* NOTE On blocks and devices

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 - State "NOTE"       from              [2024-03-02 Sat 21:30]

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 [[https://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/dev.html][/dev]]

 In Linux, everything is a file.

 /dev/ contains special device files - usually block or character

 device.

 major, minor = category, device

 0, 5

 mknod - create special device files

 [[https://www.redhat.com/en/blog/linux-block-devices-hints-debugging-and-new-developments][redhat hints]]

 #+begin_src shell

   dd if=/dev/zero of=myfile bs=1M count=32

   losetup --show -f myfile

   ls -al /dev/loop0

   losetup -d /dev/loop0 #teardown

 #+end_src

 #+begin_src shell

   echo "sup dude" > /dev/loop0

   dd if=/dev/loop0 -bs=1

   dd if=/dev/nvme0 of=/dev/null progress=true

   #pacman -S hdparm

   hdparm -T /dev/nvme0

 #+end_src

 #+begin_src shell

 modprobe scsi_debug add_host=5 max_luns=10 num_tgts=2 dev_size_mb=16

 #+end_src

 sparsefiles: create with C, dd, or truncate

 #+begin_src shell :results output

 truncate --help

 #+end_src

 #+RESULTS:

 #+begin_example

 Usage: truncate OPTION... FILE...

 Shrink or extend the size of each FILE to the specified size

 A FILE argument that does not exist is created.

 If a FILE is larger than the specified size, the extra data is lost.

 If a FILE is shorter, it is extended and the sparse extended part (hole)

 reads as zero bytes.

 Mandatory arguments to long options are mandatory for short options too.

   -c, --no-create        do not create any files

   -o, --io-blocks        treat SIZE as number of IO blocks instead of bytes

   -r, --reference=RFILE  base size on RFILE

   -s, --size=SIZE        set or adjust the file size by SIZE bytes

       --help        display this help and exit

       --version     output version information and exit

 The SIZE argument is an integer and optional unit (example: 10K is 10*1024).

 Units are K,M,G,T,P,E,Z,Y,R,Q (powers of 1024) or KB,MB,... (powers of 1000).

 Binary prefixes can be used, too: KiB=K, MiB=M, and so on.

 SIZE may also be prefixed by one of the following modifying characters:

 '+' extend by, '-' reduce by, '<' at most, '>' at least,

 '/' round down to multiple of, '%' round up to multiple of.

 GNU coreutils online help: <https://www.gnu.org/software/coreutils/>

 Full documentation <https://www.gnu.org/software/coreutils/truncate>

 or available locally via: info '(coreutils) truncate invocation'

 #+end_example

 test mkfs.btrfs on 10T dummy block device

 #+begin_src shell

   dd if=/dev/zero of=/tmp/bb1 bs=1 count=1 seek=10T

   du -sh /tmp/bb1

   losetup --show -f /tmp/bb1

   mkfs.btrfs /dev/loop0

 #+end_src

 diagnostics

 #+begin_src shell

   iostat # pacman -S sysstat

   blktrace # paru -S blktrace

   iotop # pacman -S iotop

 #+end_src

 bcc/ trace: Who/which process is executing specific functions against

 block devices?

 bcc/biosnoop: Which process is accessing the block device, how many

 bytes are accessed, which latency for answering the requests?

 at the kernel level besides BPF we got kmods and DKMS,

 compression/de-duplication can be done via VDO kernel mod

 https://en.wikipedia.org/wiki/Dynamic_Kernel_Module_Support

 * NOTE save-lisp-and-respawn

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 - State "NOTE"       from              [2024-03-02 Sat 22:57]

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 #+begin_src lisp

   sb-ext:*save-hooks*

 #+end_src

 * NOTE syslog for log

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 - State "NOTE"       from              [2024-03-03 Sun 16:35]

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 sb-posix:

 - openlog syslog closelog

 - levels: emerg alert crit err warning notice info debug

 - setlogmask