HJL's posts

Autumn Moon on the Tama River

Sun, 20 Apr 2025 02:46:40 +0800

Size: 26.0cm by 39.5cm (10.24 in by 15.55 in)
Designer: Ando Hiroshige
Era: Post-war
Printer: Ogawa / Carver: Hotta

Description: The Tama River ran along the western border of Edo, while the Sumida River flowed through the east side of the city. The Tama tended to evoke poetic associations, such as with the desolate Musashino moor and the autumnal moon. The poem reads: "I shall never get tired of looking at the moon tonight; the mountains are bound together under the brilliant moonlight."

A striking image from Hiroshige’s Edo kinko hakkei ("Eight Views of the Suburbs of Edo") series. A post-war reproduction from the O-Edo Mokuhan-sha Company.

Linux filesystem(ext4), inode and blocks

Sat, 22 Feb 2025 14:51:39 +0200

Data structure

Field	Description
block group	partition consists of multiple block groups
block group 0	a little bit special. first 1024 bytes are reserved for boot sector
super block (sb)	is a record of the characteristics of a filesystem. primary superblock is in block group 0 whose number is power of 1,3,5,7
group descriptor	records the location of block bitmap and inode bitmap. counter for free blocks and inodes. number of directories.
block bitmap	each bit represents block. max size is 8*4096(block size)
inode	index nodes
inode table	array of inodes
inode bitmap	each bit represents a inode in inodetable. max size is 8*4096(block size)

Commands

dumpe2fs(8) - retrieve filesystem information

$ sudo dumpe2fs -h /dev/nvme0n1p4
Filesystem OS type: Linux
Inode count: 24453120
Block count: 97792081
Reserved block count: 4889604
Overhead clusters: 1814658
Free blocks: 81687378
Free inodes: 23866982
First block: 0
Block size: 4096
Fragment size: 4096
Group descriptor size: 64
Reserved GDT blocks: 1024
Blocks per group: 32768
Fragments per group: 32768
Inodes per group: 8192
Inode blocks per group: 512

dumpe2fs(8) - where is primary and backup superblocks (1,3,5,7,...)

$ sudo dumpe2fs /dev/nvme0n1p4 | grep -B 1 -i superblock
dumpe2fs 1.47.2 (1-Jan-2025)
Group 0: (Blocks 0-32767) csum 0x6bd4 [ITABLE_ZEROED]
  Primary superblock at 0, Group descriptors at 1-47
--
Group 1: (Blocks 32768-65535) csum 0xab1c [ITABLE_ZEROED]
  Backup superblock at 32768, Group descriptors at 32769-32815
--
Group 3: (Blocks 98304-131071) csum 0x992a [ITABLE_ZEROED]
  Backup superblock at 98304, Group descriptors at 98305-98351
--
Group 5: (Blocks 163840-196607) csum 0x8083 [ITABLE_ZEROED]
  Backup superblock at 163840, Group descriptors at 163841-163887
--
Group 7: (Blocks 229376-262143) csum 0x84c7 [ITABLE_ZEROED]
  Backup superblock at 229376, Group descriptors at 229377-229423
--

tune2fs(8) - modify filesystem parameters

$ sudo tune2fs -l /dev/nvmen01p4
tune2fs 1.47.2 (1-Jan-2025)
Filesystem volume name:   endeavouros
Last mounted on:          /sysroot
Filesystem UUID:          9b6cd942-2a06-48b1-b9fb-cf00862cb5d0
Filesystem magic number:  0xEF53
Filesystem revision #:    1 (dynamic)
Filesystem features:      has_journal ext_attr resize_inode dir_index orphan_file filetype needs_recovery extent 64bit flex_bg metadata_csum_seed sparse_super large_file huge_file dir_nlink extra_isize metadata_csum orphan_present
Filesystem flags:         signed_directory_hash
Default mount options:    user_xattr acl
Filesystem state:         clean
Errors behavior:          Continue
Filesystem OS type:       Linux
Inode count:              24453120
Block count:              97792081
Reserved block count:     4889604
Overhead clusters:        1814658
Free blocks:              81687378
Free inodes:              23866982
First block:              0
Block size:               4096
Fragment size:            4096
Group descriptor size:    64
Reserved GDT blocks:      1024
Blocks per group:         32768
Fragments per group:      32768
Inodes per group:         8192
Inode blocks per group:   512
Flex block group size:    16
Filesystem created:       Mon Nov 27 02:07:25 2023
Last mount time:          Sun Feb 16 04:23:30 2025
Last write time:          Sun Feb 23 23:39:58 2025
Mount count:              32
Maximum mount count:      -1
Last checked:             Tue Dec 31 14:40:33 2024
Check interval:           0 (<none>)
Lifetime writes:          1030 GB
Reserved blocks uid:      0 (user root)
Reserved blocks gid:      0 (group root)
First inode:              11
Inode size:               256
Required extra isize:     32
Desired extra isize:      32
Journal inode:            8
Default directory hash:   half_md4
Directory Hash Seed:      de0781a2-b353-41ab-9a47-2cfdaefb0ec8
Journal backup:           inode blocks
Checksum type:            crc32c
Checksum:                 0xb125b965
Checksum seed:            0x0bc9aa60
Orphan file inode:        12

stat(1) - print file or file system status

stat calculates "Blocks" based on virtual block(512 bytes), that's why you see 24 blocks below.

$ stat -fc "%n type=%T freeblk=%f totalblk=%b blksz=%S" /root
/root type=ext2/ext3 freeblk=81921201 totalblk=95977423 blksz=4096

$ dd if=/dev/urandom of=a count=3 bs=4096

$ stat a
  File: a
  Size: 12288       Blocks: 24         IO Block: 4096   regular file
Device: 259,4   Inode: 21644642    Links: 1
Access: (0644/-rw-r--r--)  Uid: ( 1000/     hjl)   Gid: ( 1000/     hjl)
Access: 2025-04-19 23:19:07.283020996 +0800
Modify: 2025-04-19 23:20:00.730320206 +0800
Change: 2025-04-19 23:20:00.730320206 +0800
 Birth: 2025-04-19 23:19:07.283020996 +0800```

ls(1) - list file and directory inode

$ dd if=/dev/urandom of=a count=3 bs=4096
$ ls -i a
21644642 a

$ mkdir b

$ ls -id .
21657519 .

$ ls -id b
21774445 b

$ ls -lia b
total 8
21774445 drwxr-xr-x 2 hjl hjl 4096 Apr 20 03:18 .
21657519 drwxr-xr-x 3 hjl hjl 4096 Apr 20 03:18 ..

df(1) - filesystem space usage

$ df /dev/nvme0n1p4
Filesystem     1K-blocks     Used Available Use% Mounted on
/dev/nvme0n1p4 383909692 59225448 305109444  17% /

$ df -B 4096 /dev/nvme0n1p4
Filesystem     4K-blocks     Used Available Use% Mounted on
/dev/nvme0n1p4  95977423 14806327  76277396  17% /

blockdev(8) - block device ioctls

from "EXTENTS" we can see file a occupies 3 blocks

$ sudo blockdev --getbsz /dev/nvme0n1p4
4096

$ dd if=/dev/urandom of=a count=3 bs=4096

$ ls -i a
21644642 a

$ sudo debugfs -R "stat <21644642>" /dev/nvme0n1p4
debugfs 1.47.2 (1-Jan-2025)
Inode: 21644642   Type: regular    Mode:  0644   Flags: 0x80000
Generation: 1360945917    Version: 0x00000000:00000009
User:  1000   Group:  1000   Project:     0   Size: 12288
File ACL: 0
Links: 1   Blockcount: 24
Fragment:  Address: 0    Number: 0    Size: 0
 ctime: 0x6803bf20:ae1f3538 -- Sat Apr 19 23:20:00 2025
 atime: 0x6803beeb:437a3b10 -- Sat Apr 19 23:19:07 2025
 mtime: 0x6803bf20:ae1f3538 -- Sat Apr 19 23:20:00 2025
crtime: 0x6803beeb:437a3b10 -- Sat Apr 19 23:19:07 2025
Size of extra inode fields: 32
Inode checksum: 0x44c771d9
EXTENTS:
(0-2):5850527-5850529

Reference

Story of TTY and PTS

Sun, 09 Feb 2025 01:45:35 +0800

A little bit backgroud

As a Linux user, I am using terminal every day. One day I find the term "terminal" should be precisely named as "terminal emulator", I decide to dig more into the mystery of TTY. TTY stands for teletype and it has a long history all the way back to 1800s even before ASCII was invented. It was introduced into UNIX in 1900s for multiple users to talk to the operating system. By then the physical teletype is equipped with ticker tape (for input) and the paper printer (for output). It was gradually evolved to hardware emulated teletype. CRT and keyboard was made to replace paper printer and ticker tape. Now days keyboard and monitor are equipped with every Laptop and PC. People use software emulated terminal to talk to kernel or applications. But the legacy from the old cast-iron is till lurking beneath the surface. How kernel treats physical teletype and software emulated teletype doesn't vary as much as hardware evolves.

Physical teletype

Ken Thompson developped UNIX on PDP7 with it.

Teletype Model 33

Hardware emulated teletype

DEC VT05

DEC VT100

Software emulated teletype

Now days physical and hardware emulated teletypes are practically kept in computer museum. People install software emulated terminal in graphical desktop environment to manage day to day jobs. Some well known software terminal emulators: urxvt, xterm, konsole, iterm2, st, kitty. In modern UNIX or Linux distros, mutiple software emulated terminal processes can run concurrently by a single user or multiple users, but all of them are attached to same tty (who(1)) which usually assigned for graphical desktop environment. How to distinguish different terminal processes as a logical tty? Here comes PTS - pseudo-teletypes. In Linux there is a master multiplexor ptmx(4) /dev/ptmx manages all master PTM slave PTS /dev/pts/* pair. The slave PTS number can be checked with tty command.

IO control commands

termios(3), tty_ioctl(4), stty(1)

Print TTY device setting

$ stty -a
speed 38400 baud; rows 75; columns 254; line = 0;
intr = ^C; quit = ^\; erase = ^?; kill = ^U; eof = ^D; eol = <undef>; eol2 = <undef>; swtch = <undef>; start = ^Q; stop = ^S; susp = ^Z; rprnt = ^R; werase = ^W; lnext = ^V; discard = ^O; min = 1; time = 0;
-parenb -parodd -cmspar cs8 -hupcl -cstopb cread -clocal -crtscts
-ignbrk -brkint -ignpar -parmrk -inpck -istrip -inlcr -igncr icrnl ixon -ixoff -iuclc -ixany -imaxbel iutf8
opost -olcuc -ocrnl onlcr -onocr -onlret -ofill -ofdel nl0 cr0 tab0 bs0 vt0 ff0
isig icanon iexten echo echoe echok -echonl -noflsh -xcase -tostop -echoprt echoctl echoke -flusho -extproc

Current PTS

$ tty
/dev/pts/1

$ who
hjl      tty1         2025-02-09 13:18

$ tty -s && echo "In a tty"
In a tty

Interesting reading

https://www.linusakesson.net/programming/tty/