aix interview tips

8/11/2019 AIX Interview Tips

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1. What is ODM?

The ODM has many purposes. Its primary functions are to maintain the configuration, associated devices, and thevital product database.

In addition, it provides a more robust, secure, and sharable resource than theASCII files previously used in AI.

System data managed by the ODM includes!

" Device configuration information" Display information for SMIT #menus, selectors, and dialogs$

" %ital product data for installation and update procedures" Communications configuration information

" System resource informationMost system ob&ect classes and ob&ects are stored in the 'usr'lib'ob&repos

directory( ho)ever, ODM information is stored in three directories as follo)s!" 'usr'lib'ob&repos" 'usr'share'lib'ob&repos

" 'etc'ob&repos

The basic components of the ODM are ob&ect classes and ob&ects. To manage

ob&ect classes and ob&ects, you use the ODM commands and subroutines.

Odmadd, odmchange, odmget, odmsho) etc.

2. What is Physical Volume? *ach individual fi+eddis- drive is called a physical volume #%$ and has a

name #for e+ample! hdis-/, hdis-0, or hdis-1$.

3. What is Volume Group(VG) ?

All physical volumes belong to one volume group #%2$

4. What is Physical Partition ?

All of the physical volumes in a volume group are divided into physical

partitions #s$ of the same si3e.

. What is lo!ical Volume ?

4ithin each volume group, one or more logical volumes #5%s$ are defined.

5ogical volumes are groups of information located on physical volumes. Data

on logical volumes appear as contiguous to the user but can be discontiguouson the physical volume.

". What is lo!ical Partition ?

*ach logical volume consists of one or more logical partitions #5s$. *ach

logical partition corresponds to at least one physical partition. If mirroring isspecified for the logical volume, additional physical partitions are allocated to

store the additional copies of each logical partition.

7. What is #o!ical Volume Mana!er (#VM)? The set of operating system commands, library subroutines, and other tools that

allo) you to establish and control logical volume storage is called the 5ogical%olume Manager #5%M$. The 5%M controls dis- resources by mapping data

bet)een a more simple and fle+ible logical vie) of storage space and the actual

physical dis-s.

$. Where %oes #VM &eeps con'i!uration %ata?

Oect Data Mana!er (ODM) %ataase

The ODM database is the place )here most of the AI system configuration datais -ept. The ODM database contains information about all configured physical

volumes, volume groups, and logical volumes. This information mirrors theinformation found in the %2DA. 6or e+ample, the process of importing a %2DA

involves automatically copying the %2DA data for the imported volume group intothe ODM. 4hen a volume group is e+ported, the data held in the ODM about that

volume group is removed from the ODM database.

*. What is VGD+ ( Volume Group Descriptor area?

The %2DA, located at the beginning of each physical volume, contains

information that describes all the logical volumes and all the physical volumes

that belong to the volume group of )hich that physical volume is a member. The%2DA is updated by almost all the 5%M commands. The %2DA ma-es each


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volume group selfdescribing. An AI system can read the %2DA on a dis-, and,

from that, can determine )hat physical volumes and logical volumes are part ofthis volume group.

*ach dis- contains at least one %2DA. This is important at vary on time. The

time stamps in the %2DAs are used to determine )hich %2DAs correctly reflectthe state of the volume group. %2DAs can get out of sync )hen, for e+ample, a

volume group of four dis-s has one dis- failure. The %2DA on that dis- cannot beupdated )hile it is not operational. Therefore, you need a )ay to update this

%2DA )hen the dis- comes online, and this is )hat the vary on process )ill do.

The %2DA is allocated )hen the dis- is assigned as a physical volume #)ith thecommand m&%e,$. This &ust reserves a space for the %2DA at the start of the dis-.

The actual volume group information is placed in the %2DA )hen the physicalvolume is assigned to a volume group #using the m&,! or e-ten%,! commands$.4hen a physical volume is removed from the volume group #using the re%uce,!

command$, the volume group information is removed from the %2DA.

1. What is #V/0 ( #o!ical Volume /ontrol 0loc& )?

The 5%C7 is located at the start of every logical volume. It contains information

about the logical volume and uses a fe) hundred bytes.

11. What is Dis& uorum ?

A 8uorum is a state in )hich 90 percent or more of the physical volumes in avolume group are accessible. A 8uorum is a vote of the number of %olume 2roupDescriptor Areas and %olume 2roup Status Areas #%2DA'%2SA$ that are active.A 8uorum ensures data integrity in the event of a dis- failure.

12. What is Dis& Mirrorin! ?

Dis- mirroring is the association of t)o or three physical partitions )ith each

logical partition in a logical volume. 4hen the data is )ritten onto the logicalvolume, it is also )ritten to all the physical partitions that are associated )ith the

logical partition. Therefore, mirroring of data increases the availability of data.AI and the logical volume manager provide a dis- mirroring facility at a logical

volume level. If mirroring is established, this can be done )hen a logical volumeis created.

The m&l, command allo)s you to select one or t)o additional copies for eachlogical volume. Mirroring can also be added to an e+isting logical volume using

the m&l,copy command.

he mirror,! comman%The mirror,! command mirrors all the logical volumes on a given volume group.

This same function may also be accomplished manually if you run the m&l,copycommand for each individual logical volume in a volume group. As )ith m&l,copy,

the target physical drives to be mirrored )ith data must already be members ofthe volume group.

To get default mirroring of rootvg, run the follo)ing command.mirrorvg rootvg

To replace a failed dis- drive in a mirrored volume group, run the follo)ingcommands!

unmirrorvg )or-vg hdis-:

reducevg )or-vg hdis-:rmdev l hdis-: ;d


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The osoot command al)ays saves device configuration data for dis-. It

does not update the list of boot devices in the =%


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In many cases, restoration of the bac-up of the superbloc- to the primary

superbloc- )ill recover the file system. If this does not resolve the problem,recreate the file system and restore the data from a bac-up.

/annot unmount 'ile systems

A file system cannot be unmounted if any reference is still active )ithin that file

system. The follo)ing error message )ill be displayed!Device busy

or

A device is already mounted or cannot be unmounted

The follo)ing situations can leave an open references to a mounted file system!

" 6iles are open )ithin a file system. Close these files before the file systemcan be unmounted. The 'user command is often the best )ay to determine

21. What is a pa!in! space ?

To accommodate a large virtual memory space )ith a limited real memory space,

the system uses real memory as a )or- space and -eeps inactive data andprograms on a dis-. The area of the dis- that contains this data is called the

system paging space.A paging space, also called a s)ap space, is a logical volume )ith the attribute

type e8ual to paging.

The installation creates a default paging logical volume #hd$ on drive hdis-/,also referred as primary paging space.

22. What is !eneral recommen%ation 'or pa!in! space?

The general recommendation is that the sum of the si3es of the paging spaces

should be e8ual to at least t)ice the si3e of the real memory of the machine, upto a memory si3e of 19 M7 #901 M7 of paging space$. 6or memories larger than

19 M7, the follo)ing rule is recommended!Total paging space ? 901 M7 B #memory si3e 19 M7$ E 0.19

The lsps 9a command provides a snapshot of the current utili3ation of each of

the paging spaces on a system, )hile the lsps 9s command provides asummary of the total active paging space and its current utili3ation.

23. Mention %i''erent s5itches 'or m&sys ?

24. o5 to e-clu%e a 'ile 'rom m&sys ac&up ? 4hen you need to ma-e a m&sys bac-up of a system, and you )ant to e+clude

some data file systems from the system, you need to edit the 'etc'e+clude.rootvgfile. If, for e+ample, you )ant to e+clude the file systems 'usr and 'tmp from your

m&sys bac-up, add the follo)ing!'usr''tmp'Ma-e sure there are no empty lines in this file. Fou can list the contents of the file

as follo)s!G cat 'etc'e+clude.rootvg

'usr''tmp'

Then run the m&sys command using the e flag to e+clude the contents of thee+clude.rootvg file as follo)s!

m-sysb e 'dev'rmt/2. What is :;< 7 enhance% :;< an% 5hat is the main %i''erence?

ournaled 6ile System, or 6S, is the default file system for AI )hen running on a >1bit -ernel. A &ournaling filesystem allo)s for 8uic- file system recovery after a crash occurs by logging the metadata of files. 7y enabling filesystem logging, the system records every change in the metadata of the file into a reserved area of the file

system. The actual )rite operations are performed after the logging of changes to the metadata has beencompleted.

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*nhanced 6S, or 6S1, is another native AIH &ournaling file system that )as introduced in AI 9.0. *nhanced

6S is the default file system for bit -ernel environments. Due to address space limitations of the >1;bit-ernel, *nhanced 6S is not recommended for use in >1bit -ernel environments.

Differences

=ernel a%%ress space

AI offers t)o different types of -ernels, a >1bit -ernel and a bit -ernel. The >1bit and bit -ernels have common

libraries, commands, utilities, and header files. o)ever, the bit -ernel offers a degree of scaling for bit hard)are

that the >1bit -ernel cannot.

6S is optimi3ed for the >1bit -ernel, and )ill run on >1bit and bit hard)are. *nhanced 6S is optimi3ed to run on

the bit -ernel, allo)ing it to ta-e advantage of bit functionality.

:ournalin!

7efore )riting actual data, a &ournaling file system logs the metadata, thus incurring an overhead penalty that slo)s )rite

throughput. One )ay of improving performance under 6S is to disable metadata logging by using the nointe!ritymount

option. =ote that the enhanced performance is achieved at the e+pense of metadata integrity. Therefore, use this option

)ith e+treme caution because a system crash can ma-e a file system mounted )ith this option unrecoverable.

In contrast to 6S, *nhanced 6S does not allo) you to disable metadata logging. o)ever, the implementation of

&ournaling on *nhanced 6S ma-es it more suitable to handle metadataintensive applications. Thus, the performance

penalty is not as high under *nhanced 6S as it is under 6S.

Directory or!ani8ation

An inde+ node, or inode, is a data structure that stores all file and directory properties. 4hen a program loo-s up a file,

it searches for the appropriate inode by loo-ing up a file name in a directory. 7ecause these operations are performed

often, the mechanism used for searching is of particular importance.

6S employs a linear organi3ation for its directories, thus ma-ing searches linear as )ell. In contrast, *nhanced 6Semploys a binary tree representation for directories, thus greatly accelerating access to files.

nhance% :;1bit -ernel bit -ernel

Ma+imum file system si3e 0 terabyte petabytes0

Ma+imum file si3e gigabytes petabytes0

=umber of Inodes 6i+ed at file system creation Dynamic, limited by dis- space

5arge file support As mount option Default

Online defragmentation Fes Fes

=amefs Fes Fes

DMAI =o Fes

Compression Fes =o

Kuotas Fes Fes

Deferred update Fes =o

Direct I'O support Fes Fes

2". What #o!ical Partitionin! (#P+)?

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A logical partition #5A

partitions on the system have no visibility to the other partitions on the system or the free pool. 4ith D5A, you )ere not able to share processors among the different partitions. Starting )ith AI 9.>, you can use

shared processor partitions, or S5A


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%irtual SCSI! ///>$.

SMT! %ersion 9.> supports the SMT mode of O4*///>$.

offers a ne) volume group type called scalable volume group #%2$. The scalable %2 canaccommodate a ma+imum of 0/1 physical volumes and raises the limit for the number of logical volumes #5%s$

to /P. The ma+imum number of physical partitions #s$ is no longer defined on a per dis- basis, but applies

to the entire %2. The scalable %2 can hold up to 1,/P:,091 #1/Q R$ s. The range of the si3e starts at 0 M7

and goes up to 0>0,/:1 #01Q 27$, )hich is more than t)o orders of magnitude above the 0/1 #0 27$ ma+imum

available in AI 95 %ersion 9.1.

erformance improvement of 5%M commands

A large number of changes and enhancements have been implemented in AI 95 %ersion 9.> to reduce thee+ecution times of the follo)ing 5%M commands!

o e+tendvg

o importvg

o m-vg

o varyonvgo chlvcopy

o m-lvcopy

o lslv

o lspv

. 4hen trying to importa classical concurrent mode %2, an error message informs the system administrator to convert the %2 to an

enhanced concurrent mode capable %2. Dis- 8uotas support for 6S1

AI 95 %ersion 9.> e+tends the 6S1 functionality by implementing dis- usage 8uotas to control usage of

persistent storage.

Dis- 8uotas might be set for individual users or groups on a per file system basis.

Shrin- a file system

AI 95 %ersion 9.> supports shrin-ing a 6S1 file system. 4hen the si3e of the file system is decreased, the 5% on

)hich the file system resides is also decreased.

eliaility7 +,ailaility7 introduces a recovery method )herein the log is

recovered )hen the errdemon is started. It chec-s for the error log consistency. If the errdemon detects acorrupted error log, it ma-es a bac-up copy of the e+isting error log file to 'tmp'errlog.save and then repairs the

e+isting log.

System dump enhancementsDump information is displayed on the TTF during the creation of the system dump.

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6ollo)ing a system crash, there e+ist scenarios )here a system dump might crash or fail )ithout one byte of

data )ritten out to the dump device. 6or cases )here a failed dump doesn@t include the dump minimal table, thefailures cannot be easily diagnosed. In AI 95 %ersion 9.>, the dump procedure is enhanced to use the =%

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