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From Simon Felix ...@iru.ch>
Subject RE: Shared block storage via ZooKepper
Date Wed, 13 Jul 2011 18:13:42 GMT
Thanks for the suggestion but I gues I cannot use MapR for my purpose. I’m working on a non-commercial
hobby project that one day I might make commercial. I believe what I want to use/build is
simpler than a distributed file system because I don’t have to care about:

-          Metadata

-          Locking

-          Hierarchies

-          Access rights

-          Lookups

So if anyone knows of free, appropriately licensed alternative I’d be happy to use that.

From: Ted Dunning [mailto:ted.dunning@gmail.com]
Sent: Mittwoch, 13. Juli 2011 18:52
To: user@zookeeper.apache.org
Subject: Re: Shared block storage via ZooKepper


What you are describing is (roughly) a general read-write distributed and replicated file
system.  This is a hard problem if you want high performance, absolute consistency and significant
amounts of failure tolerance.  Building such a system from scratch is a difficult proposition.

Frankly, it also sounds just like the filesystem component of MapR (conflict alert, I work
for MapR Technologies).  You may have additional constraints on what you are looking for,
but to meet the requirements that you have already stated, you should take a look at our offering.
 I can imagine scenarios where this wouldn't be satisfactory, particularly if this is a homework
assignment, but if you are simply trying to solve a real engineering problem, it should do
very well.  I don't want to hijack this list with non-Zookeeper discussion so feel free to
contact me directly for more pointers.

Ohh... I should mention MapR uses Zookeeper prominently and is glad to do so.  The strictness
and durability of ZK are ideal as the last resort determinant of coordination.  In many areas
of our system, the ZK trade-offs are not appropriate, especially where speed is critical,
but that isn't what ZK was designed to do.  Using ZK appropriately gives extremely good results.
On Wed, Jul 13, 2011 at 5:15 AM, Simon Felix <de@iru.ch<mailto:de@iru.ch>> wrote:
Thanks for the reply. I’ll try to clarify my question a bit. I want to simulate a single,
fault-tolerant shared block storage device. This means everything should be replicated and
consistent. All that system manages is (for example) one billion blocks, each containing exactly
4096 bytes. I do not need any metadata per block or locking. There will be multiple nodes,
all reading and writing the data concurrently. If two nodes A and B write to the same block
concurrently I expect that all nodes have either version A or version B of the block afterwards.

I’m not sure which of the option is the easiest to implement and which will give me the
highest performance.

#2: Cassandra: Would you store the data in multiple rows? Columns? How much data per column?
I should probably ask the Cassandra people about this...
#3: BookKeeper: Every node is writing to the data. I’d use BookKeeper as write-ahead log.
Was BookKeeper built for that kind of workload?

Has anyone else done something similar? I couldn’t find anything in the archives...


From: Flavio Junqueira [mailto:fpj@yahoo-inc.com<mailto:fpj@yahoo-inc.com>]
Sent: Mittwoch, 13. Juli 2011 14:01
To: user@zookeeper.apache.org<mailto:user@zookeeper.apache.org>
Subject: Re: Shared block storage via ZooKepper

Hi Simon, It is not entirely clear to me what you need zookeeper for in this case. Are blocks
replicated and you need to guarantee that the updates are consistent across replicas?

On your observations, I'm quite sure people will have an opinion, so here are my thoughts,
which might not be representative of the whole community :
1- You're right, we do not recommended to use ZooKeeper directly as the data store. ZooKeeper
servers keep their state in memory.
2- Cassandra already provides replication. Are you trying to strengthen the guarantees of
Cassandra? I don't get it...
3- Sound right that you could use BK as a journal, but it is not clear which element is writing
to the journal. Are you assuming a metadata manager such as the namenode of HDFS?
4- I'm not sure what this option means. Are you proposing ZooKeeper to manage the metadata
of the file system? If so, I don't find it entirely unrealistic, since metadata updates are
supposed to be small and the performance of ZooKeeper should be good enough for your case,
but it might be awkward to have your block storage clients talking directly to ZooKeeper.
Changes to metadata management would imply in this case rolling out a new version of the client
application instead of just having the changes implemented on the service side.


On Jul 13, 2011, at 12:02 PM, Simon Felix wrote:

Hello everyone

What is the best way to build a distributed, shared storage system on top of
ZooKeeper? I'm talking about block storage in the terabyte-range (i.e. store
billions of 4k blocks). Consistency and Availability are important, as is
throughput (both read & write). I need at least 50 MB/s with 3 nodes with
two regular SATA drives each for my application.

Some options I came up with:
1. Use ZooKeeper directly as a data store (Not recommended according to the
docs - and it really leads to abysmally bad performance, I tested that)
2. Use Cassandra as data store
3. Use BookKeeper as write-ahead log and implement my own underlying store
4. Use ZooKeeper to create my own (probably buggy...) data store

What would you recommend? Are there other options?



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