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以下是转载的两篇文章，这两篇文章分别介绍了kfed工具和ASM磁盘头，两篇文章之间有很强的关联性，所以在这里一并贴出。
1.kfed - ASM metadata editor
The kfed is an undocumented ASM utility that can be used to
read and modify ASM metadata blocks. It is a standalone
utility, independent of ASM instance, so it can be used with either mounted or
dismounted disk groups. The most powerful kfed feature is its ability to fix
corrupt ASM metadata.
The kfed binary is present in the recent ASM versions, but if you don't see it
in your $ORACLE_HOME/bin directory (e.g. it may not be present in
version 10.1), it can be built as follows:
kfed read
With the kfed read command we can read a single ASM metadata block. The
syntax is:
· 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.
Use kfed to read ASM disk header block
The following is an example of using the kfed utility to read the ASM disk
header from ASM disk /dev/sda1.
Note that the above kfed command is equivalent to this one (with all parameters explicitly set to their default values):
We see that the above kfed output is nicely formatted and human readable (sort
of). The fields are grouped based on the actual content of the ASM metadata
block.
In this example, the kfbh fields show the block header data,
and the most important one is kfbh.type, which says KFBTYP_DISKHEAD, meaning
the ASM disk header. This is the expected block type for an ASM disk header.
We then see the actual content of the ASM disk header metadata block - the
kfdhdb fields. Some of those are the disk number (kfdhdb.dsknum), 0 in this
case, the group redundancy type (kfdhdb.grptyp), normal redundancy in this
case, the disk header status (kfdhdb.hdrsts), member in this case, the disk
name (kfdhdb.dskname) - DATA_0000, etc.
Please see ASM disk header for the complete
explanation of kfdhdb fields.
Use kfed to read any ASM metadata block
The next example shows how to read an ASM File Directory block. To do
that we would use the following kfed command:
Is my ASM metadata block corrupt
If you see kfbh.type=KFBTYP_INVALID, in the disk header on a disk you believe
belongs to an ASM disk group, that indicates that the ASM disk header is
corrupt. But don't jump to conclusions! Are you looking at the right disk? Is
this the right disk partition? Can you access that disk via some other name -
in a multipath setup? If you are not sure, or if the disk header is in fact
damaged, contact Oracle Support for assistance.
Note that this applies to any ASM metadata block. If ASM expects to find a
metadata block and instead finds a block that is zeroed out or contains
rubbish, it will report the block as KFBTYP_INVALID, and an error (usually
ORA-15196) will be reported in the ASM and/or database alert log (depends on
which instance discovers the problem).
kfed write
With the kfed write command we can write to a single ASM metadata block. The
syntax is:
· text - a text file with the new block contents
· checksum=yes - calculate and write the correct checksum. Note that the checksum in the text file with the new content does not have to be correct.
Use kfed to write the correct checksum to ASM metadata
block
An ASM metadata may look fine, but in fact be corrupt. For
example the block checksum (kfbh.check) could be wrong, in which case that
would need to be corrected. Indeed, if the only problem is an
incorrect checksum, that can be easily corrected by simply reading the block
and then writing it back! The kfed will calculate the new checksum and write
the block back with the correct checksum.
Here are the complete steps to correct the bad checksum for block 2 in AU0 on
disk /dev/sda1:
NOTE: Please seek Oracle Support assistance with any suspected ASM metadata block corruption.
kfed find
The kfed find will examine all blocks in an allocation unit and report back on
the block types found. The syntax is:
We see that the find command parameters are the same as for the read command,
but the difference is that the find operates on all blocks in an allocation
unit.
Use kfed find command to verify blocks in AU0
This is an example of using the kfed find to verify that all blocks in AU0 have
the expected ASM metadata.
The expected result is type 1 for block 0, type 2 for block 1 and type 3 for all other blocks, i.e.:
Data type
Value
kfbh.endian
System endianness. 0 - big endian, 1 - little endian.
kfbh.type
ASM block type. KFBTYP_DISKHEAD tells us this is an ASM disk header block.
kfbh.block.blk
ASM block number. Note the ASM disk header is block number 0.
Important ASM disk header specific data
| Data type | Value |
| kfdhdb.driver.provstr | ORCLDISK+[ASM disk name] for ASMLIB disks. ORCLDISK for non-ASMLIB disks. |
| kfdhdb.dsknum | ASM disk number. |
| kfdhdb.grptyp | Disk group redundancy. KFDGTP_EXTERNAL - external, KFDGTP_NORMAL - normal, KFDGTP_HIGH - high. |
| kfdhdb.hdrsts | ASM disk header status. For possible values see V$ASM_DISK.HEADER_STATUS. |
| kfdhdb.dskname | ASM disk name. |
| kfdhdb.grpname | ASM disk group name. |
| kfdhdb.fgname | ASM failgroup name. |
| kfdhdb.crestmp.hi|lo | The date and time disk was added to the disk group. |
| kfdhdb.mntstmp.hi|lo | Last time the disk was mounted. |
| kfdhdb.secsize | Disk sector size (bytes). |
| kfdhdb.blksize | ASM metadata block size (bytes). |
| kfdhdb.ausize | Alloocation unit size (bytes). 1 MB is the default allocation unit size. |
| kfdhdb.dsksize | Disk size (allocation units). In this case the disk size is 10239 MB. |
| kfdhdb.fstlocn | Pointer to ASM Free Space Table. 1 = ASM block 1 in this allocation unit. |
| kfdhdb.altlocn | Pointer to ASM Allocation Table. 2 = ASM block 2 in this allocation unit. |
| kfdhdb.f1b1locn | Pointer to ASM File Directory. 2 = allocation unit 2. |
| kfdhdb.dbcompat | Minimum database version. 0x0a100000 = 10.1. |
| kfdhdb.grpstmp.hi|lo | The date and time the disk group was created. |
| kfdhdb.vfstart|vfend | Start and end allocation unit number for the clusterware voting disk. If this is zero, the disk does not have voting disk data. Version 11.2 and later only. |
| kfdhdb.spfile | Allocation unit number of the ASM spfile. Version 11.2 and later only. |
| kfdhdb.spfflg | ASM spfile flag. If this is 1, the ASM spfile is on this disk in allocation unit kfdhdb.spfile. Version 11.2 and later only. |
ASM disk header backup
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. To work out the
second last block number we need to know the allocation unit
size and ASM metadata block size.
I talked about this in my post on kfed, but let's do that again - get those
values from the block header and calculate the second last block number in
allocation unit 1:
$ ausize=`kfed read
/dev/oracleasm/disks/ASMD1 | grep ausize | tr -s ' ' | cut -d' ' -f2`
$ blksize=`kfed read /dev/oracleasm/disks/ASMD1 | grep blksize | tr -s ' ' |
cut -d' ' -f2`
$ let n=$ausize/$blksize-2
$ echo $n
254
$ kfed read /dev/oracleasm/disks/ASMD1 aun=1 blkn=254
kfbh.endian: 1 ; 0x000: 0x01
kfbh.hard: 130 ; 0x001: 0x82
kfbh.type: 1 ; 0x002: KFBTYP_DISKHEAD
kfbh.datfmt: 1 ; 0x003: 0x01
kfbh.block.blk: 0 ; 0x004: T=0 NUMB=0x0
kfbh.block.obj: 2147483648 ; 0x008: TYPE=0x8 NUMB=0x0
kfbh.check: 473773689 ; 0x00c: 0x1c3d3679
kfbh.fcn.base: 0 ; 0x010: 0x00000000
kfbh.fcn.wrap: 0 ; 0x014: 0x00000000
kfbh.spare1: 0 ; 0x018: 0x00000000
kfbh.spare2: 0 ; 0x01c: 0x00000000
kfdhdb.driver.provstr: ORCLDISKASMD1 ; 0x000: length=13
...
kfdhdb.dsknum: 0 ; 0x024: 0x0000
kfdhdb.grptyp: 1 ; 0x026: KFDGTP_EXTERNAL
kfdhdb.hdrsts: 3 ; 0x027: KFDHDR_MEMBER
kfdhdb.dskname: ASMD1 ; 0x028: length=5
kfdhdb.grpname: DATA ; 0x048: length=4
kfdhdb.fgname: ASMD1 ; 0x068: length=5
...
So we see the same contents as in block 0 in allocation unit 0.
This can be very handy when the disk header is damaged or lost. All we have to do is run kfed repair [disk_name], and specify the allocation unit size if the value is not default (1MB). But as I said in the kfed post, please do not do this on your own - seek Oracle Support assistance if you suspect problems with ASM disk header.
ASM disk header in Exadata
ASM disks in Exadata are not exposed to the OS via device names. Instead they
can be accessed via special name - "o/[IP address]/[disk name]". The kfed understands that
syntax, so we can still use it in Exadata.
Let's have a look at the ASM disk header on an Exadata disk:
$ kfed read
o/192.168.10.9/DBFS_DG_CD_03_exadatacel01
kfbh.endian: 1 ; 0x000: 0x01
kfbh.hard: 130 ; 0x001: 0x82
kfbh.type: 1 ; 0x002: KFBTYP_DISKHEAD
...
kfdhdb.dskname:DBFS_DG_CD_03_EXADATACEL01 ; 0x028: length=26
kfdhdb.grpname: DBFS_DG ; 0x048: length=7
kfdhdb.fgname: EXADATACEL01 ; 0x068: length=12
...
kfdhdb.ausize: 4194304 ; 0x0bc: 0x00400000
...
Some Exadata specific values in the ASM disk header are as follows:
· ASM disk name that consists of the disk group name (DBFS_DG), cell disk label (CD), cell disk number (3) and the storage cell name (exadatacel01)
· Failgroup name is the same as the storage cell name
· Default allocation unit size in Exadata is 4 MB
Conclusion
ASM disk header contains the metadata essential for the operation and
availability of an ASM disk group. To prevent the loss and accidental damage of
the ASM disk header, Oracle recommends to protect it by partitioning the disk -
thus 'moving' it away from the physical beginning of the disk. The ASM disk
header in Exadata is protected by not exposing it to the database server OS. In
ASM version 11.1.0.7 and later, the ASM disk header is further protected by
maintaining a copy in allocation unit 1.