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From Harsh J <ha...@cloudera.com>
Subject Re: backup node question
Date Sun, 07 Apr 2013 07:31:02 GMT
Hi Lin,

My reply inline.

On Sun, Apr 7, 2013 at 12:36 PM, Lin Ma <linlma@gmail.com> wrote:
> Hi guys,
> I am reading from this paper to learn about backup nodes
> (http://www.storageconference.org/2010/Papers/MSST/Shvachko.pdf),
> It is mentioned, "It contains all file system metadata information except
> for block locations. It can perform all operations of the regular NameNode
> that do not involve modification of the namespace or knowledge of block
> locations. ", what kinds of operations do not need knowledge of block
> locations?

Operations that do not involve data reads or writes would not require
knowledge of block locations. Applying also the restriction of no
namespace mutation, an example would be listing directories and
looking up file information via FileStatus objects (perhaps the only
examples - its like a safemode but no reads either).

> It is also mentioned, "Use of a BackupNode provides the option of running
> the NameNode without persistent storage, delegating responsibility for the
> namespace state persisting to the BackupNode.", what means "running the
> NameNode without persistent storage" and "delegating responsibility for the
> namespace state persisting"?

What it means is that the NameNode need not store anything locally,
but can rely on the edits being stored at the BackupNameNode which
would continuously be receiving it. When restarted, it can grab a
current checkpoint from the BNN and boot up anywhere, since there's no
local storage requirement.

Harsh J

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