-
Red Hat Subject Matter ExpertsJosef BacikKamil Dudka Hans de
Goede Doug LedfordDaniel Novotny Nathan Straz David
WysochanskiContributors Michael Christ ie Sachin PrabhuRob Evers
David Howells David LehmanJeff Moyer Eric Sandeen Mike Snitzer
Red Hat Enterprise Linux 6Storage Administration Guide
Deploying and configuring single-node storage in Red Hat
Enterprise Linux6Edit ion 2
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Red Hat Enterprise Linux 6 Storage Administrat ion Guide
Deploying and configuring single-node storage in Red Hat
Enterprise Linux6Edit ion 2
Josef BacikServer Development Kernel File
[email protected] QuotasKamil DudkaBase Operating System
Core Services - [email protected] Contro l ListsHans de
GoedeBase Operating System
[email protected] LedfordServer
Development Hardware [email protected]
NovotnyBase Operating System Core Services -
[email protected] /proc File SystemNathan StrazQuality
Engineering QE - [email protected]
WysochanskiServer Development Kernel
[email protected]/LVM2Michael ChristieServer
Development Kernel [email protected] StorageSachin
PrabhuSoftware Maintenance [email protected]
EversServer Development Kernel Storage
-
[email protected] StorageDavid HowellsServer Development
Hardware [email protected] LehmanBase
Operating System [email protected] configuration
during installationJeff MoyerServer Development Kernel File
[email protected] DisksEric SandeenServer
Development Kernel File [email protected], ext4 , XFS,
Encrypted File SystemsMike SnitzerServer Development Kernel
[email protected]/O Stack and LimitsRed Hat Subject
Matter ExpertsContributors
Edited byJacquelynn EastEngineering Content
[email protected] DomingoEngineering Content
[email protected]
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AbstractThis guide provides instructions on how to effectively
manage storage devices and file systemson Red Hat Enterprise Linux
6 . It is intended for use by system administrators with basic
tointermediate knowledge o f Red Hat Enterprise Linux or
Fedora.
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Table of ContentsPreface
1. Do cument Co nventio ns1.1. Typ o g rap hic Co nventio ns1.2.
Pull-q uo te Co nventio ns1.3. No tes and Warning s
2. Getting Help and G iving Feed b ack2.1. Do Yo u Need Help
?2.2. We Need Feed b ack
Chapt er 1 . Overview1.1. What's New in Red Hat Enterp rise
Linux 6
File Syst em Encrypt ion (T echnology Preview)
File Syst em Caching (T echnology Preview)
Bt rfs (T echnology Preview)
I/O Limit Processing
ext 4 Support
Net work Block St orage
Part I. File Syst ems
Chapt er 2 . File Syst em St ruct ure and Maint enance2.1.
Overview o f Filesystem Hierarchy Stand ard (FHS)
2.1.1. FHS Org anizatio n2.1.1.1. Gathering File System Info
rmatio n2.1.1.2. The /b o o t/ Directo ry2.1.1.3. The /d ev/
Directo ry2.1.1.4. The /etc/ Directo ry2.1.1.5. The /l ib / Directo
ry2.1.1.6 . The /med ia/ Directo ry2.1.1.7. The /mnt/ Directo
ry2.1.1.8 . The /o p t/ Directo ry2.1.1.9 . The /p ro c/ Directo
ry2.1.1.10 . The /sb in/ Directo ry2.1.1.11. The /srv/ Directo
ry2.1.1.12. The /sys/ Directo ry2.1.1.13. The /usr/ Directo
ry2.1.1.14. The /var/ Directo ry
2.2. Sp ecial Red Hat Enterp rise Linux File Lo catio ns2.3. The
/p ro c Virtual File System2.4. Discard unused b lo cks
Chapt er 3. Encrypt ed File Syst em3.1. Mo unting a File System
as Encryp ted3.2. Ad d itio nal Info rmatio n
Chapt er 4 . Bt rfs4.1. Btrfs Features
Chapt er 5. T he Ext 3 File Syst em5.1. Creating an Ext3 File
System
8889
10101011
1 212
1 2
1 2
1 2
1 2
1 2
1 3
1 4
1 515151517171818181818191919202021232324
2 52526
2 727
2 829
T able of Cont ent s
1
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. . . . . . . . . .
5.1. Creating an Ext3 File System5.2. Co nverting to an Ext3
File System5.3. Reverting to an Ext2 File System
Chapt er 6 . T he Ext 4 File Syst em6 .1. Creating an Ext4 File
System6 .2. Mo unting an Ext4 File System
Writ e Barriers6 .3. Resiz ing an Ext4 File System6 .4. Backup
ext2/3/4 File Systems6 .5. Resto re an ext2/3/4 File System6 .6 . O
ther Ext4 File System Util i ties
Chapt er 7 . G lobal File Syst em 2
Chapt er 8 . T he XFS File Syst em8 .1. Creating an XFS File
System8 .2. Mo unting an XFS File System
Writ e Barriers8 .3. XFS Quo ta Manag ement
Set t ing Project Limit s8 .4. Increasing the Size o f an XFS
File System8 .5. Rep airing an XFS File System8 .6 . Susp end ing
an XFS File System8 .7. Backup and Resto ratio n o f XFS File
Systems
Simple Mode for xfsrest ore
Cumulat ive Mode for xfsrest ore
Int eract ive Operat ion8 .8 . O ther XFS File System Util i
ties
Chapt er 9 . Net work File Syst em (NFS)9 .1. Ho w NFS Wo
rks
9 .1.1. Req uired Services9 .2. p NFS9 .3. NFS Client Co nfig
uratio n
9 .3.1. Mo unting NFS File Systems using /etc/fs tab9 .4. auto
fs
9 .4.1. Imp ro vements in auto fs Vers io n 5 o ver Vers io n 49
.4.2. auto fs Co nfig uratio n9 .4.3. Overrid ing o r Aug menting
Site Co nfig uratio n Files9 .4.4. Using LDAP to Sto re Auto mo
unter Map s
9 .5. Co mmo n NFS Mo unt Op tio ns9 .6 . Starting and Sto p p
ing NFS9 .7. NFS Server Co nfig uratio n
9 .7.1. The /etc/exp o rts Co nfig uratio n File9 .7.2. The exp
o rtfs Co mmand
9 .7.2.1. Using exp o rtfs with NFSv49 .7.3. Running NFS Behind
a Firewall
9 .7.3.1. Disco vering NFS exp o rts9 .7.4. Ho stname Fo rmats9
.7.5. NFS o ver RDMA
292930
323334
3435353738
4 0
4 14142
4 343
4 545454646
4 8
4 8
4 849
50505152535454555658596 06 26 36 36 56 66 66 76 86 8
Red Hat Ent erprise Linux 6 St orage Administ rat ion Guide
2
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. . . . . . . . . .
9 .7.5. NFS o ver RDMA9 .8 . Securing NFS
9 .8 .1. NFS Security with AUTH_SYS and exp o rt co ntro ls9 .8
.2. NFS security with AUTH_GSS
9 .8 .2.1. NFS security with NFSv49 .8 .3. File Permissio ns
9 .9 . NFS and rp cb ind9 .9 .1. Tro ub lesho o ting NFS and rp
cb ind
9 .10 . References
Inst alled Document at ion
Useful Websit es
Relat ed Books
Chapt er 1 0 . FS- Cache10 .1. Perfo rmance Guarantee10 .2.
Setting Up a Cache10 .3. Using the Cache With NFS
10 .3.1. Cache Sharing10 .3.2. Cache Limitatio ns With NFS
10 .4. Setting Cache Cull Limits10 .5. Statis tical Info rmatio
n10 .6 . References
Part II. St orage Administ rat ion
Chapt er 1 1 . St orage Considerat ions During Inst allat
ion11.1. Up d ates to Sto rag e Co nfig uratio n During Installatio
n11.2. Overview o f Sup p o rted File Systems11.3. Sp ecial Co nsid
eratio ns
Separat e Part it ions for /home, /opt , /usr/local
DASD and zFCP Devices on IBM Syst em Z
Encrypt ing Block Devices Using LUKS
St ale BIOS RAID Met adat a
iSCSI Det ect ion and Configurat ion
FCoE Det ect ion and Configurat ion
DASD
Block Devices wit h DIF/DIX Enabled
Chapt er 1 2 . File Syst em Check12.1. Best Practices fo r
fsck12.2. Filesystem-Sp ecific Info rmatio n fo r fsck
12.2.1. ext2, ext3, and ext412.2.2. XFS12.2.3. Btrfs
Chapt er 1 3. Part it ions13.1. Viewing the Partitio n Tab
le13.2. Creating a Partitio n
6 86 96 9707071717172
7 2
7 3
7 3
7 47575767677787979
8 0
8 18 18 18 2
8 2
8 2
8 3
8 3
8 3
8 3
8 3
8 3
8 58 58 68 68 78 9
9 09 19 2
T able of Cont ent s
3
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. . . . . . . . . .
13.2. Creating a Partitio n13.2.1. Making the Partitio n13.2.2.
Fo rmatting and Lab eling the Partitio n13.2.3. Ad d to /etc/fs
tab
13.3. Remo ving a Partitio n13.4. Resiz ing a Partitio n
Chapt er 1 4 . LVM (Logical Volume Manager)14.1. What is
LVM2?14.2. Using system-co nfig -lvm
14.2.1. Uti l iz ing Uninitialized Entities14.2.2. Ad d ing
Unallo cated Vo lumes to a Vo lume Gro up14.2.3. Mig rating
Extents14.2.4. Ad d ing a New Hard Disk Using LVM14.2.5. Ad d ing a
New Vo lume Gro up14.2.6 . Extend ing a Vo lume Gro up14.2.7. Ed
iting a Lo g ical Vo lume
14.3. LVM References
Inst alled Document at ion
Useful Websit es
Chapt er 1 5. Swap Space15.1. Ad d ing Swap Sp ace
15.1.1. Extend ing Swap o n an LVM2 Lo g ical Vo lume15.1.2.
Creating an LVM2 Lo g ical Vo lume fo r Swap15.1.3. Creating a Swap
File
15.2. Remo ving Swap Sp ace15.2.1. Red ucing Swap o n an LVM2 Lo
g ical Vo lume15.2.2. Remo ving an LVM2 Lo g ical Vo lume fo r
Swap15.2.3. Remo ving a Swap File
15.3. Mo ving Swap Sp ace
Chapt er 1 6 . Disk Quot as16 .1. Co nfig uring Disk Quo tas
16 .1.1. Enab ling Quo tas16 .1.2. Remo unting the File
Systems16 .1.3. Creating the Quo ta Datab ase Files16 .1.4. Assig
ning Quo tas p er User16 .1.5. Assig ning Quo tas p er Gro up16
.1.6 . Setting the Grace Perio d fo r So ft Limits
16 .2. Manag ing Disk Quo tas16 .2.1. Enab ling and Disab ling16
.2.2. Rep o rting o n Disk Quo tas16 .2.3. Keep ing Quo tas
Accurate
16 .3. Disk Quo ta References
Chapt er 1 7 . Redundant Array of Independent Disks (RAID)17.1.
RAID Typ es
Firmware RAID
Hardware RAID
Soft ware RAID17.2. RAID Levels and Linear Sup p o rt
9 29 29 39 39 49 4
9 69 79 7
10 010 110 410 610 710 8110113
1 1 3
1 1 3
1 1 4114115115116116116117117118
1 1 9119119120120121122123123123123124125
1 2 6126
1 2 6
1 2 6
1 2 7127
Red Hat Ent erprise Linux 6 St orage Administ rat ion Guide
4
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. . . . . . . . . .
17.2. RAID Levels and Linear Sup p o rt17.3. Linux RAID Sub
systems
Linux Hardware RAID cont roller drivers
mdraid
dmraid17.4. RAID Sup p o rt in the Installer17.5. Co nfig uring
RAID Sets
mdadm
dmraid17.6 . Ad vanced RAID Device Creatio n
Chapt er 1 8 . Using t he mount Command18 .1. Lis ting Currently
Mo unted File Systems
18 .1.1. Sp ecifying the File System Typ e18 .2. Mo unting a
File System
18 .2.1. Sp ecifying the File System Typ e18 .2.2. Sp ecifying
the Mo unt Op tio ns18 .2.3. Sharing Mo unts18 .2.4. Mo ving a Mo
unt Po int
18 .3. Unmo unting a File System18 .4. mo unt Co mmand
References
18 .4.1. Manual Pag e Do cumentatio n18 .4.2. Useful Web
sites
Chapt er 1 9 . T he volume_key funct ion19 .1. Co mmand s19 .2.
Using vo lume_key as an ind ivid ual user19 .3. Using vo lume_key
in a larg er o rg anizatio n
19 .3.1. Prep aratio n fo r saving encryp tio n keys19 .3.2.
Saving encryp tio n keys19 .3.3. Resto ring access to a vo lume19
.3.4. Setting up emerg ency p assp hrases
19 .4. vo lume_key References
Chapt er 2 0 . Access Cont rol List s20 .1. Mo unting File
Systems
20 .1.1. NFS20 .2. Setting Access ACLs20 .3. Setting Default
ACLs20 .4. Retrieving ACLs20 .5. Archiving File Systems With ACLs20
.6 . Co mp atib il i ty with O ld er Systems20 .7. ACL
References
Chapt er 2 1 . Solid- St at e Disk Deployment Guidelines21.1.
Dep lo yment Co nsid eratio ns21.2. Tuning Co nsid eratio ns
I/O Scheduler
Virt ual Memory
Swap
127129
1 2 9
1 2 9
1 30130130
1 30
1 30131
1 33133133134135136137140141141142142
1 4 3143144145145146146147147
1 4 8148148148149150150151151
1 53153154
1 54
1 54
1 54
T able of Cont ent s
5
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. . . . . . . . . .
Swap
Chapt er 2 2 . Writ e Barriers22.1. Imp o rtance o f Write
Barriers
How Writ e Barriers Work22.2. Enab ling /Disab ling Write
Barriers22.3. Write Barrier Co nsid eratio ns
Disabling Writ e Caches
Bat t ery- Backed Writ e Caches
High- End Arrays
NFS
Chapt er 2 3. St orage I/O Alignment and Size23.1. Parameters fo
r Sto rag e Access23.2. Usersp ace Access
sysfs Int erface
Block Device ioct ls23.3. Stand ard s
AT A
SCSI23.4. Stacking I/O Parameters23.5. Lo g ical Vo lume Manag
er23.6 . Partitio n and File System To o ls
ut il- linux- ng's libblkid and fdisk
part ed and libpart ed
File Syst em t ools
Chapt er 2 4 . Set t ing Up A Remot e Diskless Syst em24.1. Co
nfig uring a tftp Service fo r Diskless Clients24.2. Co nfig uring
DHCP fo r Diskless Clients24.3. Co nfig uring an Exp o rted File
System fo r Diskless Clients
Chapt er 2 5. Online St orage Management25.1. Fib re Channel
25.1.1. Fib re Channel API25.1.2. Native Fib re Channel Drivers
and Cap ab il ities
25.2. iSCSI25.2.1. iSCSI API25.2.2. iSCSI Targ et Set up
25.3. Pers istent Naming25.3.1. WWID25.3.2. UUID and Other Pers
istent Id entifiers
25.4. Remo ving a Sto rag e Device25.5. Remo ving a Path to a
Sto rag e Device25.6 . Ad d ing a Sto rag e Device o r Path25.7. Co
nfig uring a Fib re-Channel Over Ethernet Interface
25.7.1. Fib re-Channel o ver Ethernet (FCo E) Targ et Set up
1 54
1 55155
1 55155156
1 56
1 56
1 57
1 57
1 58158159
1 59
1 5916 0
1 6 0
1 6 016 016 116 1
1 6 1
1 6 2
1 6 2
1 6 316 316 316 4
1 6 616 616 616 716 816 816 9170171172172173174176177
Red Hat Ent erprise Linux 6 St orage Administ rat ion Guide
6
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25.8 . Co nfig uring an FCo E Interface to Auto matically Mo unt
at Bo o t25.9 . Scanning Sto rag e Interco nnects25.10 . iSCSI
Disco very Co nfig uratio n25.11. Co nfig uring iSCSI O fflo ad and
Interface Bind ing
25.11.1. Viewing Availab le iface Co nfig uratio ns25.11.2. Co
nfig uring an iface fo r So ftware iSCSI25.11.3. Co nfig uring an
iface fo r iSCSI O fflo ad25.11.4. Bind ing /Unb ind ing an iface
to a Po rtal
25.12. Scanning iSCSI Interco nnects25.13. Lo g g ing in to an
iSCSI Targ et25.14. Resiz ing an Online Lo g ical Unit
25.14.1. Resiz ing Fib re Channel Lo g ical Units25.14.2. Resiz
ing an iSCSI Lo g ical Unit25.14.3. Up d ating the Size o f Yo ur
Multip ath Device25.14.4. Chang ing the Read /Write State o f an
Online Lo g ical Unit
25.14.4.1. Rescanning lo g ical units25.14.4.2. Up d ating the
R/W state o f a multip ath d evice25.14.4.3. Do cumentatio n
25.15. Ad d ing /Remo ving a Lo g ical Unit Thro ug h
rescan-scsi-b us.sh
Known Issues Wit h rescan- scsi- bus.sh25.16 . Mo d ifying Link
Lo ss Behavio r
25.16 .1. Fib re Channel25.16 .2. iSCSI Setting s With d
m-multip ath
25.16 .2.1. NOP-Out Interval/Timeo ut
SCSI Error Handler25.16 .2.2. rep lacement_timeo ut25.16 .3.
iSCSI Ro o t
Configuring T imeout s for a Specific Session25.17. Co ntro ll
ing the SCSI Co mmand Timer and Device Status
Device St at es
Command T imer25.18 . Online Sto rag e Co nfig uratio n Tro ub
lesho o ting
Chapt er 2 6 . Device Mapper Mult ipat hing and Virt ual St
orage26 .1. Virtual Sto rag e26 .2. DM-Multip ath
Revision Hist ory
Index
17918 118 118 218 318 418 518 518 618 918 919 019 019 119 219
319 319 419 4
1 9 419 419 419 519 5
1 9 619 619 7
1 9 719 7
1 9 8
1 9 819 8
2 0 020 020 0
2 0 2
2 0 4
T able of Cont ent s
7
-
Preface
1. Document Convent ionsThis manual uses several conventions to
highlight certain words and phrases and draw attention tospecific
pieces of information.
1.1. T ypographic Convent ionsFour typographic conventions are
used to call attention to specific words and phrases.
Theseconventions, and the circumstances they apply to, are as
follows.
Mono-spaced Bold
Used to highlight system input, including shell commands, file
names and paths. Also used tohighlight keys and key combinations.
For example:
To see the contents of the file my_next_bestselling_novel in
your currentworking directory, enter the cat
my_next_bestselling_novel command at theshell prompt and press
Enter to execute the command.
The above includes a file name, a shell command and a key, all
presented in mono-spaced bold andall distinguishable thanks to
context.
Key combinations can be distinguished from an individual key by
the plus sign that connects eachpart of a key combination. For
example:
Press Enter to execute the command.
Press Ctrl+Alt+F2 to switch to a virtual terminal.
The first example highlights a particular key to press. The
second example highlights a keycombination: a set of three keys
pressed simultaneously.
If source code is discussed, class names, methods, functions,
variable names and returned valuesmentioned within a paragraph will
be presented as above, in mono-spaced bold . For example:
File-related classes include filesystem for file systems, file
for files, and dir fordirectories. Each class has its own
associated set of permissions.
Proport ional Bold
This denotes words or phrases encountered on a system, including
application names; dialog-boxtext; labeled buttons; check-box and
radio-button labels; menu titles and submenu titles.
Forexample:
Choose System Preferences Mouse from the main menu bar to
launchMouse Preferences . In the Buttons tab, select the
Left-handed mouse checkbox and click Close to switch the primary
mouse button from the left to the right(making the mouse suitable
for use in the left hand).
To insert a special character into a gedit file, choose Applicat
ions Accessories Character Map from the main menu bar. Next, choose
Search Find from the Character Map menu bar, type the name of the
character in the Search field and click Next. The character you
sought will be highlighted in the
Red Hat Ent erprise Linux 6 St orage Administ rat ion Guide
8
-
Character Table. Double-click this highlighted character to
place it in the Text to copy field and then click the Copy button.
Now switch back to your documentand choose Edit Paste from the
gedit menu bar.
The above text includes application names; system-wide menu
names and items; application-specificmenu names; and buttons and
text found within a GUI interface, all presented in proportional
boldand all distinguishable by context.
Mono-spaced Bold Italic or Proportional Bold Italic
Whether mono-spaced bold or proportional bold, the addition of
italics indicates replaceable orvariable text. Italics denotes text
you do not input literally or displayed text that changes
dependingon circumstance. For example:
To connect to a remote machine using ssh, type ssh
[email protected] at ashell prompt. If the remote machine is
example.com and your username on thatmachine is john, type ssh
[email protected].
The mount -o remount file-system command remounts the named file
system.For example, to remount the /home file system, the command
is mount -o remount /home.
To see the version of a currently installed package, use the rpm
-q packagecommand. It will return a result as follows:
package-version-release.
Note the words in bold italics above: username, domain.name,
file-system, package, version andrelease. Each word is a
placeholder, either for text you enter when issuing a command or
for textdisplayed by the system.
Aside from standard usage for presenting the title of a work,
italics denotes the first use of a new andimportant term. For
example:
Publican is a DocBook publishing system.
1.2. Pull-quote Convent ionsTerminal output and source code
listings are set off visually from the surrounding text.
Output sent to a terminal is set in mono-spaced roman and
presented thus:
books Desktop documentation drafts mss photos stuff
svnbooks_tests Desktop1 downloads images notes scripts svgs
Source-code listings are also set in mono-spaced roman but add
syntax highlighting as follows:
static int kvm_vm_ioctl_deassign_device(struct kvm *kvm, struct
kvm_assigned_pci_dev *assigned_dev){ int r = 0; struct
kvm_assigned_dev_kernel *match;
mutex_lock(&kvm->lock);
match =
kvm_find_assigned_dev(&kvm->arch.assigned_dev_head,
assigned_dev->assigned_dev_id); if (!match) { printk(KERN_INFO
"%s: device hasn't been assigned
Preface
9
-
before, " "so cannot be deassigned\n", __func__); r = -EINVAL;
goto out; }
kvm_deassign_device(kvm, match);
kvm_free_assigned_device(kvm, match);
out: mutex_unlock(&kvm->lock); return r;}
1.3. Notes and WarningsFinally, we use three visual styles to
draw attention to information that might otherwise be
overlooked.
NoteNotes are tips, shortcuts or alternative approaches to the
task at hand. Ignoring a note shouldhave no negative consequences,
but you might miss out on a trick that makes your life easier.
ImportantImportant boxes detail things that are easily missed:
configuration changes that only apply tothe current session, or
services that need restarting before an update will apply. Ignoring
abox labeled Important will not cause data loss but may cause
irritation and frustration.
WarningWarnings should not be ignored. Ignoring warnings will
most likely cause data loss.
2. Get t ing Help and Giving Feedback
2.1. Do You Need Help?If you experience difficulty with a
procedure described in this documentation, visit the Red
HatCustomer Portal at http://access.redhat.com. From the Customer
Portal, you can:
Search or browse through a knowledge base of technical support
articles about Red Hatproducts.
Submit a support case to Red Hat Global Support Services
(GSS).
Access other product documentation.
Red Hat Ent erprise Linux 6 St orage Administ rat ion Guide
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Red Hat also hosts a large number of electronic mailing lists
for discussion of Red Hat software andtechnology. You can find a
list of publicly available mailing lists
athttps://www.redhat.com/mailman/listinfo. Click the name of any
mailing list to subscribe to that list orto access the list
archives.
2.2. We Need FeedbackIf you find a typographical error in this
manual, or if you have thought of a way to make this manualbetter,
we would love to hear from you. Please submit a report in Bugzilla:
http://bugzilla.redhat.com/against the product Red Hat Enterprise
Linux 6.
When submitting a bug report, be sure to mention the manual's
identifier: doc-Storage_Admin_Guide
If you have a suggestion for improving the documentation, try to
be as specific as possible whendescribing it. If you have found an
error, please include the section number and some of thesurrounding
text so we can find it easily.
Preface
11
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Chapter 1. OverviewThe Storage Administration Guide contains
extensive information on supported file systems and datastorage
features in Red Hat Enterprise Linux 6. This book is intended as a
quick reference foradministrators managing single-node (that is,
non-clustered) storage solutions.
The Storage Administration Guide is split into two parts: File
Systems, and Storage Administration.
The File Systems part details the various file systems Red Hat
Enterprise Linux 6 supports. Itdescribes them and explains how best
to utilize them.
The Storage Administration part details the various tools and
storage administration tasks Red HatEnterprise Linux 6 supports. It
describes them and explains how best to utilize them.
1.1. What 's New in Red Hat Enterprise Linux 6Red Hat Enterprise
Linux 6 features the following file system enhancements:
File System Encryption (Technology Preview)It is now possible to
encrypt a file system at mount using eCryptfs , providing an
encryption layeron top of an actual file system. This "pseudo-file
system" allows per-file and file name encryption,which offers more
granular encryption than encrypted block devices. For more
information about filesystem encryption, refer to Chapter 3,
Encrypted File System.
File System Caching (Technology Preview)FS-Cache[1] allows the
use of local storage for caching data from file systems served over
the network(for example, through NFS). This helps minimize network
traffic, although it does not guarantee fasteraccess to data over
the network. FS-Cache allows a file system on a server to interact
directly with aclient's local cache without creating an overmounted
file system. For more information about FS-Cache, refer to Chapter
10, FS-Cache.
Btrfs (Technology Preview)Btrfs[1] is a local file system that
is now available. It aims to provide better performance
andscalability, including integrated LVM operations. For more
information on Btrfs, refer to Chapter 4,Btrfs.
I/O Limit ProcessingThe Linux I/O stack can now process I/O
limit information for devices that provide it. This allowsstorage
management tools to better optimize I/O for some devices. For more
information on this, referto Chapter 23, Storage I/O Alignment and
Size.
ext4 Support
[1]
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The ext4 file system is fully supported in this release. It is
now the default file system of Red HatEnterprise Linux 6,
supporting an unlimited number of subdirectories. It also features
more granulartimestamping, extended attributes support, and quota
journaling. For more information on ext4, referto Chapter 6, The
Ext4 File System.
Network Block StorageFibre-channel over Ethernet is now
supported. This allows a fibre-channel interface to use
10-GigabitEthernet networks while preserving the fibre-channel
protocol. For instructions on how to set this up,refer to Section
25.7, Configuring a Fibre-Channel Over Ethernet Interface .
[1] This feature is b eing p ro vid ed in this release as a
technology preview. Techno lo g y Preview featuresare currently no
t sup p o rted und er Red Hat Enterp rise Linux sub scrip tio n
services, may no t b efunctio nally co mp lete, and are g enerally
no t suitab le fo r p ro d uctio n use. Ho wever, these features
areinc lud ed as a custo mer co nvenience and to p ro vid e the
feature with wid er exp o sure.
Yo u are free to p ro vid e feed b ack and functio nality sug g
estio ns fo r a techno lo g y p review feature b efo reit b eco mes
fully sup p o rted . Erratas wil l b e p ro vid ed fo r hig
h-severity security issues.
Net work Block St orage
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Part I. File SystemsThe File Systems section explains file
system structure followed by two technology previews: eCryptfsand
Btrfs. This is followed by the file systems Red Hat fully supports:
ext3, ext4, global file system 2,XFS, NFS, and FS-Cache.
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Chapter 2. File System Structure and MaintenanceThe file system
structure is the most basic level of organization in an operating
system. The way anoperating system interacts with its users,
applications, and security model nearly always depends onhow the
operating system organizes files on storage devices. Providing a
common file systemstructure ensures users and programs can access
and write files.
File systems break files down into two logical categories:
Shareable versus unshareable files
Variable versus static files
Shareable files can be accessed locally and by remote hosts;
unshareable files are only availablelocally. Variable files, such
as log files, can be changed at any time; static files, such as
binaries, donot change without an action from the system
administrator.
Categorizing files in this manner helps correlate the function
of each file with the permissionsassigned to the directories which
hold them. How the operating system and its users interact with
afile determines the directory in which it is placed, whether that
directory is mounted with read-only orread/write permissions, and
the level of access each user has to that file. The top level of
thisorganization is crucial; access to the underlying directories
can be restricted, otherwise securityproblems could arise if, from
the top level down, access rules do not adhere to a rigid
structure.
2.1. Overview of Filesystem Hierarchy Standard (FHS)Red Hat
Enterprise Linux uses the Filesystem Hierarchy Standard (FHS) file
system structure, whichdefines the names, locations, and
permissions for many file types and directories.
The FHS document is the authoritative reference to any
FHS-compliant file system, but the standardleaves many areas
undefined or extensible. This section is an overview of the
standard and adescription of the parts of the file system not
covered by the standard.
The two most important elements of FHS compliance are:
Compatibility with other FHS-compliant systems
The ability to mount a /usr/ partition as read-only. This is
especially crucial, since /usr/contains common executables and
should not be changed by users. In addition, since /usr/ ismounted
as read-only, it should be mountable from the CD-ROM drive or from
another machinevia a read-only NFS mount.
2.1.1. FHS Organizat ionThe directories and files noted here are
a small subset of those specified by the FHS document. Referto the
latest FHS documentation for the most complete information at
http://www.pathname.com/fhs/.
2.1 .1 .1 . Gat hering File Syst em Info rmat io nThe df command
reports the system's disk space usage. Its output looks similar to
the following:
Example 2.1. df command output
Filesystem 1K-blocks Used Available Use% Mounted
on/dev/mapper/VolGroup00-LogVol00
Chapt er 2 . File Syst em St ruct ure and Maint enance
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11675568 6272120 4810348 57% / /dev/sda1 100691 9281 86211 10%
/bootnone 322856 0 322856 0% /dev/shm
By default, df shows the partition size in 1 kilobyte blocks and
the amount of used and availabledisk space in kilobytes. To view
the information in megabytes and gigabytes, use the command df -h.
The -h argument stands for "human-readable" format. The output for
df -h looks similar to thefollowing:
Example 2.2. df -h command output
Filesystem Size Used Avail Use% Mounted
on/dev/mapper/VolGroup00-LogVol00 12G 6.0G 4.6G 57% / /dev/sda1 99M
9.1M 85M 10% /boot none 316M 0 316M 0% /dev/shm
NoteIn the above examples, the mounted partition /dev/shm
represents the system's virtualmemory file system.
The du command displays the estimated amount of space being used
by files in a directory,displaying the disk usage of each
subdirectory. The last line in the output of du shows the total
diskusage of the directory; to see only the total disk usage of a
directory in human-readable format, use du -hs. For more options,
refer to man du.
To view the system's partitions and disk space usage in a
graphical format, use the Gnome System Monitor by clicking on
Applicat ions System Tools System Monitor or using thecommand
gnome-system-monitor. Select the File Systems tab to view the
system's partitions.The figure below illustrates the File Systems
tab.
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Figure 2.1. GNOME System Monitor File Systems tab
2.1 .1 .2 . T he /boot/ Direct o ryThe /boot/ directory contains
static files required to boot the system, for example, the Linux
kernel.These files are essential for the system to boot
properly.
WarningDo not remove the /boot/ directory. Doing so renders the
system unbootable.
2.1 .1 .3. T he /dev/ Direct o ryThe /dev/ directory contains
device nodes that represent the following device types:
devices attached to the system;
virtual devices provided by the kernel.
These device nodes are essential for the system to function
properly. The udevd daemon createsand removes device nodes in /dev/
as needed.
Devices in the /dev/ directory and subdirectories are defined as
either character (providing only aserial stream of input and
output, for example, mouse or keyboard) or block (accessible
randomly, forexample, a hard drive or a floppy drive). If GNOME or
KDE is installed, some storage devices areautomatically detected
when connected (such as with a USB) or inserted (such as a CD or
DVD
Chapt er 2 . File Syst em St ruct ure and Maint enance
17
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drive), and a pop-up window displaying the contents appears.
Table 2.1. Examples of common f iles in the /dev d irectory
File Descript ion/dev/hda The master device on the primary IDE
channel./dev/hdb The slave device on the primary IDE
channel./dev/tty0 The first virtual console./dev/tty1 The second
virtual console./dev/sda The first device on the primary SCSI or
SATA
channel./dev/lp0 The first parallel port./dev/ttyS0 Serial
port.
2.1 .1 .4 . T he /etc/ Direct o ryThe /etc/ directory is
reserved for configuration files that are local to the machine. It
should containno binaries; any binaries should be moved to /bin/ or
/sbin/.
For example, the /etc/skel/ directory stores "skeleton" user
files, which are used to populate ahome directory when a user is
first created. Applications also store their configuration files in
thisdirectory and may reference them when executed. The
/etc/exports file controls which file systemsexport to remote
hosts.
2.1 .1 .5 . T he /lib/ Direct o ryThe /lib/ directory should
only contain libraries needed to execute the binaries in /bin/ and
/sbin/. These shared library images are used to boot the system or
execute commands within theroot file system.
2.1 .1 .6 . T he /media/ Direct o ryThe /media/ directory
contains subdirectories used as mount points for removable media,
such asUSB storage media, DVDs, and CD-ROMs.
2.1 .1 .7 . T he /mnt/ Direct o ryThe /mnt/ directory is
reserved for temporarily mounted file systems, such as NFS file
systemmounts. For all removable storage media, use the /media/
directory. Automatically detectedremovable media will be mounted in
the /media directory.
ImportantThe /mnt directory must not be used by installation
programs.
2.1 .1 .8 . T he /opt/ Direct o ryThe /opt/ directory is
normally reserved for software and add-on packages that are not
part of thedefault installation. A package that installs to /opt/
creates a directory bearing its name, forexample /opt/packagename/.
In most cases, such packages follow a predictable
subdirectorystructure; most store their binaries in
/opt/packagename/bin/ and their man pages in
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/opt/packagename/man/.
2.1 .1 .9 . T he /proc/ Direct o ryThe /proc/ directory contains
special files that either extract information from the kernel or
sendinformation to it. Examples of such information include system
memory, CPU information, andhardware configuration. For more
information about /proc/, refer to Section 2.3, The /proc
VirtualFile System .
2.1 .1 .10. T he /sbin/ Direct o ryThe /sbin/ directory stores
binaries essential for booting, restoring, recovering, or repairing
thesystem. The binaries in /sbin/ require root privileges to use.
In addition, /sbin/ contains binariesused by the system before the
/usr/ directory is mounted; any system utilities used after /usr/
ismounted are typically placed in /usr/sbin/.
At a minimum, the following programs should be stored in
/sbin/:
arp
clock
halt
init
fsck.*
grub
ifconfig
mingetty
mkfs.*
mkswap
reboot
route
shutdown
swapoff
swapon
2.1 .1 .11. T he /srv/ Direct o ryThe /srv/ directory contains
site-specific data served by a Red Hat Enterprise Linux system.
Thisdirectory gives users the location of data files for a
particular service, such as FTP, WWW, or CVS.Data that only
pertains to a specific user should go in the /home/ directory.
Chapt er 2 . File Syst em St ruct ure and Maint enance
19
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NoteThe default httpd install uses /var/www/html for served
content.
2.1 .1 .12. T he /sys/ Direct o ryThe /sys/ directory utilizes
the new sysfs virtual file system specific to the 2.6 kernel. With
theincreased support for hot plug hardware devices in the 2.6
kernel, the /sys/ directory containsinformation similar to that
held by /proc/, but displays a hierarchical view of device
informationspecific to hot plug devices.
2.1 .1 .13. T he /usr/ Direct o ryThe /usr/ directory is for
files that can be shared across multiple machines. The /usr/
directory isoften on its own partition and is mounted read-only.
The /usr/ directory usually contains thefollowing
subdirectories:
/usr/bin
This directory is used for binaries.
/usr/etc
This directory is used for system-wide configuration files.
/usr/games
This directory stores games.
/usr/include
This directory is used for C header files.
/usr/kerberos
This directory is used for Kerberos-related binaries and
files.
/usr/lib
This directory is used for object files and libraries that are
not designed to be directlyutilized by shell scripts or users. This
directory is for 32-bit systems.
/usr/lib64
This directory is used for object files and libraries that are
not designed to be directlyutilized by shell scripts or users. This
directory is for 64-bit systems.
/usr/libexec
This directory contains small helper programs called by other
programs.
/usr/sbin
This directory stores system administration binaries that do not
belong to /sbin/.
/usr/share
This directory stores files that are not
architecture-specific.
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/usr/src
This directory stores source code.
/usr/tmp l inked to /var/tmp
This directory stores temporary files.
The /usr/ directory should also contain a /local/ subdirectory.
As per the FHS, this subdirectoryis used by the system
administrator when installing software locally, and should be safe
from beingoverwritten during system updates. The /usr/local
directory has a structure similar to /usr/, andcontains the
following subdirectories:
/usr/local/bin
/usr/local/etc
/usr/local/games
/usr/local/include
/usr/local/lib
/usr/local/libexec
/usr/local/sbin
/usr/local/share
/usr/local/src
Red Hat Enterprise Linux's usage of /usr/local/ differs slightly
from the FHS. The FHS states that /usr/local/ should be used to
store software that should remain safe from system
softwareupgrades. Since the RPM Package Manager can perform
software upgrades safely, it is notnecessary to protect files by
storing them in /usr/local/.
Instead, Red Hat Enterprise Linux uses /usr/local/ for software
local to the machine. Forinstance, if the /usr/ directory is
mounted as a read-only NFS share from a remote host, it is
stillpossible to install a package or program under the /usr/local/
directory.
2.1 .1 .14. T he /var/ Direct o rySince the FHS requires Linux
to mount /usr/