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ecs150 Fall 2007 : Operating System #5: File Systems (chapters: 6.4~6.7, 8). Dr. S. Felix Wu Computer Science Department University of California, Davis http://www.cs.ucdavis.edu/~wu/ [email protected] File System Abstraction. Files Directories. System-call interface.

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ecs150 fall 2007 operating system 5 file systems chapters 6 4 6 7 8

ecs150 Fall 2007:Operating System#5: File Systems(chapters: 6.4~6.7, 8)

Dr. S. Felix Wu

Computer Science Department

University of California, Davis

http://www.cs.ucdavis.edu/~wu/

[email protected]

ecs150, Fall 2007

file system abstraction
File System Abstraction
  • Files
  • Directories

ecs150, Fall 2007

slide3
System-call interface

Active file entries

VNODE Layer or VFS

Local naming (UFS)

FFS

Buffer cache

Block or character device driver

Hardware

ecs150, Fall 2007

slide7
dirp = opendir(const char *filename);

struct dirent *direntp = readdir(dirp);

struct dirent {

ino_t d_ino;

char d_name[NAME_MAX+1];

};

directory

dirent

inode

file_name

dirent

inode

file_name

dirent

inode

file_name

file

file

file

ecs150, Fall 2007

local versus remote
Local versus Remote
  • System Call Interface
  • V-node
  • Local versus remote
    • NFS or i-node
    • Stackable File System
  • Hard-disk blocks

ecs150, Fall 2007

file system structure
File-System Structure
  • File structure
    • Logical storage unit
    • Collection of related information
  • File system resides on secondary storage (disks).
  • File system organized into layers.
  • File control block – storage structure consisting of information about a file.

ecs150, Fall 2007

file disk
File  Disk
  • separate the disk into blocks
  • separate the file into blocks as well
  • paging from file to disk

blocks: 4 - 7- 2- 10- 12

How to represent the file??

How to link these 5 pages together??

ecs150, Fall 2007

bit torrent pieces
Bit torrent pieces
  • 1 big file (X Gigabytes) with a number of pieces (5%) already in (and sharing with others).
  • How much disk space do we need at this moment?

ecs150, Fall 2007

hard disk
Hard Disk
  • Track, Sector, Head
    • Track + Heads  Cylinder
  • Performance
    • seek time
    • rotation time
    • transfer time
  • LBA
    • Linear Block Addressing

ecs150, Fall 2007

file disk blocks
File  Disk blocks

0

file

block

0

file

block

1

file

block

2

file

block

3

file

block

4

4

7

2

10

12

  • What are the disadvantages?
  • disk access can be slow for “random access”.
  • How big is each block? 64 bytes? 68 bytes?

ecs150, Fall 2007

kernel hacking session
Kernel Hacking Session
  • This Friday from 7:30 p.m. until midnight..
  • 3083 Kemper
    • Bring your laptop
    • And bring your mug…

ecs150, Fall 2007

a file system
A File System

partition

partition

partition

b

s

i-list

directory and data blocks

d

i-node

i-node

…….

i-node

ecs150, Fall 2007

one logical file physical disk blocks
One Logical File  Physical Disk Blocks

efficient representation & access

ecs150, Fall 2007

an i node
An i-node

A file

??? entries in

one disk block

Typical:

each block 8K or 16K bytes

ecs150, Fall 2007

inode index node structure
inode (index node) structure
  • meta-data of the file.
    • di_mode 02
    • di_nlinks 02
    • di_uid 02
    • di_gid 02
    • di_size 04
    • di_addr 39
    • di_gen 01
    • di_atime 04
    • di_mtime 04
    • di_ctime 04

ecs150, Fall 2007

slide19
System-call interface

Active file entries

VNODE Layer or VFS

Local naming (UFS)

FFS

Buffer cache

Block or character device driver

Hardware

ecs150, Fall 2007

a file system21
A File System

partition

partition

partition

b

s

i-list

directory and data blocks

d

i-node

i-node

…….

i-node

ecs150, Fall 2007

slide23
125 struct ufs2_dinode {

126 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */

127 int16_t di_nlink; /* 2: File link count. */

128 u_int32_t di_uid; /* 4: File owner. */

129 u_int32_t di_gid; /* 8: File group. */

130 u_int32_t di_blksize; /* 12: Inode blocksize. */

131 u_int64_t di_size; /* 16: File byte count. */

132 u_int64_t di_blocks; /* 24: Bytes actually held. */

133 ufs_time_t di_atime; /* 32: Last access time. */

134 ufs_time_t di_mtime; /* 40: Last modified time. */

135 ufs_time_t di_ctime; /* 48: Last inode change time. */

136 ufs_time_t di_birthtime; /* 56: Inode creation time. */

137 int32_t di_mtimensec; /* 64: Last modified time. */

138 int32_t di_atimensec; /* 68: Last access time. */

139 int32_t di_ctimensec; /* 72: Last inode change time. */

140 int32_t di_birthnsec; /* 76: Inode creation time. */

141 int32_t di_gen; /* 80: Generation number. */

142 u_int32_t di_kernflags; /* 84: Kernel flags. */

143 u_int32_t di_flags; /* 88: Status flags (chflags). */

144 int32_t di_extsize; /* 92: External attributes block. */

145 ufs2_daddr_t di_extb[NXADDR];/* 96: External attributes block. */

146 ufs2_daddr_t di_db[NDADDR]; /* 112: Direct disk blocks. */

147 ufs2_daddr_t di_ib[NIADDR]; /* 208: Indirect disk blocks. */

148 int64_t di_spare[3]; /* 232: Reserved; currently unused */

149 };

ecs150, Fall 2007

slide24
166 struct ufs1_dinode {

167 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */

168 int16_t di_nlink; /* 2: File link count. */

169 union {

170 u_int16_t oldids[2]; /* 4: Ffs: old user and group ids. */

171 } di_u;

172 u_int64_t di_size; /* 8: File byte count. */

173 int32_t di_atime; /* 16: Last access time. */

174 int32_t di_atimensec; /* 20: Last access time. */

175 int32_t di_mtime; /* 24: Last modified time. */

176 int32_t di_mtimensec; /* 28: Last modified time. */

177 int32_t di_ctime; /* 32: Last inode change time. */

178 int32_t di_ctimensec; /* 36: Last inode change time. */

179 ufs1_daddr_t di_db[NDADDR]; /* 40: Direct disk blocks. */

180 ufs1_daddr_t di_ib[NIADDR]; /* 88: Indirect disk blocks. */

181 u_int32_t di_flags; /* 100: Status flags (chflags). */

182 int32_t di_blocks; /* 104: Blocks actually held. */

183 int32_t di_gen; /* 108: Generation number. */

184 u_int32_t di_uid; /* 112: File owner. */

185 u_int32_t di_gid; /* 116: File group. */

186 int32_t di_spare[2]; /* 120: Reserved; currently unused */

187 };

ecs150, Fall 2007

bittorrent pieces
Bittorrent pieces

File size: 10 GB

Pieces downloaded: 512 MB

How much disk space do we need?

ecs150, Fall 2007

slide26
#include

#include

int

main

(void)

{

FILE *f1 = fopen("./sss.txt", "w");

int i;

for (i = 0; i < 1000; i++)

{

fseek(f1, rand(), SEEK_SET);

fprintf(f1, "%d%d%d%d", rand(), rand(),

rand(), rand());

if (i % 100 == 0) sleep(1);

}

fflush(f1);

}

# ./t

# ls –l ./sss.txt

ecs150, Fall 2007

an i node30
An i-node

A file

??? entries in

one disk block

Typical:

each block 1K

ecs150, Fall 2007

i node
i-node
  • How many disk blocks can a FS have?
  • How many levels of i-node indirection will be necessary to store a file of 2G bytes? (I.e., 0, 1, 2 or 3)
  • What is the largest possible file size in i-node?
  • What is the size of the i-node itself for a file of 10GB with only 512 MB downloaded?

ecs150, Fall 2007

answer
Answer
  • How many disk blocks can a FS have?
    • 264 or 232: Pointer (to blocks) size is 8/4 bytes.
  • How many levels of i-node indirection will be necessary to store a file of 2G (231) bytes? (I.e., 0, 1, 2 or 3)
    • 12*210 + 28 * 210 + 28 *28 *2 10 +28 *28 *28 *2 10 >? 231
  • What is the largest possible file size in i-node?
    • 12*210 + 28 * 210 + 28 *28 *2 10 +28 *28 *28 *2 10
    • 264 –1
    • 232 * 210

You need to consider three issues and find the minimum!

ecs150, Fall 2007

answer lower bound
Answer: Lower Bound
  • How many pointers?
    • 512MB divided by the block size (1K)
    • 512K pointers times 8 (4) bytes = 4 (2) MB

ecs150, Fall 2007

bittorrent pieces34
Bittorrent pieces

File size: 10 GB

Pieces downloaded: 512 MB

How much disk space do we need?

ecs150, Fall 2007

answer upper bound
Answer: Upper Bound
  • In the worst case, EVERY indirection block has at least one entry!
  • How many indirection blocks?
    • Single: 1 block
    • Double: 1 + 28
    • Tripple: 1 + 28 + 216
  • Total ~ 216 blocks times 1K = 64 MB
    • 214 times 1K = 16MB (ufs2 inode)

ecs150, Fall 2007

answer 4
Answer (4)
  • 2 MB ~ 64 MB ufs1
  • 4 MB ~ 16 MB ufs2
  • Answer: sss.txt ~17 MB
    • ~16 MB (inode indirection blocks)
    • 1000 writes times 1K ~ 1MB

ecs150, Fall 2007

an i node37
An i-node

A file

??? entries in

one disk block

Typical:

each block 1K

ecs150, Fall 2007

a file system38
A File System

partition

partition

partition

b

s

i-list

directory and data blocks

d

i-node

i-node

…….

i-node

ecs150, Fall 2007

ffs and ufs
FFS and UFS
  • /usr/src/sys/ufs/ffs/*
    • Higher-level: directory structure
    • Soft updates & Snapshot
  • /usr/src/sys/ufs/ufs/*
    • Lower-level: buffer, i-node

ecs150, Fall 2007

of i nodes
# of i-nodes
  • UFS1: pre-allocation
    • 3% of HD, about < 25% used.
  • UFS2: dynamic allocation
    • Still limited # of i-nods

ecs150, Fall 2007

di size vs di blocks
di_size vs. di_blocks
  • ???

ecs150, Fall 2007

one logical file physical disk blocks42
One Logical File  Physical Disk Blocks

efficient representation & access

ecs150, Fall 2007

di size vs di blocks43
di_size vs. di_blocks
  • Logical
  • Physical
  • fstat
  • du

ecs150, Fall 2007

extended attributes in ufs2
Extended Attributes in UFS2
  • Attributes associated with the File
    • di_extb[2];
    • two blocks, but indirection if needed.
  • Format
    • Length 4
    • Name Space 1
    • Content Pad Length 1
    • Name Length 1
    • Name mod 8
    • Content variable
  • Applications: ACL, Data Labelling

ecs150, Fall 2007

some thoughts
Some thoughts….
  • What can you do with “extended attributes”?
  • How to design/implement?
    • Should/can we do it “Stackable File Systems”?
    • Otherwise, the program to manipulate the EA’s will have to be very UFS2-dependent or FiST with an UFS2 optimization option.
  • Are there any counter examples?
    • security and performance considerations.

ecs150, Fall 2007

a file system46
A File System

partition

partition

partition

b

s

i-list

directory and data blocks

d

i-node

i-node

…….

i-node

ecs150, Fall 2007

slide47
struct dirent {

ino_t d_ino;

char d_name[NAME_MAX+1];

};

struct stat {…

short nlinks;

…};

directory

dirent

inode

file_name

dirent

inode

file_name

dirent

inode

file_name

file

file

file

ecs150, Fall 2007

slide49
root

wheel

.

2

directory

/

2

drwxr-xr-x

..

2

Apr 1 2004

usr

4

3

vmunix

5

root

wheel

.

4

drwxr-xr-x

directory

/usr

..

2

4

Apr 1 2004

bin

7

root

wheel

foo

6

rwxr-xr-x

5

file

/vmunix

Apr 15 2004

text

data

kirk

staff

6

rw-rw-r--

file

/usr/foo

Hello World!

Jan 19 2004

root

wheel

.

7

7

drwxr-xr-x

..

4

directory

/usr/bin

Apr 1 2004

ex

9

8

groff

10

bin

bin

vi

9

file

/usr/bin/vi

9

rwxr-xr-x

text

data

Apr 15 2004

ecs150, Fall 2007

what is the difference
What is the difference?
  • ln –s /usr/src/sys/sys/proc.h ppp.h
  • ln /usr/src/sys/sys/proc.h ppp.h

ecs150, Fall 2007

hard versus symbolic
Hard versus Symbolic
  • ln –s /usr/src/sys/sys/proc.h ppp.h
    • Link to anything, any mounted partitions
    • Delete a Symbolic link?
  • ln /usr/src/sys/sys/proc.h ppp.h
    • Link only to “file” (not directory)
    • Link only within the same partition -- why?
    • Delete a Hard Link?

ecs150, Fall 2007

slide52
125 struct ufs2_dinode {

126 u_int16_t di_mode; /* 0: IFMT, permissions; see below. */

127 int16_tdi_nlink;/* 2: File link count. */

128 u_int32_t di_uid; /* 4: File owner. */

129 u_int32_t di_gid; /* 8: File group. */

130 u_int32_t di_blksize; /* 12: Inode blocksize. */

131 u_int64_t di_size; /* 16: File byte count. */

132 u_int64_t di_blocks; /* 24: Bytes actually held. */

133 ufs_time_t di_atime; /* 32: Last access time. */

134 ufs_time_t di_mtime; /* 40: Last modified time. */

135 ufs_time_t di_ctime; /* 48: Last inode change time. */

136 ufs_time_t di_birthtime; /* 56: Inode creation time. */

137 int32_t di_mtimensec; /* 64: Last modified time. */

138 int32_t di_atimensec; /* 68: Last access time. */

139 int32_t di_ctimensec; /* 72: Last inode change time. */

140 int32_t di_birthnsec; /* 76: Inode creation time. */

141 int32_t di_gen; /* 80: Generation number. */

142 u_int32_t di_kernflags; /* 84: Kernel flags. */

143 u_int32_t di_flags; /* 88: Status flags (chflags). */

144 int32_t di_extsize; /* 92: External attributes block. */

145 ufs2_daddr_t di_extb[NXADDR];/* 96: External attributes block. */

146 ufs2_daddr_t di_db[NDADDR]; /* 112: Direct disk blocks. */

147 ufs2_daddr_t di_ib[NIADDR]; /* 208: Indirect disk blocks. */

148 int64_t di_spare[3]; /* 232: Reserved; currently unused */

149 };

ecs150, Fall 2007

slide53
struct dirent {

ino_t d_ino;

char d_name[NAME_MAX+1];

};

struct stat {…

short nlinks;

…};

directory

dirent

inode

file_name

dirent

inode

file_name

dirent

inode

file_name

file

file

file

ecs150, Fall 2007

file system buffer cache
File System Buffer Cache

application: read/write files

translate file to disk blocks

OS:

...

...buffer cache

maintains

controls disk accesses: read/write blocks

hardware:

Any problems?

ecs150, Fall 2007

file system consistency
File System Consistency
  • To maintain file system consistency the ordering of updates from buffer cache to disk is critical
  • Example:
    • if the directory block is written back before the i-node and the system crashes, the directory structure will be inconsistent

ecs150, Fall 2007

file system consistency56
File System Consistency
  • File system almost always use a buffer/disk cache for performance reasons
  • This problem is critical especially for the blocks that contain control information: i-node, free-list, directory blocks
  • Two copies of a disk block (buffer cache, disk)  consistency problem if the system crashes before all the modified blocks are written back to disk
  • Write back critical blocks from the buffer cache to disk immediately
  • Data blocks are also written back periodically: sync

ecs150, Fall 2007

two strategies
Two Strategies
  • Prevention
    • Use un-buffered I/O when writing i-nodes or pointer blocks
    • Use buffered I/O for other writes and force sync every 30 seconds
  • Detect and Fix
    • Detect the inconsistency
    • Fix them according to the “rules”
    • Fsck (File System Checker)

ecs150, Fall 2007

file system integrity
File System Integrity
  • Block consistency:
    • Block-in-use table
    • Free-list table
  • File consistency:
    • how many directories pointing to that i-node?
    • nlink?
    • three cases: D == L, L > D, D > L
      • What to do with the latter two cases?

0

1

1

1

0

0

0

1

0

0

0

2

1

0

0

0

1

1

1

0

1

0

2

0

ecs150, Fall 2007

file system integrity59
File System Integrity
  • File system states

(a) consistent

(b) missing block

(c) duplicate block in free list

(d) duplicate data block

ecs150, Fall 2007

metadata operations
Metadata Operations
  • Metadata operations modify thestructure of the file system
    • Creating, deleting, or renamingfiles, directories, or special files
    • Directory & I-node
  • Data must be written to disk in such a way that the file system can be recovered to a consistent state after a system crash

ecs150, Fall 2007

metadata integrity
Metadata Integrity
  • FFS uses synchronous writes to guarantee the integrity of metadata
    • Any operation modifying multiple pieces of metadata will write its data to disk in a specific order
    • These writes will beblocking
  • Guarantees integrity and durability of metadata updates

ecs150, Fall 2007

deleting a file i
Deleting a file (I)

i-node-1

abc

def

i-node-2

ghi

i-node-3

Assume we want to delete file “def”

ecs150, Fall 2007

deleting a file ii
Deleting a file (II)

i-node-1

abc

?

def

ghi

i-node-3

Cannot delete i-node before directory entry “def”

ecs150, Fall 2007

deleting a file iii
Deleting a file (III)
  • Correct sequence is
      • Write to disk directory block containing deleted directory entry “def”
      • Write to disk i-node block containing deleted i-node
  • Leaves the file system in a consistent state

ecs150, Fall 2007

creating a file i
Creating a file (I)

i-node-1

abc

ghi

i-node-3

Assume we want to create new file “tuv”

ecs150, Fall 2007

creating a file ii
Creating a file (II)

i-node-1

abc

ghi

i-node-3

tuv

?

Cannot write directory entry “tuv” before i-node

ecs150, Fall 2007

creating a file iii
Creating a file (III)
  • Correct sequence is
      • Write to disk i-node block containing new i-node
      • Write to disk directory block containing new directory entry
  • Leaves the file system in a consistent state

ecs150, Fall 2007

synchronous updates
Synchronous Updates
  • Used by FFS to guarantee consistency of metadata:
    • All metadata updates are done through blocking writes
  • Increases the cost of metadata updates
  • Can significantly impact the performance of whole file system

ecs150, Fall 2007

soft updates
SOFT UPDATES
  • Use delayed writes (write back)
  • Maintain dependency informationabout cached pieces of metadata:

This i-node must be updated before/after this directory entry

  • Guarantee that metadata blocks are written to disk in the required order

ecs150, Fall 2007

3 soft update rules
3 Soft Update Rules
  • Never point to a structure before it has been initialized.
  • Never reuse a resource before nullifying all previous pointers to it.
  • Never reset the old pointer to a live resource before the new pointer has been set.

ecs150, Fall 2007

problem 1 with s u
Problem #1 with S.U.
  • Synchronous writes guaranteed that metadata operations were durable once the system call returned
  • Soft Updates guarantee that file system will recover into a consistent state but not necessarily the most recent one
    • Some updates could be lost

ecs150, Fall 2007

slide73
What are the dependency relationship?

We want to delete file “foo” and create new file “bar”

Block A

Block B

foo

i-node-2

NEW bar

NEW i-node-3

ecs150, Fall 2007

slide74
Circular Dependency

X-2nd

Y-1st

We want to delete file “foo” and create new file “bar”

Block A

Block B

foo

i-node-2

NEW bar

NEW i-node-3

ecs150, Fall 2007

problem 2 with s u
Problem #2 with S.U.
  • Cyclical dependencies:
    • Same directory block contains entries to be created and entries to be deleted
    • These entries point to i-nodes in the same block
  • Brainstorming:
    • How to resolve this issue in S.U.?

ecs150, Fall 2007

fs buffer or disk
FS: buffer or disk??
  • They appear in both and we try to synchronize them..

ecs150, Fall 2007

slide77
Disk

Block A-Dir

Block B-i-Node

foo

i-node-2

ecs150, Fall 2007

buffer
Buffer

Block A-Dir

Block B-i-Node

NEW bar

NEW i-node-3

ecs150, Fall 2007

synchronize
Synchronize??

Block A

Block B

foo

i-node-2

NEW bar

NEW i-node-3

ecs150, Fall 2007

solution in s u
def Solution in S.U.
  • Roll back metadata in one of the blocks to an earlier, safe state

(Safe state does not contain new directory entry)

Block A’

ecs150, Fall 2007

slide83
Write first block with metadata that were rolled back (block A’ of example)
  • Write blocks that can be written after first block has been written (block B of example)
  • Roll forward block that was rolled back
  • Write that block
  • Breaks the cyclical dependency but must nowwrite twice block A

ecs150, Fall 2007

slide84
Before any Write Operation

SU Dependency Checking

(roll back if necessary)

After any Write Operation

SU Dependency Processing

(task list updating)

(roll forward if necessary)

ecs150, Fall 2007

slide85
two most popular approaches for improving the performance of metadata operations and recovery:
    • Journaling
    • Soft Updates
  • Journaling systems record metadata operations on an auxiliary log
  • Soft Updates usesordered writes

ecs150, Fall 2007

journaling
JOURNALING
  • Journaling systems maintain an auxiliary log that records all meta-data operations
  • Write-ahead loggingensures that the log is written to disk beforeany blocks containing data modified by the corresponding operations.
    • After a crash, can replay the log to bring the file system to a consistent state

ecs150, Fall 2007

journaling87
JOURNALING
  • Log writes are performed in addition to the regular writes
  • Journaling systems incur log write overhead but
    • Log writes can be performed efficiently because they are sequential (block operation consideration)
    • Metadata blocks do not need to be written back after each update

ecs150, Fall 2007

journaling88
JOURNALING
  • Journaling systems can provide
    • same durability semantics as FFS if log is forced to disk after each meta-data operation
    • the laxer semantics of Soft Updates if log writes are buffered until entire buffers are full

ecs150, Fall 2007

soft updates vs journaling
Soft Updates vs. Journaling
  • Advantages
  • disadvantages

ecs150, Fall 2007

with soft updates
With Soft Updates??

Do we still need “FSCK”? at boot time?

CPU

ecs150, Fall 2007

recover the missing resources
Recover the Missing Resources
  • In the background, in an active FS…
    • We don’t want to wait for the lengthy FSCK process to complete…
  • A related issue:
    • the virus scanning process
    • what happens if we get a new virus signature?

ecs150, Fall 2007

snapshot of the fs
Snapshot of the FS
  • backup and restore
  • dump reliably an active File System
    • what will we do today to dump our 40GB FS “consistent” snapshots? (in the midnight…)
  • “background FSCK checks”

ecs150, Fall 2007

what is a snapshot i mean conceptually
What is a snapshot?(I mean “conceptually”.)
  • Freeze all activities related to the FS.
  • Copy everything to “some space”.
  • Resume the activities.

How do we efficiently implement this concept such that the activities will only be blocked for about 0.25 seconds, and we don’t have to buy a really big hard drive?

ecs150, Fall 2007

slide96
Copy-on-Write

ecs150, Fall 2007

snapshot a file
Snapshot: a file

Logical size

Versus physical size

ecs150, Fall 2007

example
Example

# mkdir /backups/usr/noon

# mount –u –o snapshot /usr/snap.noon /usr

# mdconfig –a –t vnode –u 0 –f /usr/snap.noon

# mount –r /dev/md0 /backups/usr/noon

/* do whatever you want to test it */

# umount /backups/usr/noon

# mdconfig –d –u 0

# rm –f /usr/snap.noon

ecs150, Fall 2007

slide101
#include

#include

int

main

(void)

{

FILE *f1 = fopen("./sss.txt", "w");

int i;

for (i = 0; i < 1000; i++)

{

fseek(f1, rand(), SEEK_SET);

fprintf(f1, "%d%d%d%d", rand(), rand(),

rand(), rand());

if (i % 100 == 0) sleep(1);

}

fflush(f1);

}

ecs150, Fall 2007

example102
Example

# mkdir /backups/usr/noon

# mount –u –o snapshot /usr/snap.noon /usr

# mdconfig –a –t vnode –u 0 –f /usr/snap.noon

# mount –r /dev/md0 /backups/usr/noon

/* do whatever you want to test it */

# umount /backups/usr/noon

# mdconfig –d –u 0

# rm –f /usr/snap.noon

ecs150, Fall 2007

example110
Example

# mkdir /backups/usr/noon

# mount –u –o snapshot /usr/snap.noon /usr

# mdconfig –a –t vnode –u 0 –f /usr/snap.noon

# mount –r /dev/md0 /backups/usr/noon

/* do whatever you want to test it */

# umount /backups/usr/noon

# mdconfig –d –u 0

# rm –f /usr/snap.noon

ecs150, Fall 2007

slide111
Copy-on-Write

ecs150, Fall 2007

a file system113
A File System

A file

??? entries in

one disk block

ecs150, Fall 2007

a snapshot i node
A Snapshot i-node

A file

??? entries in

one disk block

Not used or

Not yet copy

ecs150, Fall 2007

copy on write
Copy-on-write

A file

??? entries in

one disk block

Not used or

Not yet copy

ecs150, Fall 2007

copy on write116
Copy-on-write

A file

??? entries in

one disk block

Not used or

Not yet copy

ecs150, Fall 2007

multiple snapshots
Multiple Snapshots
  • about 20 snapshots
  • Interactions/sharing among snapshots

ecs150, Fall 2007

snapshot of the fs118
Snapshot of the FS
  • backup and restore
  • dump reliably an active File System
    • what will we do today to dump our 40GB FS “consistent” snapshots? (in the midnight…)
  • “background FSCK checks”

ecs150, Fall 2007

vfs the fs switch
user space

syscall layer (file, uio, etc.)

Virtual File System (VFS)

network

protocol

stack

(TCP/IP)

FFS

LFS

NFS

*FS

etc.

etc.

device drivers

VFS: the FS Switch
  • Sun Microsystems introduced the virtual file system interface in 1985 to accommodate diverse filesystem types cleanly.
      • VFS allows diverse specific file systems to coexist in a file tree, isolating all FS-dependencies in pluggable filesystem modules.

VFS was an internal kernel restructuring

with no effect on the syscall interface.

Incorporates object-oriented concepts:

a generic procedural interface with

multiple implementations.

Based on abstract objects with dynamic

method binding by type...in C.

Other abstract interfaces in the kernel: device drivers,

file objects, executable files, memory objects.

ecs150, Fall 2007

vnode
syscall layer

free vnodes

NFS

UFS

vnode
  • In the VFS framework, every file or directory in active use is represented by a vnode object in kernel memory.

Each vnode has a standard

file attributes struct.

Generic vnode points at

filesystem-specific struct

(e.g., inode, rnode), seen

only by the filesystem.

Each specific file system maintains a cache of its resident vnodes.

Vnode operations are

macros that vector to

filesystem-specific

procedures.

ecs150, Fall 2007

vnode operations and attributes
vnode Operations and Attributes

vnode attributes (vattr)

type (VREG, VDIR, VLNK, etc.)

mode (9+ bits of permissions)

nlink (hard link count)

owner user ID

owner group ID

filesystem ID

unique file ID

file size (bytes and blocks)

access time

modify time

generation number

directories only

vop_lookup (OUT vpp, name)

vop_create (OUT vpp, name, vattr)

vop_remove (vp, name)

vop_link (vp, name)

vop_rename (vp, name, tdvp, tvp, name)

vop_mkdir (OUT vpp, name, vattr)

vop_rmdir (vp, name)

vop_symlink (OUT vpp, name, vattr, contents)

vop_readdir (uio, cookie)

vop_readlink (uio)

files only

vop_getpages (page**, count, offset)

vop_putpages (page**, count, sync, offset)

vop_fsync ()

generic operations

vop_getattr (vattr)

vop_setattr (vattr)

vhold()

vholdrele()

ecs150, Fall 2007

network file system nfs
Network File System (NFS)

server

client

syscall layer

user programs

VFS

syscall layer

NFS

server

VFS

UFS

NFS

client

UFS

network

ecs150, Fall 2007

vnode cache
vnode Cache

VFS free list head

HASH(fsid, fileid)

Active vnodes are reference- counted by the structures that hold pointers to them.

- system open file table

- process current directory

- file system mount points

- etc.

Each specific file system maintains its own hash of vnodes (BSD).

- specific FS handles initialization

- free list is maintained by VFS

vget(vp): reclaim cached inactive vnode from VFS free list

vref(vp): increment reference count on an active vnode

vrele(vp): release reference count on a vnode

vgone(vp): vnode is no longer valid (file is removed)

ecs150, Fall 2007

slide127
struct vnode {

struct mtx v_interlock; /* lock for "i" things */

u_long v_iflag; /* i vnode flags (see below) */

int v_usecount; /* i ref count of users */

long v_numoutput; /* i writes in progress */

struct thread *v_vxthread; /* i thread owning VXLOCK */

int v_holdcnt; /* i page & buffer references */

struct buflists v_cleanblkhd; /* i SORTED clean blocklist */

struct buf *v_cleanblkroot; /* i clean buf splay tree */

int v_cleanbufcnt; /* i number of clean buffers */

struct buflists v_dirtyblkhd; /* i SORTED dirty blocklist */

struct buf *v_dirtyblkroot; /* i dirty buf splay tree */

int v_dirtybufcnt;

ecs150, Fall 2007

distributed fs
Distributed FS

ftp.cs.ucdavis.edu fs0: /dev/hd0a

/

usr

sys

dev

etc

bin

/

local

adm

home

lib

bin

Server.yahoo.com fs0: /dev/hd0e

ecs150, Fall 2007

logical disks
fs0: /dev/hd0alogical disks

/

usr

sys

dev

etc

bin

mount -t ufs /dev/hd0e /usr

/

local

adm

home

lib

bin

fs1: /dev/hd0e

mount -t nfs 152.1.23.12:/export/cdrom /mnt/cdrom

ecs150, Fall 2007

correctness
Correctness
  • One-copy Unix Semantics
    • every modification to every byte of a file has to be immediately and permanently visible to every client.

ecs150, Fall 2007

correctness131
Correctness
  • One-copy Unix Semantics
    • every modification to every byte of a file has to be immediately and permanently visible to every client.
    • Conceptually FS sequent access
      • Make sense in a local file system
      • Single processor versus shared memory
  • Is this necessary?

ecs150, Fall 2007

dfs architecture
DFS Architecture
  • Server
    • storage for the distributed/shared files.
    • provides an access interface for the clients.
  • Client
    • consumer of the files.
    • runs applications in a distributed environment.

open close

read write

opendir stat

readdir

applications

ecs150, Fall 2007

nfs sun 1985
NFS (SUN, 1985)
  • Based on RPC (Remote Procedure Call) and XDR (Extended Data Representation)
  • Server maintains no state
    • a READ on the server opens, seeks, reads, and closes
    • a WRITE is similar, but the buffer is flushed to disk before closing
  • Server crash: client continues to try until server reboots – no loss
  • Client crashes: client must rebuild its own state – no effect on server

ecs150, Fall 2007

rpc xdr
RPC - XDR
  • RPC: Standard protocol for calling procedures in another machine
  • Procedure is packaged with authorization and admin info
  • XDR: standard format for data, because manufacturers of computers cannot agree on byte ordering.

ecs150, Fall 2007

rpcgen
data

structure

data

structure

rpcgen

RPC program

rpcgen

RPC client.c

RPC.h

RPC server.c

ecs150, Fall 2007

nfs operations
NFS Operations
  • Every operation is independent: server opens file for every operation
  • File identified by handle -- no state information retained by server
  • client maintains mount table, v-node, offset in file table etc.

What do these imply???

ecs150, Fall 2007

slide137
Client computer

NFS

Client

Kernel

Application

Application

program

program

UNIX

system calls

UNIX kernel

Operationson

remote files

Operations on local files

NFS

Client

Other file system

NFSprotocol (remote operations)

Client computer

Server computer

Application

Application

program

program

Virtual file system

Virtual file system

UNIX

UNIX

NFS

NFS

file

file

client

server

system

system

mount –t nfs home.yahoo.com:/pub/linux /mnt/linux

ecs150, Fall 2007

*

state ful vs state less
State-ful vs. State-less
  • A server is fully aware of its clients
    • does the client have the newest copy?
    • what is the offset of an opened file?
    • “a session” between a client and a server!
  • A server is completely unaware of its clients
    • memory-less: I do not remember you!!
    • Just tell me what you want to get (and where).
    • I am not responsible for your offset values (the client needs to maintain the state).

ecs150, Fall 2007

the state
The State

open

read

stat

lseek

applications

open

read

stat

lseek

offset

applications

ecs150, Fall 2007

network file sharing
Network File Sharing
  • Server side:
    • Rpcbind (portmap)
    • Mountd - respond to mount requests (sometimes called rpc.mountd).
      • Relies on several files
        • /etc/dfs/dfstab,
        • /etc/exports,
        • /etc/netgroup
    • nfsd - serves files - actually a call to kernel level code.
    • lockd – file locking daemon.
    • statd – manages locks for lockd.
    • rquotad – manages quotas for exported file systems.

ecs150, Fall 2007

network file sharing142
Network File Sharing
  • Client Side
    • biod - client side caching daemon
    • mount must understand the hostname:directory convention.
    • Filesystem entries in /etc/[v]fstab tell the client what filesystems to mount.

ecs150, Fall 2007

unix file semantics
Unix file semantics
  • NFS:
    • open a file with read-write mode
    • later, the server’s copy becomes read-only mode
    • now, the application tries to write it!!

ecs150, Fall 2007

problems with nfs
Problems with NFS
  • Performance not scaleable:
    • maybe it is OK for a local office.
    • will be horrible with large scale systems.

ecs150, Fall 2007

slide145
Similar to UNIX file caching for local files:
    • pages (blocks) from disk are held in a main memory buffer cache until the space is required for newer pages. Read-ahead and delayed-write optimisations.
    • For local files, writes are deferred to next sync event (30 second intervals)
    • Works well in local context, where files are always accessed through the local cache, but in the remote case it doesn't offer necessary synchronization guarantees to clients.
  • NFS v3 servers offers two strategies for updating the disk:
    • write-through - altered pages are written to disk as soon as they are received at the server. When a write() RPC returns, the NFS client knows that the page is on the disk.
    • delayed commit - pages are held only in the cache until a commit() call is received for the relevant file. This is the default mode used by NFS v3 clients. A commit() is issued by the client whenever a file is closed.

ecs150, Fall 2007

*

slide146
Server caching does nothing to reduce RPC traffic between client and server
    • further optimisation is essential to reduce server load in large networks
    • NFS client module caches the results of read, write, getattr, lookup and readdir operations
    • synchronization of file contents (one-copy semantics) is not guaranteed when two or more clients are sharing the same file.
  • Timestamp-based validity check
    • reduces inconsistency, but doesn't eliminate it
    • validity condition for cache entries at the client:

(T - Tc < t) v (Tmclient = Tmserver)

    • t is configurable (per file) but is typically set to 3 seconds for files and 30 secs. for directories
    • it remains difficult to write distributed applications that share files with NFS

t freshness guarantee

Tc time when cache entry was last validated

Tm time when block was last updated at server

T current time

ecs150, Fall 2007

*

slide147
AFS
  • State-ful clients and servers.
  • Caching the files to clients.
    • File close ==> check-in the changes.
  • How to maintain consistency?
    • Using “Callback” in v2/3 (Valid or Cancelled)

open

read

applications

invalidate and re-cache

ecs150, Fall 2007

why afs
Why AFS?
  • Shared files are infrequently updated
  • Local cache of a few hundred mega bytes
    • Now 50~100 giga bytes
  • Unix workload:
    • Files are small, Read Operations dominated, sequential access is common, read/written by one user, reference bursts.
    • Are these still true?

ecs150, Fall 2007

fault tolerance in afs
Fault Tolerance in AFS
  • a server crashes
  • a client crashes
    • check for call-back tokens first.

ecs150, Fall 2007

problems with afs
Problems with AFS
  • Availability
  • what happens if call-back itself is lost??

ecs150, Fall 2007

gfs google file system
GFS – Google File System
  • “failures” are norm
  • Multiple-GB files are common
  • Append rather than overwrite
    • Random writes are rare
  • Can we relax the consistency?

ecs150, Fall 2007

the master
The Master
  • Maintains all file system metadata.
    • names space, access control info, file to chunk mappings, chunk (including replicas) location, etc.
  • Periodically communicates with chunkservers in HeartBeat messages to give instructions and check state

ecs150, Fall 2007

the master154
The Master
  • Helps make sophisticated chunk placement and replication decision, using global knowledge
  • For reading and writing, client contacts Master to get chunk locations, then deals directly with chunkservers
    • Master is not a bottleneck for reads/writes

ecs150, Fall 2007

chunkservers
Chunkservers
  • Files are broken into chunks. Each chunk has a immutable globally unique 64-bit chunk-handle.
    • handle is assigned by the master at chunk creation
  • Chunk size is 64 MB
  • Each chunk is replicated on 3 (default) servers

ecs150, Fall 2007

clients
Clients
  • Linked to apps using the file system API.
  • Communicates with master and chunkservers for reading and writing
    • Master interactions only for metadata
    • Chunkserver interactions for data
  • Only caches metadata information
    • Data is too large to cache.

ecs150, Fall 2007

chunk locations
Chunk Locations
  • Master does not keep a persistent record of locations of chunks and replicas.
  • Polls chunkservers at startup, and when new chunkservers join/leave for this.
  • Stays up to date by controlling placement of new chunks and through HeartBeat messages (when monitoring chunkservers)

ecs150, Fall 2007

slide159
CODA
  • Server Replication:
    • if one server goes down, I can get another.
  • Disconnected Operation:
    • if all go down, I will use my own cache.

ecs150, Fall 2007

disconnected operation
Disconnected Operation
  • Continue critical work when that repository is inaccessible.
  • Key idea: caching data.
    • Performance
    • Availability
  • Server Replication

ecs150, Fall 2007

consistency
Consistency
  • If John update file X on server A and Mary read file X on server B….

Read-one & Write-all

ecs150, Fall 2007

read x write n x 1
Read x & Write (N-x+1)

read

write

ecs150, Fall 2007

example r3w4 6 1
Example: R3W4 (6+1)

Initial 0 0 0 0 0 0

Alice-W2 2 0 2 2 0

Bob-W 2 3 3 3 3 0

Alice-R 2 3 3 3 3 0

Chris-W 2 1 1 1 1 0

Dan-R 2 1 1 1 1 0

Emily-W7 7 1 1 1 7

Frank-R 7 7 1 1 1 7

ecs150, Fall 2007

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