[Gluster-users] EC clarification

[Xavier Hernandez]

Hi,

On 27/08/16 10:57, Gandalf Corvotempesta wrote:
> In short: how can i set the node hosting the erasure codes? In a 16+4 EC
> (or bigger) i would like to put the 4 bricks hosting the ECs on 4
> different servers so that i can loose 4 servers and still be able to
> access/recover data

EC builds several fragments of data for each file. In the case of a 16+4 
configuration, a file of size 1MB is transformed into 20 smaller files 
(fragments) of 64KB.

To recover the original file *any* subset of 16 fragments is enough. 
There aren't special fragments with more information or importance.

If you put more than one fragment into the same server, you will lose 
all the fragments if the server goes down. If there are more than 4 
fragments on that server, the file will be unrecoverable until the 
server is brought up again.

Putting more than one fragment into a single server only makes sense to 
account for disk failures, since the protection against server failures 
is lower.

Xavi

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