Lazer's questions

I understand semaphores, but what are these semaphore arrays being used on my Linux box?

$ ipcs

------ Shared Memory Segments --------
key        shmid      owner      perms      bytes      nattch     status      
0x00000000 327681     root      644        80         2                       
0x00000000 360450     root      644        16384      2                       
0x00000000 393219     root      644        280        2                       
...

------ Semaphore Arrays --------
key        semid      owner      perms      nsems     
0x4172d4f4 290914305  lazer     660        104       
0x3b87b970 291045378  lazer     660        104       
0xa97eb380 293928963  lazer     660        104       
0x1fde2040 294191108  lazer     660        104       

------ Message Queues --------
key        msqid      owner      perms      used-bytes   messages    

$

Also, which OS resource are they guarding?

When I type a Fully Qualified Domain Name (FQDN) in my URL bar, the browser connects to some mapping table (where is this table stored? with the service provider?) that determines where the file requested is hosted.

So, it depends on the service provider (who I am assuming has the conversion table) whether IPv6 addresses are supported or not. Is that true?

Are they shared servers or just low quality (what exactly is a low quality server)?

How are they cheaper than mainframe servers?

Technically, what is the difference between the way of hosting (on the server) of

• a file that can be paused while downloading and resumed again, and
• a file that needs to be downloaded in one go?

If the robots.txt file is missing in the root directory of a website, how are things treated as:

1. the site is not indexed at all
2. the site is indexed without any restrictions

It should logically be the second one according to me. I ask in reference to this question.

I am wondering why websites crash at all.

If a server has too many requests, it might queue up the request in its waiting lists and serve it when all the earlier requests have been served. That means that the request for the website will be taken care of, although it may take some more time than expected.

Then, how do websites crash due to server overload?