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Title: STORAGE ARCHITECTURE/ GETTING STARTED: SAN SCHOOL 101


1
STORAGE ARCHITECTURE/GETTING STARTEDSAN SCHOOL
101
  • Marc Farley
  • President of Building Storage, Inc
  • Author, Building Storage Networks, Inc.

2
Agenda
  • Lesson 1 Basics of SANs
  • Lesson 2 The I/O path
  • Lesson 3 Storage subsystems
  • Lesson 4 RAID, volume management and
    virtualization
  • Lesson 5 SAN network technology
  • Lesson 6 File systems

3
Basics of storage networking
Lesson 1
4
Connecting
5
Connecting
  • Networking or bus technology
  • Cables connectors
  • System adapters network device drivers
  • Network devices such as hubs, switches, routers
  • Virtual networking
  • Flow control
  • Network security

6
Storing
7
Storing
  • Device (target) command and control
  • Drives, subsystems, device emulation
  • Block storage address space manipulation
    (partition management)
  • Mirroring
  • RAID
  • Striping
  • Virtualization
  • Concatentation

8
Filing
9
Filing
  • Namespace presents data to end users and
    applications as files and directories (folders)
  • Manages use of storage address spaces
  • Metadata for identifying data
  • file name
  • owner
  • dates

10
Connecting, storing and filing as a complete
storage system
Connecting
11
NAS and SAN analysis
NAS is filing over a network SAN is storing over
a network NAS and SAN are independent
technologies They can be implemented
independently They can co-exist in the same
environment They can both operate and provide
services to the same users/applications
12
Protocol analysis for NAS and SAN
Filing
NAS SAN Network
Storing
Connecting
13
Integrated SAN/NAS environment
NAS ServerSAN Initiator NAS Head
Storing
Filing
Connecting
Connecting
14
Common wiring with NAS and SAN
NAS Head
Filing
Storing
Connecting
15
The I/O path
Lesson 2
16
  • Host hardware path components

MemoryBus
System I/O Bus
Storage Adapter (HBA)
Processor
Memory
17
Host software path components
Application
Multi-Pathing
VolumeManager
Device Driver
Filing System
CacheManager
Operating System
18
Network hardware path components
  • Switches, hubs, routers, bridges, gatways
  • Port buffers, processors
  • Backplane, bus, crossbar, mesh, memory
  • Cabling
  • Fiber optic
  • Copper

19
Network software path components
Access and Security
Virtual Networking
FlowControl
Fabric Services
Routing
20
Subsystem path components
Access and Security
NetworkPorts
Cache
Resource Manager
Internal Busor Network
21
Device and media path components
Disk drives
Tape drives
Solid state devices
Tape Media
22
The end to end I/O path picture
Storage Adapter (HBA)
Device Driver
System I/O Bus
CacheManager
MemoryBus
Memory
VolumeManager
Filing System
Multi-Pathing
Operating System
Processor
App
Virtual Networking
FlowControl
Access and Security
Routing
Fabric Services
Network Systems
Cabling
SubsystemNetwork Poirt
Access and Security
Disk drives
Tape drives
Resource Manager
Internal Busor Network
Cache
23
Storage subsystems
Lesson 3
24
Generic storage subsystem model
Controller (logicprocessors) Access
control Resource manager
Storage Resources
NetworkPorts
Internal Bus or Network
Cache Memory
Power
25
Redundancy for high availability
  • Multiple hot swappable power supplies
  • Hot swappable cooling fans
  • Data redundancy via RAID
  • Multi-path support
  • Network ports to storage resources

26
Physical and virtual storage
Subsystem Controller Resource Manager (RAID,
mirroring, etc.)
HotSpare Device
27
SCSI communications architectures determine SAN
operations
  • SCSI communications are independent of
    connectivity
  • SCSI initiators (HBAs) generate I/O activity
  • They communicate with targets
  • Targets have communications addresses
  • Targets can have many storage resources
  • Each resource is a single SCSI logical unit (LU)
    with a universal unique ID (UUID) - sometimes
    referred to as a serial number
  • An LU can be represented by multiple logical unit
    numbers (LUNs)
  • Provisioning associates LUNs with LUs subsystem
    ports
  • A storage resource is not a LUN, its an LU

28
Provisioning storage
LUN 0
LUN 1
Port S1
LUN 1
LUN 2
Port S2
LUN 2
Port S3
LUN 3
LUN 3
Port S4
LUN 0
Controller functions
29
Multipathing
LUN X
MP SW
LUN X
30
Caching
Write Caches 1. Write Through (to disk) 2.
Write Back (from cache)
Read Caches 1. Recently Used 2. Read Ahead
31
Tape subsystems
TapeDrive
TapeDrive
Tape Subsystem Controller
TapeDrive
TapeDrive
Robot
Tape Slots
32
Subsystem management
Now with SMIS
Management station browser-based network mgmt
software
Ethernet/TCP/IP
Out-of-band management port
In-band management
ExportedStorage Resource
Storage Subsystem
33
Data redundancy
  • Duplication
  • Parity
  • Difference

2n
n1
-1
d(x) f(x) f(x-1)
f(x-1)
34
Duplication redundancy with mirroring
Host-based
Within a subsystem
I/O PathA
MirroringOperator
I/O Path
I/O PathB
Terminate I/O regenerate new I/Os Error
recovery/notification
35
Duplication redundancy with remote copy
Host
Uni-directional (writes only)
A
B
A
A
A
36
Subsystem Snapshot
Point-in-time snapshot
Host
A
B
A
A
A
C
37
RAID, volume managementand virtualization
Lesson 4
38
RAID parity redundancy
  • Duplication
  • Parity
  • Difference

2n
n1
-1
d(x) f(x) f(x-1)
f(x-1)
39
History of RAID
  • Late 1980s RD project at UC Berkeley
  • David Patterson
  • Garth Gibson (independent)
  • Redundant array of inexpensive disks
  • Striping without redundancy was not defined (RAID
    0)
  • Original goals were to reduce the cost and
    increase the capacity of large disk storage

40
Benefits of RAID
  • Capacity scaling
  • Combine multiple address spaces as a single
    virtual address
  • Performance through parallelism
  • Spread I/Os over multiple disk spindles
  • Reliability/availability with redundancy
  • Disk mirroring (striping to 2 disks)
  • Parity RAID (striping to more than 2 disks)

41
Capacity scaling
Storage extent 1
Storage extent 2
Storage extent 3
Storage extent 4
Combined extents 1 - 12
Storage extent 5
Storage extent 6
Storage extent 7
Storage extent 8
Storage extent 9
Storage extent10
Storage extent11
Storage extent12
RAID Controller(resourcemanager)
42
Performance
RAID controller (microsecond performance)
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Disk drives (Millisecond performance)from
rotational latency and seek time
43
Parity redundancy
  • RAID arrays use XOR for calculating parity
    Operand 1 Operand 2 XOR Result False
    False False False True True True
    False True True True False
  • XOR is the inverse of itself
  • Apply XOR in the table above from right to left
  • Apply XOR to any two columns to get the third

44
Reduced mode operations
  • When a member is missing, data that is accessed
    must be reconstructed with xor
  • An array that is reconstructing data is said to
    be operating in reduced mode
  • System performance during reduced mode operations
    can be significantly reduced

XOR M1M2M3P
45
RAID Parity Rebuild
Parity rebuild
  • The process of recreating data on a replacement
    member is called a parity rebuild
  • Parity rebuilds are often scheduled for
    non-production hours because performance
    disruptions can be so severe

XOR M1M2M3P
46
Hybrid RAID 01
RAID 01, 10
RAID Controller
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
Diskdrive
1
2
3
4
5
Mirrored pairs of striped members
47
Volume management and virtualization
  • Storing level functions
  • Provide RAID-like functionality in host systems
    and SAN network systems
  • Aggregation of storage resources for
  • scalability
  • availability
  • cost / efficiency
  • manageability

48
OS kernel
File system
Volume management
Volume Manager
Volume Manager
  • RAID partition management
  • Device driver layer between the kernel and
    storage I/O drivers

HBA drivers
HBAs
49
Server system
Volume managers can use all available connections
and resources and can span multiple SANs as well
as SCSI and SAN resources
Virtual Storage
SCSI disk resource
SCSI HBA
SCSI Bus
SAN disk resources
HBA drivers
SAN cable
SAN Switch
SAN HBA
50
SAN storage virtualization
  • RAID and partition management in SAN systems
  • Two architectures
  • In-band virtualization (synchronous)
  • Out-of-band virtualization (asynchronous)

51
In-band virtualization
Exported virtual storage
I/O Path
System(s), switch or router
Disk subsystems
52
Out-of-band virtualization
  • Distributed volume management
  • Virtualization agents are managed from a central
    system in the SAN

Virtualizationagents
Disk subsystems
53
SAN networks
Lesson 5
54
Fibre channel
  • The first major SAN networking technology
  • Very low latency
  • High reliability
  • Fiber optic cables
  • Copper cables
  • Extended distance
  • 1, 2 or 4 Gb transmission speeds
  • Strongly typed

55
Fibre channel
  • A Fibre Channel fabric presents a consistent
    interface and set of services across all switches
    in a network
  • Host and subsystems all 'see' the same resources

StorageSubsystem
StorageSubsystem
StorageSubsystem
56
Fibre channel port definitions
  • FC ports are defined by their network role
  • N-ports end node ports connecting to fabrics
  • L-ports end node ports connecting to loops
  • NL-ports end node ports connecting to fabrics or
    loops
  • F-ports switch ports connecting to N ports
  • FL-ports switch ports connecting to N ports or
    NL ports in a loop
  • E-ports switch ports connecting to other switch
    ports
  • G ports generic switch ports that can be F, FL
    or E ports

57
Ethernet / TCP / IP SAN technologies
  • Leveraging the install base of Ethernet and
    TCP/IP networks
  • iSCSI native SAN over IP
  • FC/IP FC SAN extensions over IP

58
iSCSI
  • Native storage I/O over TCP/IP
  • New industry standard
  • Locally over Gigabit Ethernet
  • Remotely over ATM, SONET, 10Gb Ethernet

iSCSI
TCP
IP
MAC
PHY
59
iSCSI equipment
  • Storage NICs (HBAs)
  • SCSI drivers
  • Cables
  • Copper and fiber
  • Network systems
  • Switches/routers
  • Firewalls

60
  • FC/IP
  • Extending FC SANs over TCP/IP networks
  • FCIP gateways operate as virtual E-port
    connections
  • FCIP creates a single fabric where all resources
    appear to be local

TCP/IPLAN, MANor WAN
FCIPGateway
FCIPGateway
E-port
E-port
61
SAN switching fabrics
  • High-end SAN switches have latencies of 1 - 3
    µsec
  • Transaction processing requires lowest latency
  • Most other applications do not
  • Transaction processing requires non-blocking
    switches
  • No internal delays preventing data transfers

62
Switches and directors
  • Switches
  • 8 48 ports
  • Redundant power supplies
  • Single system supervisor
  • Directors
  • 64 ports
  • HA redundancy
  • Dual system supervisor
  • Live SW upgrades

63
SAN topologies
  • Star
  • Simplest
  • single hop
  • Dual star
  • Simple network redundancy
  • Single hop
  • Independent or integrated fabric(s)

64
SAN topologies
  • N-wide star
  • Scalable
  • Single hop
  • Independent or integrated fabric(s)
  • Core - edge
  • Scalable
  • 1 3 hops
  • integrated fabric

65
SAN topologies
  • Ring
  • Scalable
  • integrated fabric
  • 1 to N2 hops
  • Ring Star
  • Scalable
  • integrated fabric
  • 1 to 3 hops

66
Lesson 6
File systems
67
File system functions
68
Storing
Filing
69
Think of the storage address space as a sequence
of storage locations (a flat address space)
70
Superblocks
  • Superblocks are known addresses used to find file
    system roots (and mount the file system)

71
Filing and Scaling
  • File systems must have a known and dependable
    address space
  • The fine print in scalability - How does the
    filing function know about the new storing
    address space?

Storing
? ? ?
Storing
Filing
72
SCSI's role in storage networking
Lesson 2
  • Legacy open systems server storage
  • Physical parallel bus
  • Independent master/slave protocol
  • Storing in SANs
  • Compatibility requirements with system software
    force the use of the SCSI protocol
  • Storing and wiring in NAS
  • SCSI and ATA (IDE) used with NAS

73
Parallel SCSI bus technologies
A bus, with address lines and data lines
  • 8-bit and 16-bit (narrow and wide)
  • Single ended, differential, low voltage
    differential (LVD) electronics
  • 5MB, 10MB, 20MB, 40MB, 80MB, 160MB, 320MB
  • Ultra SCSI 3 is 320 MB/sec
  • Distances vary from 3 to 25 meters
  • Current LVD SCSI is 12 meters

74
SCSI command protocol
  • Master/slave relationships
  • host master, device slave
  • Independent of physical connectivity
  • CDBs Command Descriptor Blocks
  • Command format
  • Used for both device operations and data xfers
  • Serial SCSI standard created and implemented as
  • Fibre Channel Protocol (FCP)
  • iSCSI

75
SCSI addressing model
Host system
LUN
Targetstoragesubsystem
16 bus addresses with LUN sub-addressing
76
SCSI daisy chain connectivity
Target devices or subsystems
Host system
Storageinterface
Storageinterface
Storageinterface
Storageinterface
In / Out
In / Out
In / Out
In /
77
SCSI arbitration
Host system ID 7
Target IDs 6 5 4 3 2 1 0
The highest number address 'wins' arbitration to
access the bus next
78
SCSI resource discovery
SCSI inquiry CDBtell me your resources
Host system
LUN
Targetstoragesubsystem
There is no domain server concept in SCSI
79
SCSI performance capabilities
write
  • Overlapped I/O
  • Tagged command queuing (Reshuffled I/Os)

status
read
80
Parallel SCSI bus shortcomings
  • Bus length
  • servers and storage tied together
  • Single initiator
  • access to data depends on server
  • A standard full of variations
  • change is the only constant

81
Disk drives
Lesson 4
  • Disk drive components
  • Areal density
  • Rotational latency
  • Seek time
  • Buffer memory
  • Dual porting

82
Disk drives
  • Complex electro-mechanical devices
  • Media
  • Motor and speed control logic
  • Bearings and suspension
  • Actuator (arm)
  • Read/write heads
  • Read/write channels
  • I/O controller (ext interface int operations)
  • Buffer memory
  • Power

83
Disk drive areal density
  • Amount of signal per unit area of media
  • Keeps pace with Moore's law
  • Areal density doubles approximately every 18
    months
  • Increasingly smaller magnetic particles
  • Continued refinement of head technology
  • Electro-magnetic physics research

84
Rotational latency
  • Time for data on media to rotate underneath heads
  • faster rotational speed lower rotational
    latency
  • 2 to 10 milliseconds are common
  • Application level I/O operations can generate
    multiple disk accesses, each impacted by
    rotational latency

Memory SAN switch Disk drive nanoseconds micros
econds milliseconds 10 -9 10 -6
10 -3
85
Rotational latency filing systems
  • Filing systems determine contiguous data lengths
  • (file systems and databases)
  • Block size definitions
  • 512
  • 2k
  • 4k
  • 16k
  • 512k
  • 2M

Disk media
86
Seek Time
  • Time needed to position the actuator over the
    track
  • Equivalent to rotational latency in time

Disk head
Disk actuator
Disk media
87
Disk drive buffer memory
  • FIFO memory for data transfers
  • not cache
  • Overcome mechanical latencies with faster memory
    storage
  • Enables overlapped I/Os to multiple drives
  • Performance metrics
  • Burst transfer rate transfer in/out buffer
    memory
  • (Sustained transfer rate transfer with track
    changes)

88
Dual-ported disk drives
  • Redundant connectivity interfaces
  • Only FC to date

Controller A
Controller B
89
Forms of data redundancy
  • Duplication
  • Parity
  • Difference

2n
n1
-1
d(x) f(x) f(x-1)
f(x-1)
90
Business Continuity
  • 24 x 7 data access is the goal
  • 5 nines through planning and luck
  • There are many potential threats
  • People
  • Power
  • Natural disasters
  • Fires
  • Redundancy is the key
  • Multiple techniques cover different threats

91
Backup and recovery
Lesson 8
  • Removable media, usually tape
  • removable redundancy
  • Backup systems
  • Backup operations
  • Media rotation
  • Backup metadata
  • Backup challenges

92
Forms of data redundancy in backup
  • Duplication
  • Parity
  • Difference

2n
n1
-1
d(x) f(x) f(x-1)
f(x-1)
93
Backup and recovery tape media
  • Magnetic 'ribbon'
  • multiple layers of backing, adhesive, magnetic
    particles and lube/coating
  • corrodes and cracks
  • requires near-perfect conditions for long-term
    storage
  • Sequential access
  • slow load and seek times
  • reasonable transfer rates
  • can hold multiple versions of files

94
Tape drives
  • Two primary geometries
  • Longitudinal tracking
  • Helical tracking
  • Highly differentiated
  • Speeds (3MB/s to 30MB/s)
  • Capacities (20MB to 160MB)
  • Physical formats (layouts)
  • Compatibility is a constant issue
  • Mostly parallel SCSI

95
Tape drive formats
  • Two primary geometries
  • Longitudinal tracking
  • Helical tracking
  • Highly differentiated
  • Speeds (3MB/s to 30MB/s)
  • Capacities (20MB to 160MB)
  • Physical formats (layouts)
  • 4mm, 8mm, ¼ inch, DLT, LTO, 19mm
  • Cartridge construction, tape lengths
  • Compatibility is a constant issue
  • Mostly parallel SCSI

96
Longitudinal tracking
  • Parallel data tracks written lengthwise on tape
    by a 'stack' of heads

Data tracks
Tape heads
Technologies DLT, SDLT, LTO, QIC
97
Helical tracking
  • Single data tracks written diagonally across tape
    by a rotating cylindrical head assembly

Tape head
Data tracks
Tape
Technologies 4mm, 8mm, 19mm
98
Tape subsystems
  • Tape libraries autoloaders

Tape Subsystem Controller
Tapes
Robot
Tape drive
Tape drive
Tape drive
99
Generic backup system components
  • Tape subsystems
  • I/O bus/network subsystem
  • Work scheduler manager
  • Data mover
  • Metadata (database or catalog)
  • Media manager (rotation scheduler)
  • File system and database backup agents

100
Generic Network Backup System
File server
Web server
DB server
APP server
Backupagent
Backupagent
Backupagent
Backupagent
Ethernet network
Work scheduler Data mover Metadata system Media
manager
SCSI bus
Tape drive(s) or Tape subsystem
Backup server
101
Backup operations
  • Full (all data)
  • Longest backup operations
  • Usually done over/on weekends
  • Easiest recovery with 1 tape set
  • Incremental (changed data)
  • Shortest backup operation
  • Often done on days of the week
  • Most involved recovery
  • Differential (accumulated changed data)
  • Compromise for easier backups and recovery
  • Max 2 tape set restore

102
Backup operations and data redundancy
  • Full
  • Duplication redundancy
  • One backup for complete redundancy
  • Incremental
  • Difference redundancy
  • Multiple backups for complete redundancy
  • Differential
  • Difference redundancy
  • Two backups for complete redundancy

103
Media rotations
  • Change of tapes with common names and purposes
  • Tape sets - not individual tapes
  • Backup job schedules anticipate certain tapes
  • Monday, Tuesday, Wednesday, etc..
  • Even days, odd days
  • 1st Friday, 2nd Friday, etc..
  • January, February, March, etc...
  • 1st Qtr, 2nd Qtr, etc....

104
Media rotation problems
  • What happens when wrong tapes are used by
    mistake?
  • Say you use the last Friday's tape on the next
    Tuesday?
  • Data you might need to restore sometime can be
    overwritten!
  • Backup system logic may have to choose between
  • Not completing backup (restore will fail)
  • Deleting older backup files (restore might fail)

105
Backup metadata
  • A database for locating data on tape
  • Version create/modify date size
  • Date/time of backup job
  • Tape names backup job ID on tape
  • Owner
  • Delete records (don't restore deleted data!)
  • Transaction processing during backup
  • Many small files creates heavy processor loads
  • This is where backup fails to scale
  • Backup databases need to be pruned
  • Performance and capacity problems

106
Traditional backup challenges
  • Completing backups within the backup window
  • Backup window time allotted for daily backups
  • Starts after daily processing finishes
  • Ends before next day's processing begins
  • Media management and administration
  • Thousands of tapes to manage
  • Audit requirements are increasing
  • On/offsite movement for disaster protection
  • Balancing backup time against restore complexity

107
LAN-free backup in SANs
Tape drives or tape subsystem
SAN
SAN switch
Backupsoftware
Backupsoftware
Backupsoftware
Backupsoftware
File server
Web server
DB server
APP server
Ethernet client network
LAN
108
Advantages of LAN-free backup
  • Consolidated resources (especially media)
  • Centralized administration
  • Performance
  • Offloads LAN traffic
  • Platform optimization

SAN
109
Path management
  • Dual pathing
  • Zoning
  • LUN masking
  • Reserve / release
  • Routing
  • Virtual networking

110
Dual pathing
  • System software for redundant paths
  • Path management is a super-driver process
  • Redirects I/O traffic over a different path to
    the same storage resource
  • Typically invoked after SCSI timeout errors
  • Active / active or active / passive
  • Static load balancing only

111
Zoning 1
  • I/O segregation
  • Switch function that restricts forwarding
  • Zone membership is based on port or address

Address zoning
Port zoning
  • Zone 1
  • Addr 1
  • Addr 2
  • Zone 2
  • Addr 3
  • Addr 4
  • Zone 3
  • Addr 5
  • Addr 6

112
Zoning 2
  • Address zoning allows nodes to belong to more
    than one zone
  • For example, tape subsystems can belong to all
    zones
  • Zone 1
  • Addr 1 (server A)
  • Addr 2 (disk subsystem port target address A)
  • Addr 7 (tape subsystem port target address A)
  • Zone 2
  • Addr 3 (server B)
  • Addr 4 (disk subsystem port target address B)
  • Addr 7 (tape subsystem port target address A)
  • Zone 3
  • Addr 5 (server C)
  • Addr 6 (disk subsystem port target address C)
  • Addr 7 (tape subsystem port target address A)

Addr1 Addr 2 Addr 7
1
Addr 3 Addr 4 Addr 7
2
Addr 5 Addr 6 Addr 7
3
113
Zoning 3
  • Zones (or zone memberships) can be 'swapped' to
    reflect different operating environments

Changingzones
114
LUN masking
  • Restricts subsystem access to defined servers
  • Target or LUN level masking
  • Non-response to SCSI inquiry CDB
  • Can be used with zoning for multi-level control

No responseto SCSI inquiry
115
Reserve / Release
  • SCSI function
  • Typically implemented in SCSI/SAN storage
    routers
  • Used to reserve tape resources during backups
  • tape drives
  • robotics

1st access
2nd access blocked
Reserved
Storage router
116
Routing
  • Path decisions made by switches
  • Large TCP/IP networks require routing in switches
    instead of in end nodes
  • Looping is avoided by spanning tree algorithms
    that ensure a single path
  • OSPF is spanning tree technology for Fibre
    Channel
  • Routing is not HA failover technology

117
Name Space
  • The Name Space is the representation of data to
    end users and applications
  • Identification and searching
  • Organizational structure
  • Directories or folders in file systems
  • Rows and columns in databases
  • Associations of data
  • Database indexing
  • File system linking

118
Metadata and Access Control (Security)
  • Metadata is the description of data
  • Intrinsic information and accounting information
  • Access control determines how (or if) a user or
    application can use the data
  • for example, read-only
  • Access control is often incorporated with
    metadata but can be a separate function

119
  • Data has attributes that describe it
  • Storage is managed based on data attributes
  • Activity info
  • Owner info
  • Capacity info
  • Whatever info
  • Data can have security associated with it.
  • Data can be erased, copied, renamed, etc.

120
Locking
  • Managing multiple users or applications with
    concurrent access to data
  • Locking has been done in multi-user systems for
    decades
  • Locking in NAS has been a central issue
  • NFS advisory locks provide no guarantees
  • CIFS oplocks are enforced
  • Lock persistence

121
File systems organize data in blocks
  • Blocks are SCSIs address abstraction layer
  • Filing functions use block addresses to
    communicate with storing level entities
  • Filing systems manage the utilization of block
    address spaces (space management)
  • Block address structures typically are uniform
  • Block address boundaries are static for efficient
    and error-free space management

122
Journaling
  • File system structure has to be verified when
    mounting (FSCHECK)
  • FSCheck can take hours on large file systems
  • Journaling file systems keep a log of file system
    updates
  • Like a database log file, journal updates can be
    checked against actual structures
  • Incomplete updates can be rolled forward or
    backward to maintain system integrity

123
V/VM and Filing
  • Filing is a filing function
  • Virtualization volume management (V/VM) is a
    storing function
  • V/VM manipulates block addresses and creates real
    and virtual address spaces
  • Filing manages the placement of data in the
    address spaces exported by virtualization
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