Question

maxal on Wed, 06 Jan 2016 19:26:35


Recently, January 3rd, we observed interesting behavior with Redis Cache in Azure. It happened just once, and I'm trying to make sense of it.

We got alert that CPU went above 80% on Redis Cache service. Looking closely we discovered that used memory was dropped from typical 100MB to almost 0. Then it was quickly populated back to normal, I assume by normal usage of the application. While it was being populated, there was this CPU spike.

It looked like if cache was reset. However, this is production environment with very limited people having access to it, and we sure 100% that nobody reset it. There were no any deployment around that time. I couldn't find anything in diagnostic logs.

Updated: we are on standard price tier (C1)

Questions:

1. Any ideas what could happen?

2. Where can I look, what to look for?

No customers reported any problems, I just hate when I don't understand what is going on.



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Replies

Edward Bakker on Wed, 06 Jan 2016 20:26:40


Hi,

What tier of Azure Redis are you using? The basic tier runs only on one VM. Any issue with the VM can result in a loss of the cache (which is stored in memory).

Standard tier uses replication to store the cache on two VMs. In this case it is less likely that you lose the data in the cache. In the premium tier your cache is persited so it won't suffer from any issues on the VMs.

So, in case you are using the basic tier, issues with the VM are probably the reason for this behavior.

Hope this helps!

Edward

maxal on Wed, 06 Jan 2016 21:46:27


Edward, thanks for idea. I missed this important part, we are on Standard Tier, so there is replication and SLA.