tips
2015-05-24
Background: Virtualbox virtual machine on Windows. Ubuntu 14.04.1 LTS, 3.13 kernel. ext4 file system.
Mistake: A few days ago, I was developing a website on this virtual machine, made N commits, thought I had git pushed, but in fact, the push failed.
Mystery: Today, my colleague git pulled and found no updates, then said I hadn’t been working these days.
Tragedy: Logged into the virtual machine and saw that a few newly written files in the project directory had become 0-byte empty files. (ext4 is so stable, it must be the evil NTFS and Virtualbox in the host machine that caused the trouble)
Many files in the .git directory also became 0-byte empty files. Git prompts that the repository is damaged.
1 | $ git status |
1 | $ git fsck |
Has the code written in the past few days just vanished like this? We know that when you delete something, you just delete the reference to this thing in the current three-dimensional space, and the entity of this thing still exists in the four-dimensional space-time. Let’s time travel!
2014-11-25
Many people are confused about the difference between these two sets of concepts. Let’s use the example of Xiao Ming downloading a file.
- Synchronous Blocking: Xiao Ming keeps staring at the download progress bar until it reaches 100%.
- Synchronous Non-blocking: After Xiao Ming starts the download, he goes to do other things, occasionally glancing at the progress bar. When it reaches 100%, the download is complete.
- Asynchronous Blocking: Xiao Ming switches to a software that notifies him when the download is complete. However, he still waits for the “ding” sound (which seems silly, doesn’t it?)
- Asynchronous Non-blocking: Still using the software that “dings” when the download is complete, Xiao Ming starts the download and then goes to do other things. When he hears the “ding”, he knows the download is complete.
In other words, synchronous/asynchronous refers to the notification method of the download software, or the API being called. Blocking/non-blocking refers to Xiao Ming’s waiting method, or the API caller’s waiting method.
2014-07-06
Those who use the ext file system to store data may have noticed that a 200 G partition can only use 190 G, and a 2000 GB partition can only use 1900 GB. Where did this 5% disk space go? Some people think this is the difference between the decimal system (1000) and the binary system (1024), but this is not the case.
1 | $ df /mnt/sdb/ |
The available space of the above-mentioned 2 T disk is 0, which seems to be full. Try to create a directory and find that there is no space left.
1 | $ mkdir hello |
But sudo mkdir hello can succeed! What’s going on? The answer lies in the “reserved space” of the ext file system.
2014-02-10
With the improvement of computer processing capabilities and the increasing complexity of software, performance is often not the most important criterion for measuring software. But sometimes we do need to squeeze the performance of the computer. Especially when doing research, in order to make the performance indicators surpass the opponent, not only the algorithm (asymptotic complexity) needs to be optimized, but also the implementation (the constant in the complexity) needs to be optimized. This article tries to summarize some rules and hopes to discuss with everyone:
Do not use open source software
Open source software often considers a general problem, so there are many configuration parameters and conditional judgments that are almost never used; open source software often requires code readability and maintainability higher than performance, so it generally does not use so-called “tricks”.
2013-11-22
Sometimes, we need to remotely access websites that can only be accessed from the server’s network, such as the router’s Web control panel. Using links on the server is obviously unreliable. We can establish a tunnel between the local machine and the server using ssh, allowing the local browser to access restricted websites through the tunnel.
First, use ssh -D to establish a socks5 tunnel between the local machine and the server: (60000 is an arbitrary integer greater than 1024 and less than 65536)
1 | ssh -C2qTnN -D 60000 user@remote-host |
Then, let Chrome access through the socks5 tunnel. The Chrome plugin ProxySwitchy no longer works because Chrome no longer supports NPAPI, and ProxySwitchySharp sometimes doesn’t work. Some articles online say that chrome –proxy-server is enough, but in fact, it doesn’t work when you have another Chrome instance open, because Chrome will automatically find the open instance.
A more reliable method is to open Incognito mode and use a non-existent Chrome user data directory to prevent it from finding open instances. After use, it is best to delete the newly created user data directory (in the following example, it is C:\Temp\Chrome). Note that the following socks5 cannot be replaced with http, as they are different protocols.
1 | PS C:\Program Files (x86)\Google\Chrome\Application> .\chrome.exe --proxy-server="socks5://127.0.0.1:60000" -incognito -user-data-dir=C:\Temp\Chrome |
2013-10-07
Those who have used Windows Vista/7/8 may have had this experience: after modifying a file on the C drive with a 32-bit program (such as cygwin), when you look at it from the Windows Explorer, it’s still the version before the modification! Does the file system have different views for different programs? You’re right, since Vista introduced UAC and VirtualStore, don’t trust the changes made by 32-bit programs in the C drive.
After Windows Vista introduced stronger security mechanisms, some important system directories are not modifiable by everyone. These directories include the C drive root directory, Program Files, Program Files (x86), Windows, and the registry’s HKEY_LOCAL_MACHINE, etc. But some old applications still assume these directories are writable, and if the system API simply returns access denied, these programs can’t run.
Therefore, Vista provides VirtualStore. For 32-bit programs running without administrator privileges, as long as there are write operations to these directories, the modified or added files will be copied to this user’s VirtualStore. The file at this path seen by the 32-bit program running under this user’s identity is the corresponding file in VirtualStore, and it knows nothing about any modifications to the file at the original path.
2013-07-25
The wireless network at Vantone Inn requires username and password authentication. On Windows, as soon as you connect to the “Vantone Inn” access point, a browser window will pop up, displaying the user authentication interface. Visiting any HTTP address will also redirect to the authentication interface. Once authenticated, the wireless network is unobstructed. Today, a classmate asked me, how can this access point be so NB, can force the computer to pop up a window, is it a virus?
I don’t know the answer to this question, so I captured some packets and then Googled it. For ease of understanding, let’s not discuss the automatic pop-up of the browser first, let’s think about how to implement “visiting any HTTP address will redirect to the authentication interface”.