Friday, August 16, 2019

Alexa, where's my RAM?

I regularly use very small AWS EC2 instances for things like jump servers. In particular, I use both t2.nano, and t3a.nano instances running CentOS 7. These instance types are both sold as having 0.5 GB RAM.

So I was a little surprised when I took a t2.nano instance, and upgraded it to t3a.nano, and found it to have less RAM available. This was the exact same OS boot volume, so no changes in kernel or anything like that:

[rah@t2.nano ~]$ grep MemTotal /proc/meminfo 
MemTotal:         497116 kB

[rah@t3a.nano ~]$ grep MemTotal /proc/meminfo 
MemTotal:         478304 kB

That's a reduction of almost 20 MB RAM when moving to the newer platform. Normally, I wouldn't care about 20 MB, but when you're dealing with an OS running on supposedly 512 MB to start with, this reduction is not good.

So the question is, why? They're both sold as being 0.5 GB, and since it's all virtual, Amazon can allocate however much they like per instance type. If there is a valid technical reason, e.g. the same amount of RAM presents differently due to different mapping, maybe that's OK.

But this just feels slightly icky to me, as if Amazon are trying to squeeze an extra instance or two onto their next generation hypervisors, with the same physical capacity.

No comments: