[SA] ProFTPd

因為有人推薦 ProFTPd，所以這次就捨棄 PureFTPd 來用用看 ProFTPd 吧。

先來簡介一下 ProFTPd 。

ProFTPD (short for Pro FTP daemon) is a FTP server. ProFTPD is Free and open-source software, compatible to Unix-like systems and Microsoft Windows (via Cygwin). Along with vsftpd and Pure-FTPd, ProFTPD is among the most popular FTP servers in UNIX-like environments today. Compared to those, which focus e.g. on simplicity, speed or security, ProFTPD's primary design goal is to be a highly feature rich FTP server, exposing a large amount of configuration options to the user.

ProFTPd 在 FreeBSD 裏面的位置是 /usr/ports/ftp/proftpd
cd 到那裡然後 make install clean 之後就能順利安裝了：Ｄ

[SA] ZFS

ZFS 概觀

ZFS 是 Sun Microsystems 為 Solaris 作業系統 (Unix 的一支) 開發的檔案系統。
ZFS 具備 logical volume management 的能力 (稱為 Zpool)、也提供快照(snapshots)、copy-on-write 副本與自動修復的功能。

不同於傳統的檔案系統是建立在單一的磁碟分割之上，ZFS 是建立在虛擬的儲存池(pool)之上，稱為 Zpool。Zpool 可以是由多個 volume 所組成，之後檔案系統再透過 zpool 取得使用空間。ZFS的另一項特點就是 128 位元的設計，理論上可以 2 的 48 次方個檔案，每個檔案最大可以有 16EB，最大單一 volume 可達 16EB。

在資料安全與穩定性方面，ZFS 採用 copy- on-write 與 snapshots 計術來進一步維護檔案系統的正確性。所謂 copy-on-write 簡單的說就是每當檔案有變更時，不是直接覆蓋舊 有的資料，而是將使用中的區塊複製出來，而這些變更是在這複製的區塊上。Copy-on-Write 最大的好處就是當資料變更時，舊有的資料依然能夠維持，方便作為復原之用。Snapshot 就是保存著這些就有資料的映像副本的技術。ZFS 有著另一項先進的計術，就是透過刪除重複的資料 (Deduplication) 來節省儲存的空間。

[IOS] Ch1 Introduction note #3

Process Management

• A process is a program in execution. It is a unit of work within the system. Program is a passive entity, process is an active entity.
• Process needs resources to accomplish its task
• CPU, memory, I/O, files
• Initialization data
• Process termination requires reclaim of any reusable resources
• Single-threaded process has one program counter specifying location of next instruction to execute
• Process executes instructions sequentially, one at a time, until completion
• Multi-threaded process has one program counter per thread
• Typically system has many processes, some user, some operating system running concurrently on one or more CPUs
• Concurrency by multiplexing the CPUs among the processes / threads

[IOS] Ch1 Introduction note #2

Caching

• Important principle, performed at many levels in a computer (in hardware, operating system, software)
• Information in use copied from slower to faster storage temporarily
• Faster storage (cache) checked first to determine if information is there 
• If it is, information used directly from the cache (fast)
• If not, data copied to cache and used there
• Cache smaller than storage being cached
• Cache management important design problem
• Cache size and replacement policy

[IOS] Ch1 Introduction note #1

Overview

       An operating system acts as an intermediary between the user of a
computer and the computer hardware. The purpose of an operating
system is to provide an environment in which a user can execute
programs in a convenient and efficient manner.

        An operating system is software that manages the computer hard-
ware. The hardware must provide appropriate mechanisms to ensure the

correct operation of the computer system and to prevent user programs
from interfering with the proper operation of the system.

[C] argc & argv

一般來說 main function 長這樣：

int main(int argc, char *argv[])

其中，argc 代表的是參數個數，argv[argc] 代表的是指向參數，以下就用簡單的小程式來解釋。