[chord] replication in chord + dhash

[Frank Dabek]

Srinivas,

    I'm assuming here that you are using the most recent CVS. In this
version the -n flag is unused.

    The current CVS of DHash replicates blocks by creating 14 IDA encoded
fragments, 7 of which are necessary for reconstruction. These parameters
aren't currently adjustable.

    When half of the nodes in a system die, we would expect that some blocks
will become unavailable, namely those blocks which had more than 7of their
fragments stored on the failed nodes. The probability is a little messy due
to the use of erasure codes, but you get the idea.

How many blocks did you see unavailable?

--Frank

----- Original Message ----- 
From: "Srinivas V." <srini at cs.arizona.edu>
To: <chord at amsterdam.lcs.mit.edu>
Sent: Tuesday, May 27, 2003 11:22 AM
Subject: [chord] replication in chord + dhash


> Hello
>
>
> I recently built and set up CFS and have been trying to get it up and
> running. I have been running the lsd daemon on a few machines and
> storing/fetching blocks using the dbm client.
>
> Initially I ran some simple tests where I started lsd on two hosts. I used
> the dbm program on the hosts to store and fetch blocks i.e. I would store
> blocks on the ring in one host and was able to fetch them using dbm on the
> other host.
>
> When I kill lsd, on one of the the hosts and use dbm on the other host to
> try fetching blocks, some of the blocks are unavailable.I get error
> messages like:
>
> dbm: dhashclient::retrieve failed:
24ec132e88b234835fd9aa3f7c9c4fa91357d3f4: DHASH_NOENT
> dbm: dhashclient::retrieve failed:
7b94623bccedaf84c537684d62a21b9ff51d2b99: DHASH_CHORDERR
>
> I noticed that in lsd, one could pass the replication factor with the -n
> command line flag (I didn't set this initially since lsd's usage message
> didn't display this option). In later tests I did use this flag to set the
> replication factor at 8. For each of the two chord nodes I started, I set
> the no. of virtual nodes to 16. However, I still get the error message
> about blocks being unavailable. while starting lsd, I use the following
> options/flags:
> ./lsd -d /tmp/db-something -j host:port [-p port_for_bootstrap] -v
vnodes -n nreplicas
>
> I wanted to know if I was doing anything wrong and if there's anything
> else I have to set.
>
>      thanks
>
>      srinivas
>
>
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