1. Chap 4: Distributed File Systems
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2. 4.1 Review of conventional FS
Directory
File service
Block service
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3. Moving-Head Disk Mechanism
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4. Block service interface
Obtain new block:
b=AllocateBlock( );
Release block:
FreeBlock(b);
Read/Write block:
&buf = GetBlock(b);
PutBlock(b, &data);
Must manage:
Free block list
Caching
Block service
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5. File service interface
Realizes flat file system
fid = CreateFile( );
DeleteFile(fid);
&buf = Read(fid, position, #bytes);
Write(fid, position, &buf);
Need to maintain File Control Block (FCB),
e.g., Unix inode, containing index, owner, creation time, last modify time, etc.
File Service
Block Service
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6. Directory service Directory service File service Block service
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7. 4.2 Unix (s5fs) example Boot area Sup- block inode blocks Data blocks
(a) Disk layout
Free inodes cache
Free data blocks cache
empty
empty
Link to next pointers
block (next slide)
(b) Superblock structure
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8. Linked free blocks list
In Super-
block a b c
Data blocks are used to store addresses of free blocks.
Sep 26
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9. Block buffer cache
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10. Inode u g s r w x r w x r w x Set uid owner group other Type (4 bits)
sticky bit
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11. Inode and 3-level index Inode 4 KB 1 1K pointers 4 KB 12 13 1415
1K pointers
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12. Managing open files in File Service layer
Swappable
per process
Kernel-resident
Disk-resident 1
inode 2 3
Parent’s OFT
data 1
System
OFT
(stores
position
pointers) 2
In-core
inode
table
Child’s OFT
OFT: Open File Table (one entry per open)
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13. Digression
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14. Digression
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15. Logical structure of directory hierarchy
foo
usr
bin /
vmunix
Root
directory
etc
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16. Implementing directories.
Lnk_cnt=1
Lnk_cnt=2 .. 4
foo 6 6
Hello world! .
bin ..
usr 2 4 5
VMUNIX
vmunix 5 .
local 3 .. 3
foo 6
bin2
/usr/bin 8 8
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17. Unix links %ln file hlink /*file is in current directory*/
%ln –s file slink
%ls –li *
rw-r- - r- - 2 tiko May 23 00:05 file
rw-r- - r- - 2 tiko May 23 00:05 hlink
10607 lrwxrwxrwx 1 tiko May 23 00:59 slink
file
%ln –s nonesuch devoid /*nonsuch non-existent
%ls –l devoid
…………..devoid nonesuch
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18. Mapping file systems to physical devices
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19. Mounting / Root file system bin etc usr cc date sh passwd getty /
Mount point
cc date sh passwd getty /
/dev/sd0g
bin src include
yacc ban awk uts stdio.h
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