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From Steve Loughran <ste...@apache.org>
Subject Re: recommendation on HDDs
Date Mon, 14 Feb 2011 11:23:13 GMT
On 12/02/11 16:26, Michael Segel wrote:
> All,
> I'd like to clarify somethings...
> First the concept is to build out a cluster of commodity hardware.
> So when you do your shopping you want to get the most bang for your buck. That is the
'sweet spot' that I'm talking about.
> When you look at your E5500 or E5600 chip sets, you will want to go with 4 cores per
CPU, dual CPU and a clock speed around 2.53GHz or so.
> (Faster chips are more expensive and the performance edge falls off so you end up paying
a premium.)

Interesting choice; the 7 core in a single CPU option is something else 
to consider. Remember also this is a moving target, what anyone says is 
valid now (Feb 2011) will be seen as quaint in two years time. Even a 
few months from now, what is the best value for a cluster will hve moved on.

> Looking at your disks, you start with using the on board SATA controller. Why? Because
it means you don't have to pay for a controller card.
> If you are building a cluster for general purpose computing... Assuming 1U boxes you
have room for 4 3.5" SATA which still give you the best performance for your buck.
> Can you go with 2.5"? Yes, but you are going to be paying a premium.
> Price wise, a 2TB SATA II 7200 RPM drive is going to be your best deal. You could go
with SATA III drives if your motherboard supports the SATA III ports, but you're still paying
a slight premium.
> The OP felt that all he would need was 1TB of disk and was considering 4 250GB drives.
(More spindles...yada yada yada...)
> My suggestion is to forget that nonsense and go with one 2 TB drive because its a better
deal and if you want to add more disk to the node, you can. (Its easier to add disk than it
is to replace it.)
> Now do you need to create a spare OS drive? No. Some people who have an internal 3.5
space sometimes do. That's ok, and you can put your hadoop logging there. (Just make sure
you have a lot of disk space...)

One advantage of a specific drive for OS and log (in a separate 
partition) is you can re-image it without losing data you care about, 
and swap in a replacement fast. If you have a small cluster set up for 
hotswap, that reduces the time a node is down -just have a spare OS HDD 
ready to put in. OS disks are the ones you care about when they fail, 
the others are more "mildly concerned about the failure rate" than 
something to page you over.

> The truth is that there really isn't any single *right* answer. There are a lot of options
and budget constraints as well as physical constraints like power, space, and location of
the hardware.

+1. don't forget weight either.

> Also you may be building out a cluster who's main purpose is to be a backup location
for your cluster. So your production cluster has lots of nodes. Your backup cluster has lots
of disks per node because your main focus is as much storage per node.
> So here you may end up buying a 4U rack box, load it up with 3.5" drives and a couple
of SATA controller cards. You care less about performance but more about storage space. Here
you may say 3TB SATA drives w 12 or more per box. (I don't know how many you can fit in to
a 4U chassis these days.  So you have 10 DN backing up a 100+ DN cluster in your main data
center. But that's another story.

You can get 12 HDDs in a 1U if you ask nicely. but in a small cluster 
there's a cost, that server can be a big chunk of your filesystem, and 
if it goes down there's up to 24TB worth of replication going to take 
place over the rest of the network, so you'll need at least 24TB of 
spare capacity on the other machines, ignoring bandwidth issues.

> I think the main take away you should have is that if you look at the price point...
your best price per GB is on a 2TB drive until the prices drop on 3TB drives.
> Since the OP believes that their requirement is 1TB per node... a single 2TB would be
the best choice. It allows for additional space and you really shouldn't be too worried about
disk i/o being your bottleneck.

One less thing to worry about is good.

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