question How does active namenode write to journal nodes in a HA configuration? in Archives of Support Questions (Read Only)

How does active namenode write to journal nodes in a HA configuration?

keagles — Wed, 12 Oct 2016 21:03:50 GMT

I am not 100% sure how an active namenode write to Jounral nodes. Considering the folloiwng example:

2 namenodes(active+standby)

3 journal nodes

When the active namenode updates edit file, does it write 1/3 copies of the updated edits to each jn? or it write a complete copy to each jn?

When standby nn read the edites from jn, does it read 1/3 copy from each jn or the entire copy from each jn? If it's later, what if the edit md5 doesn't match across the jn nodes?

Thanks!

Re: How does active namenode write to journal nodes in a HA configuration?

nt89 — Wed, 12 Oct 2016 23:28:52 GMT

The latter. Active NN will write every edit to all available journal nodes. A majority is confirm is required before the transaction is committed - that is, 2 out of your 3 journal nodes will have to have accepted the write.

In the case of the standby NN reading from the journal nodes, it will again look for a majority and discard the discrepancy.

I think if one out of three JN are down, and the final two don't agree, it will accept the most up to date JN's answer. But I could be completely wrong on this.