A closer Look inside Oracle ASM - CERN

CERN - IT DepartmentCH-1211 Genève 23

Switzerlandwww.cern.ch/it

A Closer Look inside Oracle ASM

UKOUG Conference 2007Luca Canali, CERN IT

LCG


Switzerlandwww.cern.ch/it Inside Oracle ASM, UKOUG Dec 2007 - 2

Outline

• Oracle ASM for DBAs– 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



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



ASM for a Clustered Architecture

• Oracle architecture of redundant low-cost components

Servers

SAN

Storage



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

Mirroring

Striping Striping

Failgroup1 Failgroup2

ASM Diskgroup



Files, Extents, and Failure Groups

Files and extent pointers

Failgroupsand ASMmirroring



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



Selected V$ Views and X$ Tables

View Name X$ Table DescriptionV$ASM_DISKGROUP X$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

N.A. X$KFDPARTNER lists disk-to-partner relationships

N.A. X$KFFXP extent map table for all ASM files

N.A. X$KFDAT allocation table for all ASM disks



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



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



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



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.



Example 1: Direct File Access 1/2

Goal: Reading ASM files with OS tools, using metadata information from X$ tables

1. Example: find the 2 mirrored extents of the RDBMS spfile

2. sys@+ASM1> 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#---------- ---------- ---------- ---------- 1 16 17528 0 1 4 14838 0



Example 1: Direct File Access 2/2

3. Find the disk path sys@+ASM1> select disk_number,path fromv$asm_disk where GROUP_NUMBER=1 and disk_numberin (16,4); DISK_NUMBER PATH ----------- ------------------------------------

4 /dev/mpath/itstor417_1p1 16 /dev/mpath/itstor419_6p1

4. Read data from disk using ‘dd’ dd if=/dev/mpath/itstor419_6p1 bs=1024kcount=1 skip=17528 |strings



X$KFFXP

Column Name DescriptionNUMBER_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)



Example 2: A Different Way

A different metadata table to reach the same goal of reading ASM files directly from OS:

sys@+ASM1> 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#---------- ---------- ---------- 1 4 14838 1 16 17528



X$KFDAT

Column Name (subset) DescriptionGROUP_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



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;/



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)



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



Strace and ASM 1/3

Goal: understand strace output when using ASM storage

• Example:read64(15,"#33\0@\"..., 8192, 473128960)=8192 • This is a read operation of 8KB from FD 15 at offset

473128960• What is the segment name, type, file# and block# ?



Strace and ASM 2/3

1. From /proc/<pid>/fd I find that FD=15 is/dev/mpath/itstor420_1p1

2. This is disk 20 of D.G.=1 (from v$asm_disk)3. From x$kffxp I find the ASM file# and extent#:

• Note: offset 473128960 = 451 MB + 27 *8KBsys@+ASM1>select number_kffxp,

xnum_kffxp from x$kffxp where group_kffxp=1 and disk_kffxp=20 and au_kffxp=451;

NUMBER_KFFXP XNUM_KFFXP------------ ---------- 268 17



Strace and ASM 3/3

4. From v$asm_alias I find the file alias for file 268: USERS.268.612033477

5. From v$datafile view I find the RDBMS file#: 96. From dba extents finally find the owner and

segment name relative to the original IO operation:

sys@TEST1>select owner,segment_name,segment_type from dba_extents where FILE_ID=9 and 27+17*1024*1024 between block_id and block_id+blocks;

OWNER SEGMENT_NAME SEGMENT_TYPE ----- ------------ ------------SCOTT EMP TABLE



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

= 1

MB



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---------- ---------- ------------------- ------------- 1 1 2097152 6291456 1 2 1048576 3145728 1 3 264241152 795869184 1 4 1392640 6291456 1 5 1048576 3145728 1 6 1048576 3145728



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



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



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



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



ASM Rebalancing Performance

• Tracing ASM rebalancing operations– 10046 trace of the +arbx processes

• Oradebug setospid … • oradebug event 10046 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)



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



Rebalancing, an Example

ASM Rebalancing Performance (RAC)

0

1000

2000

300040005000

6000

7000

0 2 4 6 8 10 12

Diskgroup Rebalance Parallelism

Rate

, MB/

min

Oracle 11g

Oracle 10g

Data: D.Wojcik, CERN IT



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



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



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



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



Read Performance, Random I/O

IOPS measured with SQL (synthetic test)

Small Random I/O (8KB block, 32GB probe table) 64 SATA HDs (4 arrays, 1 instance)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Workload, number of oracle sessions

I/O s

mal

l rea

d pe

r sec

(IO

PS)

8675 IOPS

~130 IOPS per disk Destroking, only the external part of the disks is used



Read Performance, Sequential I/O

Sequential I/O Throughput, 80GB probe table64 SATA HDs (4 arrays and 4 RAC nodes)

0

100

200

300

400

500

600

700

800

1 2 4 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40

Workload, number of parallel query slaves

MB

/sec

Limited by HBAs -> 4 x 2 Gb

(measured with parallel query)



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, 10.2.0.3 on RHEL4,

RAC of 4 to 8 nodes



Conclusions

• CERN deploys RAC and ASM on Linux on commodity HW– 2.5 years of production, 110 Oracle 10g RAC

nodes and 300TB of raw disk space (Dec 2007)• ASM metadata

– Most critical part, especially rebalancing– Knowledge of some ASM internals helps

troubleshooting• ASM on VLDB

– Know and work around pitfalls in 10g– 11g has important manageability and

performance improvements



Q&A

Q&A• Links:

– http://cern.ch/phydb– http://twiki.cern.ch/twiki/bin/view/PSSGroup/ASM_Internals– http://www.cern.ch/canali

A closer Look inside Oracle ASM - CERN

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