Transcript -log-file system

Aneela Tabassum
1
Basic Problem in UFSS

Most file systems now have large memory caches
(buffers) to hold recently-accessed blocks.
 Most reads are thus satisfied from the buffer cache.
 Most traffic is to write traffic on the disk.
 We need to make writes go faster.
 But disk performance is limited according to disk
head movement.
2
Motivation for Log-FS

Technology:
 Disk is the bottleneck.
 Faster CPU.
 no faster disk.
 Faster memory (thus cache).
 Larger disks.

Workloads:
 Random access to the disk.
 Small reads and writes.
3
Disk is the Bottleneck (1)
cylinder k
surface 0
surface 1
surface 2
surface 3
surface 4
surface 5
platter 0
platter 1
Track
spindle
spindle
spindle
spindle
platter 2
spindle
,
The read/write
head attach with
the end of arm .
the arm can
position the
read/write head
over any track.
4
Bottleneck
5
LFS: Basic Idea
LFS is use as alternative of UFS.
 A log is a data structure that is written only at the
head.
 If the disk were managed as a log, there would be
effectively no head seeks
 The “file” is always added to sequentially
 New data and metadata (inodes, directories) are
accumulated in the buffer cache.
 This would greatly increase disk throughput.

6
Issues (1): Retrieving Information
from Logs

(since data and metadata are written together,
sequentially)
 inode map records current location of each inode
 the inode map itself is divided into blocks written on the disk
 a fixed region on each disk keeps track of all inode map
blocks
 inode map small enough to fit into the memory
7
Structure File System
Disk
Write sequentially : never overwrite data
8
Unix Structure File System
Disk
File 1
File 2
Update
File 1
Write sequentially : never overwrite data
9
Log-Structure File System
Disk
Block 0 Block 1 Block 2
10
Log-Structure File System
Disk
Node A
Block 0 Block 1 Block 2
11
Where does the meta-data go?
Disk
Node A
Block 0 Block 1 Block 2
12
Log-Structure File System
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 6 ……
13
Log-Structure File System
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node B
Block 7
14
Updating a file
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node B
Block 7
Block 1
update
15
Updating a file
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A’
Node B
Block 7
Block 1
update
16
Updating a file
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A’
Node B
Block 7
Block 1
update
17
Updating a file
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A`
Node B
Block 7
Block 1
update
18
Updating a file
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A`
Node B
Block 7
Block 1
update
19
Imap
2
1
0
Pointer to most recant version of node
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A`
Node B
Block 7
Block 1
update
20
Imap
2
1
0
Pointer to most recant version of node
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Node A`
Node B
Block 7
Block 1
update
21
Once the disk fill up with lot of old junk
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Imap
Block 5
Block 8 update
Node B’
Imap
Node A`
Node B
Block 7
Block 1
update
….
22
Issues(2): Manage Free Space

Through a combination of threading and copying
○ fixed-size extents called segments (512 KB or 1 MB)
○ identify live data from segments
○ copy live data in a compacted form and clean the remaining
segments
23
Garbage collection in LSFS
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Imap
Block 5
Block 8 update
Node B’
Imap
Node A`
Node B
Block 7
Block 1
update
….
24
Dead memory or garbage collection
Disk
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Imap
Block 5
Block 8 update
Node B’
Imap
Node A`
Node B
Block 7
Block 1
update
….
25
Dead memory or garbage collection
segment
Node A
Block 0 Block 1 Block 2
Block 3 Block 4 Block 5 Block 6
Disk, continued
Imap
Block 5
Block 8 update
Node B’
Imap
Node A`
Node B
Block 7
Block 1
update
….
26
Disk Defragmentation
Block 6
Imap
Block 5
Block 8 update
Node B’
Imap
Node A`
Node B
Block 7
Block 1
update
….
Disk
Block 0 Block 2 Block 3 Block 4
27
LFS Data Structures


inodes: as in Unix, inodes contain physical block
pointers for files.
inode map: a table indicating where each inode is
on the disk.
 inode map blocks are written as part of the segment.


segment summary: info on every block in a
segment.
segment usage table: info on the amount of “live”
data in a block.
28
LFS vs. UFS
file2
file1
inode
directory
dir1
dir2
Unix File
System
inode map
dir2
dir1
Log
file1
data
file2
Log-Structured
File System
Blocks written to
create two 1-block
files: dir1/file1 and
dir2/file2, in UFS and
LFS
29
Group Work: File System
Operations
Open an existing file
2. Create a new file
3. Remove a file
4. Rename a file
5. Modify an existing file
6. Modify a directory
1.
30
THE END
31