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From Tapas Sarangi <tapas.sara...@gmail.com>
Subject Re: disk used percentage is not symmetric on datanodes (balancer)
Date Tue, 19 Mar 2013 15:04:52 GMT

On Mar 19, 2013, at 5:00 AM, Алексей Бабутин <zorlaxpokemonych@gmail.com>

> node A=12TB
> node B=72TB
> How many A nodes  and B from 200 do you have?

We have more number of A nodes than B. The ratio of the number is about 80, 20. Note that
not all the B nodes are 72TB, that's a max value. Similarly for A it is a min. value.

> If you have more B than A you can deactivate A,clear it and apply again.

Apply what ? It may not be a choice for an active system and it may cripple us for days.

> I suppose that cluster about 3-5 Tb.Run balancer with threshold 0.2 or 0.1.

You meant 3.5 PB, then you are about right.  What this threshold does exactly ? We are not
setting the threshold manually, but isn't hadoop's default 0.1 ?

> Different servers in one rack is bad idea.You should rebuild cluster with multiple racks.

Why bad idea ? We are using hadoop as a file system not as a scheduler. How multiple racks
are going to help in balancing the disk-usage across datanodes ?


> 2013/3/19 Tapas Sarangi <tapas.sarangi@gmail.com>
> Hello,
> I am using one of the old legacy version (0.20) of hadoop for our cluster. We have scheduled
for an upgrade to the newer version within a couple of months, but I would like to understand
a couple of things before moving towards the upgrade plan.
> We have about 200 datanodes and some of them have larger storage than others. The storage
for the datanodes varies between 12 TB to 72 TB.
> We found that the disk-used percentage is not symmetric through all the datanodes. For
larger storage nodes the percentage of disk-space used is much lower than that of other nodes
with smaller storage space. In larger storage nodes the percentage of used disk space varies,
but on average about 30-50%. For the smaller storage nodes this number is as high as 99.9%.
Is this expected ? If so, then we are not using a lot of the disk space effectively. Is this
solved in a future release ?
> If no, I would like to know  if there are any checks/debugs that one can do to find an
improvement with the current version or upgrading hadoop should solve this problem.
> I am happy to provide additional information if needed.
> Thanks for any help.
> -Tapas

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