If you are using a ufs filesystem, the system attempts to optimize the time it takes to perform operations on the filesystem. In certain circumstances, the system attempts to optimize the space on the storage media being used by the filesystem.

Space optimization occurs when file fragmentation exceeds: (free – 2) % of free disk space. When file fragmentation exceeds this value, this message is displayed:

Optimization changed from TIME to <Space>

and the system will begin to optimize the space used by the filesystem.

The value of free is, by default, 10%, but it can be set to any value using the mkfs(1M) command. The main use of free is to specify the minimum percentage of free disk space allowed. It is also used to limit the fragmentation of the filesystem.

When a file grows past an 8K boundary under time optimization, the system allocates a fragment (which is 1K in size) to contain the data. When the boundary of the fragment is exceeded, the system allocates a new 8K block, and the fragment is copied to it. This means that there could be nearly 7K of wasted space at the end of some files.

With space optimization, however, whenever a file grows past a 1K boundary, all the fragments since the last 8K boundary are copied into a new, larger fragment.

If your system has swapped to space optimization, and you want to revert to time optimization, you must increase the value of free. Use tunefs(1M) to do this. You can increase free to any value you want; however, keep in mind the two functions of the free parameter if you want to set realistic values. free is used to do the following:

 

• Set the minimum percentage of free disk space allowed. Therefore, if you set the value of free too high, ordinary (nonprivileged) users will not be able to write files (as described in “Creating a filesystem with mkfs”).

   

• Set the maximum percentage of free disk space that can be fragments. Therefore, if you set the value of free too low, the optimization will change from time to space and file writes will be slower.

Set the value of free carefully, taking into account the typical file size and the number of directories on your system.

A switch works on a low level: it only knows MAC addresses. When it receives a broadcast message it will send that message to all of its ports (like it is supposed to do). Your machine maneges an arp table (try 'arp -a'). This table makes sure that your machine knows on which MAC address which ipaddress is so it can do an Internet-to-Ethernet translation. When the broadcast reaches your machine it will be received on both your NIC's. Arp notices that it gets a package from a MAC address of which it thought was on interface 0 on interface 1 and reports this problem. I think you might agree with me thatthis is a good thing. But everything works fine and there are situations where such a setup is desired and this problem generates allot of messages. These messages are going to hide some of the important other messages because of the amount of them so you realy want to get rid of them.

 意思是这样的：交换机只会认MAC地址。当收到广播信息时，它会把广播消息发送到本交换机的每个端口上。你的机器维护着一个路由表(命令行下输入arp -a可以查看这个路由表)，这个表确保你的机器能识别某一个MAC地址对应的IP地址，并因此可以完全Internet到Ethernet的转换。从交换机传来发广播信息会发送到两个网卡上，arp发现有一个错误的数据包，它以为是从网卡0上发送过来的，但事实上却是网卡1，于是就向系统报告了这个问题。你可能跟我想的一样，认为这是个好事情。一切工作正常，而这里却不停止的发送大量的错误报告(作者笔误，应该是 a lots of messages)。大量这些信息频繁出现以至于你严重影响到你阅读其他的一些重要的信息，于是你特别特别想把这些信息除去。

解决办法：
这里就不多说了，作者提供了四种办法，但明显我们不会使用其他办法：1是忽略这些信息，2是从源代码中去掉这些信息，3是改进一下网络连接，连接路由器（作者原话是rewire，中文是我猜的，路由器可以有效的过滤一些广播信息），4是调整内核参数。有效而且容易实施的办法是4，在FreeBSD下可以如下操作：

root@host:/#sysctl -w net.link.ether.inet.log_arp_wrong_iface=0然后在/etc/sysctl.conf(如果文件不存在，请自行创建)添加一行：net.link.ether.inet.log_arp_wrong_iface=0

其他*nix系统也是相同操作，不过大部分Linux默认内核设置里就已经设置为了0原文地址：http://www.freebsdhowtos.com/102.html

        tr -d \r < dosfile > newfile
                — Originally by Dru <genesis@istar.ca>

就这一句，NB，谢谢mudone ，谢谢那位我不知道的混smth的牛。

        sed -i.bak s/e/o/g foo

And you’ll get a backup of the original in a file named ‘foo.bak’, but if you
want no backup:

        sed -i ” s/e/o/g foo

        alias lf="ls -FA"
        alias ll="ls -lA"
        alias su="su -m"

In csh or tcsh, these would be

        alias lf ls -FA
        alias ll ls -lA
        alias su su -m

To remove an alias, you can usually use ‘unalias aliasname’.  To list all
aliases, you can usually type just ‘alias’.

        TERM=xterm-color; export TERM

in Bourne-derived shells, and

        setenv TERM xterm-color

in csh-derived shells.