1. A Closer Look inside Oracle ASM
UKOUG Conference 2007
Luca Canali, CERN IT

2. Outline Oracle ASM for DBAs ASM is not a black box ASM and VLDB
Introduction and motivations
ASM is not a black box
Investigation of ASM internals
Focus on practical methods and troubleshooting
ASM and VLDB
Metadata, rebalancing and performance
Lessons learned from CERN’s production DB services

3. ASM Oracle Automatic Storage Management
Provides the functionality of a volume manager and filesystem for Oracle (DB) files
Works with RAC
Oracle 10g feature aimed at simplifying storage management
Together with Oracle Managed Files and the Flash Recovery Area
An implementation of S.A.M.E. methodology
Goal of increasing performance and reducing cost

4. ASM for a Clustered Architecture
Oracle architecture of redundant low-cost components
Servers
SAN
Storage
This is the architecture deployed at CERN for the Physics DBs, more on

5. ASM Disk Groups Example: HW = 4 disk arrays with 8 disks each
An ASM diskgroup is created using all available disks
The end result is similar to a file system on RAID 1+0
ASM allows to mirror across storage arrays
Oracle RDBMS processes directly access the storage
RAW disk access
ASM Diskgroup
Mirroring
Striping
Striping
Failgroup1
Failgroup2

6. Files, Extents, and Failure Groups
Files and extent pointers Failgroups and ASM mirroring

7. ASM Is not a Black Box ASM is implemented as an Oracle instance
Familiar operations for the DBA
Configured with SQL commands
Info in V$ views
Logs in udump and bdump
Some ‘secret’ details hidden in X$TABLES and ‘underscore’ parameters

8. Selected V$ Views and X$ Tables
View Name
X$ Table
Description
V$ASM_DISKGROUP
X$KFGRP
performs disk discovery and lists diskgroups
V$ASM_DISK
X$KFDSK, X$KFKID
performs disk discovery, lists disks and their usage metrics
V$ASM_FILE
X$KFFIL
lists ASM files, including metadata
V$ASM_ALIAS
X$KFALS
lists ASM aliases, files and directories
V$ASM_TEMPLATE
X$KFTMTA
ASM templates and their properties
V$ASM_CLIENT
X$KFNCL
lists DB instances connected to ASM
V$ASM_OPERATION
X$KFGMG
lists current rebalancing operations
N.A.
X$KFKLIB
available libraries, includes asmlib
X$KFDPARTNER
lists disk-to-partner relationships
X$KFFXP
extent map table for all ASM files
X$KFDAT
allocation table for all ASM disks
A complete list in
View Name X$ Table name Description
V$ASM_DISKGROUP X$KFGRP performs disk discovery and lists diskgroups
V$ASM_DISKGROUP_STAT X$KFGRP_STAT diskgroup stats without disk discovery
V$ASM_DISK X$KFDSK, X$KFKID performs disk discovery, lists disks and their usage metrics
V$ASM_DISK_STAT X$KFDSK_STAT, X$KFKID lists disks and their usage metrics
V$ASM_FILE X$KFFIL lists ASM files, including metadata/asmdisk files
V$ASM_ALIAS X$KFALS lists ASM aliases, files and directories
V$ASM_TEMPLATE X$KFTMTA lists the available templates and their properties
V$ASM_CLIENT X$KFNCL lists DB instances connected to ASM
V$ASM_OPERATION X$KFGMG lists rebalancing operations
N.A. X$KFKLIB available libraries, includes asmlib path
N.A. X$KFDPARTNER lists disk-to-partner relationships
N.A. X$KFFXP extent map table for all ASM files
N.A. X$KFDAT extent list for all ASM disks
N.A. X$KFBH describes the ASM cache (buffer cache of ASM in blocks of 4K (_asm_blksize)
N.A. X$KFCCE a linked list of ASM blocks. to be further investigated
Additional in 11g:
V$ASM_ATTRIBUTE X$KFENV ASM attributes, the X$ table shows also 'hidden' attributes
V$ASM_DISK_IOSTAT X$KFNSDSKIOST I/O statistics
N.A. X$KFDFS
N.A. X$KFDDD
N.A. X$KFGBRB
N.A. X$KFMDGRP
N.A. X$KFCLLE
N.A. X$KFVOL
N.A. X$KFVOLSTAT
N.A. X$KFVOFS
N.A. X$KFVOFSV

9. ASM Parameters Notable ASM instance parameters:
*.asm_diskgroups='TEST1_DATADG1','TEST1_RECODG1'
*.asm_diskstring='/dev/mpath/itstor*p*'
*.asm_power_limit=5
*.shared_pool_size=70M
*.db_cache_size=50M
*.large_pool_size=50M
*.processes=100
A longer list:
*.asm_diskgroups='TEST1_DATADG1','TEST1_RECODG1'
*.asm_diskstring='/dev/mpath/itstor*p*'
*.asm_power_limit=5
*.cluster_database=true
*.cluster_database_instances=6
*.shared_pool_size=70M
*.db_cache_size=50M
*.large_pool_size=50M
*.processes=100
+ASM1.instance_number=1 (..repeat for each instance)
*.instance_type='asm'
+ASM1.local_listener='LISTENER_+ASM1‘ (..repeat for each instance)
*.remote_login_passwordfile='exclusive'
*.user_dump_dest='/ORA/dbs00/oracle/admin/+ASM/udump‘
*.core_dump_dest='/ORA/dbs00/oracle/admin/+ASM/cdump'
*.background_dump_dest='/ORA/dbs00/oracle/admin/+ASM/bdump'

10. More ASM Parameters Underscore parameters
Several undocumented parameters
Typically don’t need tuning
Exception: _asm_ausize and _asm_stripesize
May need tuning for VLDB in 10g
New in 11g, diskgroup attributes
V$ASM_ATTRIBUTE, most notable
disk_repair_time
au_size
X$KFENV shows ‘underscore’ attributes
Query to display underscore parameters
select a.ksppinm "Parameter", c.ksppstvl "Instance Value" from x$ksppi a, x$ksppcv b, x$ksppsv c where a.indx = b.indx and a.indx = c.indx and ksppinm like '%asm%' order by a.ksppinm;

11. ASM Storage Internals ASM Disks are divided in Allocation Units (AU)
Default size 1 MB (_asm_ausize)
Tunable diskgroup attribute in 11g
ASM files are built as a series of extents
Extents are mapped to AUs using a file extent map
When using ‘normal redundancy’, 2 mirrored extents are allocated, each on a different failgroup
RDBMS read operations access only the primary extent of a mirrored couple (unless there is an IO error)
In 10g the ASM extent size = AU size
11g implements variable extent size for extent#>20000 (similar to the automatic extent size management in the DB)

12. ASM Metadata Walkthrough
Three examples follow of how to read data directly from ASM.
Motivations:
Build confidence in the technology, i.e. ‘get a feeling’ of how ASM works
It may turn out useful one day to troubleshoot a production issue.

13. Example 1: Direct File Access 1/2
Goal: Reading ASM files with OS tools, using metadata information from X$ tables
Example: find the 2 mirrored extents of the RDBMS spfile
select GROUP_KFFXP Group#, DISK_KFFXP Disk#, AU_KFFXP AU#, XNUM_KFFXP Extent# from X$KFFXP where number_kffxp=(select file_number from v$asm_alias where name='spfiletest1.ora');
GROUP# DISK# AU# EXTENT#

14. Example 1: Direct File Access 2/2
Find the disk path
select disk_number,path from
v$asm_disk where GROUP_NUMBER=1 and disk_number
in (16,4);
DISK_NUMBER PATH
4 /dev/mpath/itstor417_1p1
16 /dev/mpath/itstor419_6p1
Read data from disk using ‘dd’
dd if=/dev/mpath/itstor419_6p1 bs=1024k
count=1 skip=17528 |strings
Device mapper multipathing is used under Linux
Block devices are given a name /dev/mpath/itstor.. to reflect the storage array name
The name is a symbolic link (handled by device mapper) to to /dev/dm-xxx (device mapper block device)
Device names and device mapper configurations are set up with /etc/multipath.conf

15. X$KFFXP Column Name Description NUMBER_KFFXP
ASM file number. Join with v$asm_file and v$asm_alias
COMPOUND_KFFXP
File identifier. Join with compound_index in v$asm_file
INCARN_KFFXP
File incarnation id. Join with incarnation in v$asm_file
XNUM_KFFXP
ASM file extent number (mirrored extent pairs have the same extent value)
PXN_KFFXP
Progressive file extent number
GROUP_KFFXP
ASM disk group number. Join with v$asm_disk and v$asm_diskgroup
DISK_KFFXP
ASM disk number. Join with v$asm_disk
AU_KFFXP
Relative position of the allocation unit from the beginning of the disk.
LXN_KFFXP
0->primary extent,1->mirror extent, 2->2nd mirror copy (high redundancy and metadata)
‘File extent pointer’ X$ table. This is the author interpretation of the table based on investigation of actual instances.
X$KFFXP Column Name Description
ADDR x$ table address/identifier
INDX row unique identifier
INST_ID instance number (RAC)
NUMBER_KFFXP ASM file number. Join with v$asm_file and v$asm_alias
COMPOUND_KFFXP File identifier. Join with compound_index in v$asm_file
INCARN_KFFXP File incarnation id. Join with incarnation in v$asm_file
PXN_KFFXP Progressive file extent number
XNUM_KFFXP ASM file extent number (mirrored AU have the same extent value)
GROUP_KFFXP ASM disk group number. Join with v$asm_disk and v$asm_diskgroup
DISK_KFFXP Disk number where the extent is allocated. Join with v$asm_disk
AU_KFFXP Relative position of the allocation unit from the beginning of the disk. The allocation unit size (1 MB) in v$asm_diskgroup
LXN_KFFXP 0->primary extent, ->mirror extent, 2->2nd mirror copy (high redundancy and metadata)
FLAGS_KFFXP N.K.
CHK_KFFXP N.K.

16. Example 2: A Different Way
A different metadata table to reach the same goal of reading ASM files directly from OS:
select GROUP_KFDAT Group# ,NUMBER_KFDAT Disk#, AUNUM_KFDAT AU# from X$KFDAT where fnum_kfdat=(select file_number from v$asm_alias where name='spfiletest1.ora');
GROUP# DISK# AU#
This is much slower that reading X$KFFXP since it scans all the disks’ allocation tables

17. X$KFDAT Column Name (subset) Description GROUP_KFDAT
Diskgroup number, join with v$asm_diskgroup
NUMBER_KFDAT
Disk number, join with v$asm_disk
COMPOUND_KFDAT
Disk compund_index, join with v$asm_disk
AUNUM_KFDAT
Disk allocation unit (relative position from the beginning of the disk), join with x$kffxp.au_kffxp
V_KFDAT
Flag: V=this Allocation Unit is used; F=AU is free
FNUM_KFDAT
File number, join with v$asm_file
XNUM_KFDAT
Progressive file extent number join with x$kffxp.pxn_kffxp
‘Disk allocation table’ X$ table. This is the author interpretation of the table based on investigation of actual instances.
X$KFDAT Column Name Description
ADDR x$ table address/identifier
INDX row unique identifier
INST_ID instance number (RAC)
GROUP_KFDAT diskgroup number, join with v$asm_diskgroup
NUMBER_KFDAT disk number, join with v$asm_disk
COMPOUND_KFDAT disk compund_index, join with v$asm_disk
AUNUM_KFDAT Disk allocation unit (relative position from the beginning of the disk), join with x$kffxp.au_kffxp
V_KFDAT V=this Allocation Unit is used; F=AU is free
FNUM_KFDAT file number, join with v$asm_file
I_KFDAT N.K.
XNUM_KFDAT progressive file extent number join with x$kffxp.pxn_kffxp
RAW_KFDAT raw format encoding of the disk,and file extent information

18. Example 3: Yet Another Way
Using the internal package dbms_diskgroup
declare
fileType varchar2(50); fileName varchar2(50);
fileSz number; blkSz number; hdl number; plkSz number;
data_buf raw(4096);
begin
fileName := '+TEST1_DATADG1/TEST1/spfiletest1.ora';
dbms_diskgroup.getfileattr(fileName,fileType,fileSz, blkSz);
dbms_diskgroup.open(fileName,'r',fileType,blkSz, hdl,plkSz, fileSz);
dbms_diskgroup.read(hdl,1,blkSz,data_buf);
dbms_output.put_line(data_buf);
end; /

19. DBMS_DISKGROUP Can be used to read/write ASM files directly
It’s an Oracle internal package
Does not require a RDBMS instance
11g’s asmcmd cp command uses dbms_diskgroup
Procedure Name
Parameters
dbms_diskgroup.open
(:fileName, :openMode, :fileType, :blkSz, :hdl,:plkSz, :fileSz)
dbms_diskgroup.read
(:hdl, :offset, :blkSz, :data_buf)
dbms_diskgroup.createfile
(:fileName, :fileType, :blkSz, :fileSz, :hdl, :plkSz, :fileGenName)
dbms_diskgroup.close
(:hdl)
dbms_diskgroup.commitfile
(:handle)
dbms_diskgroup.resizefile
(:handle,:fsz)
dbms_diskgroup.remap
(:gnum, :fnum, :virt_extent_num)
dbms_diskgroup.getfileattr
(:fileName, :fileType, :fileSz, :blkSz)

20. File Transfer Between OS and ASM
The supported tools (10g)
RMAN
DBMS_FILE_TRANSFER
FTP (XDB)
WebDAV (XDB)
They all require a RDBMS instance
In 11g, all the above plus asmcmd
cp command
Works directly with the ASM instance
ftp into asm diskgroups can be used together with transportable tablespace.
CERN’s experience is the migration and consolidation of 10 DBs from 9i to a multi TB RAC DB on 10g
11g asmcmd is improved from the 10g version: Look in $OH/bin and $OH/rdbms/lib
(find $ORACLE_HOME -name *asmcmd*)

21. Strace and ASM 1/3
Goal: understand strace output when using ASM storage
Example:
8192, )=8192
This is a read operation of 8KB from FD 15 at offset
What is the segment name, type, file# and block# ?

22. Strace and ASM 2/3 From /proc/<pid>/fd I find that FD=15 is
/dev/mpath/itstor420_1p1
This is disk 20 of D.G.=1 (from v$asm_disk)
From x$kffxp I find the ASM file# and extent#:
Note: offset = 451 MB + 27 *8KB
number_kffxp, xnum_kffxp from x$kffxp where group_kffxp=1 and disk_kffxp=20 and au_kffxp=451;
NUMBER_KFFXP XNUM_KFFXP

23. Strace and ASM 3/3
From v$asm_alias I find the file alias for file 268: USERS
From v$datafile view I find the RDBMS file#: 9
From dba extents finally find the owner and segment name relative to the original IO operation:
owner,segment_name,segment_type from dba_extents where FILE_ID=9 and *1024*1024 between block_id and block_id+blocks;
OWNER SEGMENT_NAME SEGMENT_TYPE
SCOTT EMP TABLE

24. Investigation of Fine Striping
An application: finding the layout of fine-striped files
Explored using strace of an oracle session executing ‘alter system dump logfile ..’
Result: round robin distribution over 8 x 1MB extents
Fine striping size = 128KB (1MB/8) … A0 B0 A1 B1 A2 B2 A3 B3 A4 B4 A5 B5 A6 B6 A7 B7
AU = 1MB

25. Metadata Files ASM diskgroups contain ‘hidden files’
Not listed in V$ASM_FILE (file# <256)
Details are available in X$KFFIL
In addition the first 2 AUs of each disk are marked as file#=0 in X$KFDAT
Example (10g):
GROUP# FILE# FILESIZE_AFTER_MIRR RAW_FILE_SIZE
select group_kffil group#, number_kffil file#, filsiz_kffil filesize_after_mirr, filspc_kffil raw_file_size from x$kffil where number_kffil <256;
Metadata files have 3-way mirroring

26. ASM Metadata 1/2
File#0, AU=0: disk header (disk name, etc), Allocation Table (AT) and Free Space Table (FST)
File#0, AU=1: Partner Status Table (PST)
File#1: File Directory (files and their extent pointers)
File#2: Disk Directory
File#3: Active Change Directory (ACD)
The ACD is analogous to a redo log, where changes to the metadata are logged.
Size=42MB * number of instances
Source: Oracle Automatic Storage Management, Oracle Press Nov 2007, N. Vengurlekar, M. Vallath, R.Long
A note on File#1 and file extent pointers:
The extent information available in X$KFFXP is
In File#1 for the first 60 extents (short files)
Extent# > 60 make use of indirect pointers
Indirect pointers are allocated in files
Direct investigation:
Using X$KFFXP count the extents allocated for:
create tablespace test1 datafile size 30712k;
60 extents (normal redundancy)
create tablespace test2 datafile size 30713k;
65 extents (normal redundancy)
The extra extents are indirect pointers

27. ASM Metadata 2/2 File#4: Continuing Operation Directory (COD).
The COD is analogous to an undo tablespace. It maintains the state of active ASM operations such as disk or datafile drop/add. The COD log record is either committed or rolled back based on the success of the operation.
File#5: Template directory
File#6: Alias directory
11g, File#9: Attribute Directory
11g, File#12: Staleness registry, created when needed to track offline disks

28. ASM Rebalancing
Rebalancing is performed (and mandatory) after space management operations
Goal: balanced space allocation across disks
Not based on performance or utilization
ASM spreads every file across all disks in a diskgroup
ASM instances are in charge of rebalancing
Extent pointers changes are communicated to the RDBMS
RDBMS’ ASMB process keeps an open connection to ASM
This can be observed by running strace against ASMB
In RAC, extra messages are passed between the cluster ASM instances
LMD0 of the ASM instances are very active during rebalance

29. ASM Rebalancing and VLDB
Performance of Rebalancing is important for VLDB
An ASM instance can use parallel slaves
RBAL coordinates the rebalancing operations
ARBx processes pick up ‘chunks’ of work. By default they log their activity in udump
Does it scale?
In 10g serialization wait events can limit scalability
Even at maximum speed rebalancing is not always I/O bound
Gmon is also involved at the beginning and the end of the rebalancing operation to take and release the diskgroup lock

30. ASM Rebalancing Performance
Tracing ASM rebalancing operations
10046 trace of the +arbx processes
Oradebug setospid …
oradebug event trace name context forever, level 12
Process log files (in bdump) with orasrp (tkprof will not work)
Main wait events from my tests with RAC (6 nodes)
DFS lock handle
Waiting for CI level 5 (cross instance lock)
Buffer busy wait
‘unaccounted for’
enq: AD - allocate AU
enq: AD - deallocate AU
log write(even)
log write(odd)

31. ASM Single Instance Rebalancing
Single instance rebalance
Faster in RAC if you can rebalance with only 1 node up (I have observed: 20% to 100% speed improvement)
Buffer busy wait can be the main event
It seems to depend on the number of files in the diskgroup.
Diskgroups with a small number of (large) files have more contention (+arbx processes operate concurrently on the same file)
Only seen in tests with 10g
11g has improvements regarding rebalancing contention

32. Rebalancing, an Example
Rebalancing speed is measured in MB/minute to conform to V$ASM_OPERATION units
Test conditions may vary the results (OS, storage, Oracle version, number of ASM files, etc)
It’s a good idea to repeat the measurements when several parameters of the environment change to get meaningful results.
Data: D.Wojcik, CERN IT

33. Rebalancing Workload
When ASM mirroring is used (e.g. with normal redundancy)
Rebalancing operations can move more data than expected
Example:
5 TB (allocated): ~100 disks, 200 GB each
A disk is replaced (diskgroup rebalance)
The total IO workload is 1.6 TB (8x the disk size!)
How to see this: query v$asm_operation, the column EST_WORK keeps growing during rebalance
The issue: excessive repartnering
Rebalancing in RAC is failed over when an instance crashes, but
Does not restart if all instance are down (typical of single instance)
No obvious way to tell if a diskgroup has a pending rebalance op
A partial work around is to query v$ASM_DISK to see if there are disk occupation imbalances
total_mb and free_mb

34. ASM Disk Partners ASM diskgroup with normal redundancy
Two copies of each extents are written to different ‘failgroups’
Two ASM disks are partners:
When they have at least one extent set in common (they are the 2 sides of a mirror for some data)
Each ASM disk has a limited number of partners
Typically 10 disk partners: X$KFDPARTNER
Helps to reduce the risk associated with 2 simultaneous disk failures

35. Free and Usable Space
When ‘ASM mirroring’ is used not all the free space should be occupied
V$ASM_DISKGROUP.USABLE_FILE_MB:
Amount of free space that can be safely utilized taking mirroring into account, and yet be able to restore redundancy after a disk failure
it’s calculated for the case of the worst scenario, anyway it is a best practice not to have it go negative (it can)
This can be a problem when deploying a small number of large LUNs and/or failgroups

36. Fast Mirror Resync
ASM 10g with normal redundancy does not allow to offline part of the storage
A transient error in a storage array can cause several hours of rebalancing to drop and add disks
It is a limiting factor for scheduled maintenances
11g has new feature ‘fast mirror resync’
Redundant storage can be put offline for maintenance
Changes are accumulated in the staleness registry (file#12)
Changes are applied when the storage is back online

37. Read Performance, Random I/O
IOPS measured with SQL (synthetic test)
8675 IOPS
~130 IOPS per disk
Destroking, only the external part of the disks is used
From other tests with less ad-hoc tuning and from application deployment: ~100 IOPS random read operations per disk (SATA 7200 rpm)
More info here:

38. Read Performance, Sequential I/O
Limited by HBAs -> 4 x 2 Gb
(measured with parallel query)

39. Implementation Details
Multipathing
Linux Device Mapper (2.6 kernel)
Block devices
RHEL4 and 10gR2 allow to skip raw devices mapping
External half of the disk for data disk groups
JBOD config
No HW RAID
ASM used to mirror across disk arrays
HW:
Storage arrays (Infortrend): FC controller, SATA disks
FC (Qlogic): 4Gb switch and HBAs (2Gb in older HW)
Servers are 2x CPUs, 4GB RAM, on RHEL4, RAC of 4 to 8 nodes

40. Q&A Q&A Links: http://cern.ch/phydb