All Questions(server)

Let's Encrypt are providing free SSL certificates. Are there any downsides compared to other, paid certificates e.g. AWS Certificate Manager?

There is a server that is used from 4:30 am in the morning until ~ 22:00.

Should it be turned off? I think that it is a server and that it won't have a problem to stay on, but serious professors are telling me that it is dangerous and that HD can fail within 2 years. The server owner believes that his old server running from 1995 without backup and a single hard disk (if the hard disk fails he is screwed) had no problem because he used to turn it off at nights.

What do you believe for this?

Now it has a RAID 1 array, external hard disk backup, and serveral full hard disk backups on DVD and over the internet.

This is a software design question

I used to work on the following rule for speed

cache memory > memory > disk > network

With each step being 5-10 times the previous step (e.g. cache memory is 10 times faster than main memory).

Now, it seems that gigabit ethernet has latency less than local disk. So, maybe operations to read out of a large remote in-memory DB are faster than local disk reads. This feels like heresy to an old timer like me. (I just spent some time building a local cache on disk to avoid having to do network round trips - hence my question)

Does anybody have any experience / numbers / advice in this area?

And yes I know that the only real way to find out is to build and measure, but I was wondering about the general rule.

edit:

This is the interesting data from the top answer:

• Round trip within same datacenter 500,000 ns

• Disk seek 10,000,000 ns

This is a shock for me; my mental model is that a network round trip is inherently slow. And its not - its 10x faster than a disk 'round trip'.

Jeff attwood posted this v good blog on the topic http://blog.codinghorror.com/the-infinite-space-between-words/

Sometimes I forget how the exact syntax of a CMD command looks and then I would like to search my own CMD history. Clearly, within the same session, you can browse it with the up and down arrow keys but what about the history of former CMD sessions? Is there a file, a log the history gets written to or does it all go to digital Nirvana?

Thanks!

I heard recently that Nginx has added caching to its reverse proxy feature. I looked around but couldn't find much info about it.

I want to set up Nginx as a caching reverse proxy in front of Apache/Django: to have Nginx proxy requests for some (but not all) dynamic pages to Apache, then cache the generated pages and serve subsequent requests for those pages from cache.

Ideally I'd want to invalidate cache in 2 ways:

1. Set an expiration date on the cached item
2. To explicitly invalidate the cached item. E.g. if my Django backend has updated certain data, I'd want to tell Nginx to invalidate the cache of the affected pages

Is it possible to set Nginx to do that? How?