Community Blog Friday Q&A - Week 16 - Questions From The Vault

Friday Q&A - Week 16 - Questions From The Vault

We answer some database-related user questions this week. Ever wondered which disk type to choose, or how to do cross-region HA? Join us!

By Jeremy Pedersen

Welcome back for the 16th installment in our weekly blog series! This week we're doing something we haven't done in a while: answering those burning user questions!

Alibaba Cloud RDS instances can have both "Local disks" and "SSD disks" : what's the difference?

This is a really excellent question, since there's a lot to think about!

First, let's clear up some terminology:

  1. "Local disk" = an SSD drive attached to the physical server hosting your RDS instance.
  2. "SSD disk" = Cloud Disk. That's right, the same Cloud Disk service used by Alibaba Cloud ECS.

First, when you purchase an ApsaraDB for RDS database, you must choose four things:

  1. Database engine (MySQL, MariaDB, MS SQL, or PostgreSQL)
  2. Edition (Basic, High Availability, Enterprise)
  3. Storage type (local disk, SSD cloud disk)
  4. Instance specification (CPU, RAM)

Choices 1, 2, and 3 are permanent: once you settle on a database type, edition, and storage type, you cannot change these later (except by backing up your database and restoring onto a new, different RDS instance). Choice #4 is less permanent: you can upgrade or downgrade your instance's CPU and RAM settings as needed.

Further, some choices you make will affect others. For instance:

  1. If you choose "Basic Edition" RDS, you can only select the "SSD cloud disk" storage type.
  2. If you choose "High Availability Edition" RDS, you can select either "SSD cloud disk" or "local disk" as your storage type.
  3. If you choose "Enterprise Edition" RDS, you can only choose the "local disk" storage type.

Why do disk types matter?

The disk type you choose will affect:

  1. What types of backups you can make
  2. Your database's I/O performance
  3. The maximum size of your database

Let's take a look at the differences here:

Local disk pros: Extreme performance (I/O, latency), flexible backup and restore.
Local disk cons: More expensive, database size limited to 2 TB (MySQL). Does not support "Basic Edition" RDS.

Cloud disk pros: Cheap, I/O performance still quite good, database sizes up to 32 TB (MySQL).
Cloud disk cons: No physical or logical backup option. Does not support "Enterprise Edition" RDS.

As it turns out, the only time you have to make a decision about what disk types to choose is when using "High Availability Edition". With both "Basic Edition" and "Enterprise Edition", the choice is made for you by RDS:


What else should I think about before I hit that "Buy" button?

The RDS "Edition" you select (Basic, HA, or Enterprise) may affect which database engines and versions are available, so take a close look at the database engines being offered as you switch between editions.

What's the difference between PolarDB and RDS? It looks like they both support common engines like PostgreSQL and MySQL?

This is true: the database types supported are similar. From a user perspective, there isn't any (obvious) difference between them. Under the hood, however, there is a big difference: PolarDB is built with a decoupled storage/compute architecture that makes it possible for PolarDB databases to grow larger (up to 100 TB), handle more loads (up to millions of QPS), and even scale in and out horizontally (for some configurations). If you are planning to really scale up, PolarDB is a better choice than RDS.

Better still, PolarDB offers good support for migrations away from Oracle (see PolarDB-O).

Can I move from Alibaba Cloud RDS to a self-built database?

Yes! We aren't trying to lock you in. You can use Data Transmission Service (DTS) to:

  1. Migrate to RDS from your own non-RDS database
  2. Migrate from RDS to your own non-RDS database
  3. Migrate between two RDS databases
  4. Migrate between two non-RDS databases

DTS can even migrate between different database engines, say from MySQL to PostgreSQL. We're flexible that way.

I am using High Availability Edition RDS: can I put my primary and secondary database nodes in different Alibaba Cloud regions?

Unfortunately the answer is no. However, you can put them in different zones within a region, which will still allow your database to survive the failure of a single Alibaba Cloud datacenter within that region.

If you really really need multi-region redundancy, you can achieve it using DTS. There's a YouTube Video explaining how as well as this blog post by our Alibaba Cloud MVP Alberto which explains how to do active-active cross-region synchronization using DTS.

I've Got A Question!

Great! Reach out to me at jierui.pjr@alibabacloud.com and I'll do my best to answer in a future Friday Q&A blog.

You can also follow the Alibaba Cloud Academy LinkedIn Page. We'll re-post these blogs there each Friday.

Not a LinkedIn person? We're also on Twitter and YouTube.

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