了解和使用kfed維護ASM元數據

      以下是轉載的兩篇文章，這兩篇文章分別介紹了kfed工具和ASM磁盤頭，兩篇文章之間有很強的關聯性，所以在這里一并貼出。

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

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.