False memory claims

Started by Kik84, Jun 17, 2022, 10:14 AM

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Kik84Topic starter

Hey,

I'm currently doing my 3 days free test of Upcloud and so far, I'm not pleased at all : their supposedly 1GB RAM plan only has... 907MB...

For comparison 1GB RAM VPS at:

Aruba offers:
Mem: 982M

Vultr offers:
Mem: 990M

Upcloud offers:
Mem: 907M

I don't even know how it can be legal to market 907M as being 1GB... you can't even "round" 907 to 1000 lol!!
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Sevad

I am not entirely sure why you think the providers are being "dishonest"?

free -m or another commands will not show ram which is reserved for the kernel and other applications. Depending on the software they use virtualize the VPS, this will be different per provider.
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praktik_man

Yeah, the storage marketing folks always use GB or TB for capacity since it appears larger than GiB or TiB, respectively. It could drive someone mad trying to figure out which unit people/providers actually mean.  I had always used MB, GB, etc. for base-2 sizing until a decade or more ago. I went to university in 1992, and the largest unit I needed to worry about was a Megabyte. 
When I was a kid, that was Kilobyte. Crazy how times have changed and we now need to know Petabyte and Exabyte, and possibly a Zettabyte!
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samfrank

RAM overselling is when neighboring virtual servers (nodes) are allocated more RAM in total than is physically available on the server. The situation is possible, since not all virtual machines and do not always use the maximum of available resources. Thanks to that, the provider can offer cheap VPS.

Problems arise if overselling is too strong, and at some point there is not enough memory for everybody. Then the systems use the swap file (swap file), that is, they access the disk, which has a very significant effect on the speed of work.

What does it look like in practice? Linux has an operating system-level cache (page cache) that resides in RAM. It stores frequently used files. If some process needs a file, and it is in the cache, everything works quickly. If the file does not appear in the cache, then it is read from disk. If the file is large, or you need a lot of them at once, and the disk is slow, there will be brakes.

So, when some nodes suddenly need RAM for processes, but it is already physically absent, the obvious candidate for uploading to disk is the web page cache of unloaded server neighbors. Perhaps excessive RAM overselling is bad in the first place because it causes accelerated and unpredictable flushing of the OS cache. But, of course, with insufficient allocation of space for swap, systems can generally crash.

If we talk about a site hosted on a VPS, then the page cache can contain, for instance, files with MySQL tables. And although MySQL has its own cache, it is not at all equivalent to the OS cache. Constant reading of large tables from the disk - the slow operation of the website.

Determining if your VPS is suffering from overselling is not as easy as one would like. You need to fill the page cache with files and access them periodically, monitoring disk activity.
Such a script was written for the "VPS performance tests" section. RAM overselling data is available in most advanced reports.
generally, based on the results of testing two dozen low-cost KVM VPS, we can say that excessive RAM overselling is not a very common occurrence, although it does occur.
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