   
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000            <-- ASM block size
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000        <-- AU size : 1 Mbyte 
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff        <-- ASM disk size : 1 GByte  

Check ASM block types for the first 2 AUs 
AU[0] :
[root@grac41 Desktop]# kfed find /dev/asm_test_1G_disk1
Block 0 has type 1
Block 1 has type 2
Block 2 has type 3
Block 3 has type 3
Block 4 has type 3
Block 5 has type 3
Block 6 has type 3
Block 7 has type 3
Block 8 has type 3
Block 9 has type 3
Block 10 has type 3
..
Block 252 has type 3
Block 253 has type 3
Block 254 has type 3
Block 255 has type 3

AU[1] :
[root@grac41 Desktop]#  kfed find /dev/asm_test_1G_disk1 aun=1
Block 256 has type 17
Block 257 has type 17
Block 258 has type 13
Block 259 has type 18
Block 260 has type 13
..
Block 508 has type 13
Block 509 has type 13
Block 510 has type 1
Block 511 has type 19

Summary :
--> Disk header size is 512 bytes 
    AU size = 1Mbyte  --> AU block size = 4096 
    This translates to 1048576 / 4096 = 256 blocks to read an AU ( start with block 0 - 255 )
    Block 510 and block 0 storing an ASM disk header ( == type 1 )

Run the kfed command below if you interested in a certain ASM block type ( use output from kfed find to the type info )
[root@grac41 Desktop]#  kfed read  /dev/asm_test_1G_disk1 aun=1 blkn=255  | egrep 'type'
kfbh.type:                           19 ; 0x002: KFBTYP_HBEAT

Some ASM block types 
[root@grac41 Desktop]# kfed read  /dev/asm_test_1G_disk1 aun=0 blkn=0  | egrep 'type' 
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfbh.type:                            2 ; 0x002: KFBTYP_FREESPC
kfbh.type:                            3 ; 0x002: KFBTYP_ALLOCTBL
kfbh.type:                            5 ; 0x002: KFBTYP_LISTHEAD
kfbh.type:                           13 ; 0x002: KFBTYP_PST_NONE
kfbh.type:                           18 ; 0x002: KFBTYP_PST_DTA
kfbh.type:                           19 ; 0x002: KFBTYP_HBEAT

Repair ASM disk header block in AU[0] with kfed repair

  • In ASM versions 11.1.0.7 and later, the ASM disk header block is backed up in the second last ASM metadata block in the allocation unit 1.
Verify ASM DISK Header block located in  AU[0] and AU[1]
AU[0] :
[root@grac41 Desktop]# kfed read  /dev/asm_test_1G_disk1 aun=0 blkn=0 | egrep 'name|size|type'
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9
kfdhdb.grpname:                    TEST ; 0x048: length=4
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff

AU[1] : 
[root@grac41 Desktop]# kfed read  /dev/asm_test_1G_disk1 aun=1 blkn=254  | egrep 'name|size|type'
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9
kfdhdb.grpname:                    TEST ; 0x048: length=4
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff

Erase Disk header block in first AU ( aun=0 blkn=0 )
# dd if=/dev/zero of=/dev/asm_test_1G_disk1  bs=4096 count=1

Verify ASM disk header
# kfed read /dev/asm_test_1G_disk1 aun=0 blkn=0
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]
--> Corrupted ASM disk header detected in AU [0]

Repair disk header in AU[0] with kfed 
[grid@grac41 ASM]$ kfed repair  /dev/asm_test_1G_disk1 
[grid@grac41 ASM]$ kfed read /dev/asm_test_1G_disk1 aun=0 blkn=0
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9
kfdhdb.grpname:                    TEST ; 0x048: length=4
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff
--> kfed repair worked - Disk header restored 

Can kfed repair the Disk header block stored in the 2.nd AU ?

Delete  Disk header block in AU[1]
First use dd to figure out whether we are getting the correct block
[grid@grac41 ASM]$  dd if=/dev/asm_test_1G_disk1 of=-  bs=4096 count=1 skip=510 ; strings block1
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.000464628 s, 8.8 MB/s
ORCLDISK
TEST_0000
TEST
TEST_0000
--> looks like an ASM disk header - go ahead and erase that block

[grid@grac41 ASM]$  dd if=/dev/zero of=/dev/asm_test_1G_disk1  bs=4096 count=1  seek=510 
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.00644028 s, 636 kB/s

Verify ASM disk header block in AU[1]
[grid@grac41 ASM]$ kfed read /dev/asm_test_1G_disk1 aun=1 blkn=254
kfbh.type:                            0 ; 0x002: KFBTYP_INVALID
KFED-00322: Invalid content encountered during block traversal: [kfbtTraverseBlock][Invalid OSM block type][][0]
--> Corrupted ASM disk header detected

[grid@grac41 ASM]$ kfed repair  /dev/asm_test_1G_disk1 
KFED-00320: Invalid block num1 = [0], num2 = [1], error = [endian_kfbh]
--> kfed repair doesn' work 

Repair block with dd
grid@grac41 ASM]$ dd if=/dev/asm_test_1G_disk1  bs=4096  count=1 of=/dev/asm_test_1G_disk1  bs=4096 count=1  seek=510 
1+0 records in
1+0 records out
4096 bytes (4.1 kB) copied, 0.0306682 s, 134 kB/s
[grid@grac41 ASM]$ kfed read /dev/asm_test_1G_disk1 aun=0 blkn=0
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9
kfdhdb.grpname:                    TEST ; 0x048: length=4
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff

# kfed read /dev/asm_test_1G_disk1 aun=1 blkn=254
kfbh.type:                            1 ; 0x002: KFBTYP_DISKHEAD
kfdhdb.dskname:               TEST_0000 ; 0x028: length=9
kfdhdb.grpname:                    TEST ; 0x048: length=4
kfdhdb.fgname:                TEST_0000 ; 0x068: length=9
kfdhdb.capname:                         ; 0x088: length=0
kfdhdb.secsize:                     512 ; 0x0b8: 0x0200
kfdhdb.blksize:                    4096 ; 0x0ba: 0x1000
kfdhdb.ausize:                  1048576 ; 0x0bc: 0x00100000
kfdhdb.dsksize:                    1023 ; 0x0c4: 0x000003ff

Summary:
 to fix the backup block or the ASM disk header in AU 1 block you need to use dd

Reference

3 thoughts on “Using kfed/dd to repair ASM disk header corruption

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