Re: [Moosefs-users] How are chunks spread across MFS?

[Ricardo J. B. <ric...@da...>]

El Lunes 01 Agosto 2011, Darren Webb escribió:
> Hello,

Hi!

> I understand that files are split into chunks and each chunk is a file
> on a drive specified mfshdd.cfg on one of the file servers.   Two
> questions:
>
> 1) For any given file, are all chunks for a single copy kept on the same

I asssume you mean all copies of a single chunk :)

> chunkserver, or can they be spread across different servers?  If so, is
> each chunk copy guaranteed* to be stored on different chunk servers? And
> if not, would this imply that setting higher goal means improved
> performance?

Every copy of a chunk is always stored on a different chunkserver, so if you 
lose one chunkserver you can still use the copy from another chunkserver, as 
long as you have set a goal > 1.

A higher goal doesn't necessarily mean increased performance, at least that's 
not MFS's goal (I think this is in the FAQ if you want more about it).

> 2) Can chunks from two copies of the same file be stored on the same
> chunkserver?

Not in the normal case. You might have two chunkserver processes running on 
the same server and defeat both, my answer and you high availability :)

> 3) If my goal for all files is N, is it guaranteed* to withstand N-1
> simultaneous chunkserver failures?

Hey, you said *two* questions! :)

A file with goal = N is guaranteed to withstand N-1 chunkserver failures, yes.

Also, when a chunkserver goes down, MFS replicates undergoal chunks to other 
chunkservers to achieve goal = N again.

Cheers,
-- 
Ricardo J. Barberis
Senior SysAdmin / ITI
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