According to Wikipedia Erasure Coding means:
In coding theory, an erasure code is a forward error correction (FEC) code under the assumption of bit erasures (rather than bit errors), which transforms a message of k symbols into a longer message (code word) with n symbols such that the original message can be recovered from a subset of the n symbols. The fraction r = k/n is called the code rate. The fraction k’/k, where k’ denotes the number of symbols required for recovery, is called reception efficiency.
So Raid systems applied to drives are Erasure Code too.
But I want to talk about Erasure Code for the needs of organizations like Instagram, that need to store huge amount of files and they cannot afford to lose the data simply because several drives, or all the Server, fails.
So what is the way to make this sure if you have thousands of Servers?.
Many Start ups that require to host files, cannot afford to have every file duplicated or triplicated in other systems.
So how to do this in a cheap an efficient way?.
Here is where Erasure Coding comes to play.
Erasure Coding work so simply as:
- Given a given file, for example, 1 video of 10 MB
- We apply the Erasure Coding to encode the file
- We select, for example, to generate 3 additional chunks
- So our original 10MB file fill be split in 13 blocks (13 new files), each block will have approx. 1MB
- We can rebuild the original file by combining any 10 of those 13 files
That means that we can afford to loss 3 blocks (1MB files) and we will still be able to reconstruct the original file.
- Ok, so now imagine we have 13 identical Servers, and we encode all our files, using Erasure Coding. Imagine that we store each block in a different Server. That means that we can lose 3 Servers and still have all our information intact.
- Imagine we have 100 Servers, and we split all those files to the Servers that have more free space available. We could lose 3 Serversand still not having lose any information. If we are really lucky (or the SDS – Software Defined Storage is very clever) we could lose more than 3 Servers.
- Now imagine we have 100 Racks full of Servers. Our SDS selects the Rack that has more free space and places one of the blocks in there, and the same for the other 12 blocks. We could afford to lose 3 racks without losing any Data. That’s more manageable for Google or Yahoo than managing at Server Level.
We can use Erasure coding with different configs like 8+3, or 10+4… The sample I choose 10+3 is easy to understand, as we clearly see that will occupy only 30% of additional space.
Those blocks can conveniently be stored in different Servers, across different regions too, for example, using a config of 9+3 you can have 4 different Cloud Providers in different geographic regions, and each holding 25% of the required files, so 3 files each. Then, you only require 3 Cloud providers to rebuild the original file (you only precise 9 surviving blocks, not all 12). Possibilities are infinite.
When one Rack is down, you can rebalance all the blocks that were there to another rack.
Also you can have different Servers, with different capacity… your SDS should be clever enough to accommodate the blocks for protection and space efficiency. To checksum them to ensure no corruption in the block as was stored or transported over the network. Your SDS Software should be clever enough to be able to add new nodes and Racks, and to substract nodes, to Rebalance, to checksum the blocks in the Servers… and to store the information effectively on the local Servers (not many files per folder…), to use Commodity Hardware with low memory, or even VM’s… if your System is good enough it will even put to sleep, to save energy, the Servers that are not in use (typically the Servers that are full), until required.
Also, when in need to recover a file, the clever SDS Software, using multithread, will ask to the 9 locations at the same time, in parallel, so using all the available bandwidth, in order to fetch the blocks and rebuild the original file really quick. This can also be implemented with no single point of failure, will all the nodes being able to be the headnode.
That’s exactly what my Erasure Coding solution did.
I invented a lot of technologies to scale out since I created my messenger in 1996.
You can do it yourself, or use existing Erasure Coding solutions. The most known is OpenStack Swift, although in my opinion is a pain to configure and to maintain.