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HDFS >> mail # user >> Mutiple dfs.data.dir vs RAID0

Jean-Marc Spaggiari 2013-02-11, 01:57
Michael Katzenellenbogen 2013-02-11, 02:12
Jean-Marc Spaggiari 2013-02-11, 02:19
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Re: Mutiple dfs.data.dir vs RAID0
thanks all for your feebacks.

I have updated with hdfs config to add another dfs.data.dir entry and
restarted the node. Hadoop is starting to use the entry, but is not
spreading the existing data over the 2 directories.

Let's say you have a 2TB disk on /hadoop1, almost full. If you add
another 2TB disk on /hadoop2 and add it on dfs.data.dir, hadoop will
start to write into /hadoop1 and /hadoop2, but /hadoop1 will stay
almost full. It will not balance the already existing data over the 2

I have deleted all the content of /hadoop1 and /hadoop2 and restarted
the node and now the data is spread over the 2. Just need to wait for
the replication to complete.

So what I will do instead is, I will add 2 x 2TB drives, mount them as
raid0 then move the existing data into this drive and remove the
reprious one. That way hadoop will see still one directory under
/hadoop1 but it will be 4TB instead of 2TB...

Is there anywhere where I can read about hadoop vs the different kind
of physical data storage configuration? (Book, web, etc.)


2013/2/11, Ted Dunning <[EMAIL PROTECTED]>:
> Typical best practice is to have a separate file system per spindle.  If
> you have a RAID only controller (many are), then you just create one RAID
> per spindle.  The effect is the same.
> MapR is unusual able to stripe writes over multiple drives organized into a
> storage pool, but you will not normally be able to achieve that same level
> of performance with ordinary Hadoop by using LVM over JBOD or controller
> level RAID.  The problem is that the Java layer doesn't understand that the
> storage is striped and the controller doesn't understand what Hadoop is
> doing.  MapR schedules all of the writes to individual spindles via a very
> fast state machine embedded in the file system.
> The comment about striping increasing the impact of a single disk drive is
> exactly correct and it makes modeling the failure modes of the system
> considerably more complex.  The net result of the modeling that I and
> others have done is that moderate to large RAID groups in storage pools for
> moderate sized clusters (< 2000 nodes or so) is just fine.  For large
> clusters of up to 10,000 nodes, you should probably limit RAID groups to 4
> drives or less.
> On Sun, Feb 10, 2013 at 7:39 PM, Marcos Ortiz <[EMAIL PROTECTED]> wrote:
>>  We have seen in several of our Hadoop clusters that LVM degrades
>> performance of our M/R jobs, and I remembered a message where
>> Ted Dunning was explaining something about this, and since
>> that time, we don't use LVM for Hadoop data directories.
>> About RAID volumes, the best performance that we have achieved
>> is using RAID 10 for our Hadoop data directories.
>> On 02/10/2013 09:24 PM, Michael Katzenellenbogen wrote:
>> Are you able to create multiple RAID0 volumes? Perhaps you can expose
>> each disk as its own RAID0 volume...
>> Not sure why or where LVM comes into the picture here ... LVM is on
>> the software layer and (hopefully) the RAID/JBOD stuff is at the
>> hardware layer (and in the case of HDFS, LVM will only add unneeded
>> overhead).
>> -Michael
>> On Feb 10, 2013, at 9:19 PM, Jean-Marc Spaggiari<[EMAIL PROTECTED]>
>> <[EMAIL PROTECTED]> wrote:
>>  The issue is that my MB is not doing JBOD :( I have RAID only
>> possible, and I'm fighting for the last 48h and still not able to make
>> it work... That's why I'm thinking about using dfs.data.dir instead.
>> I have 1 drive per node so far and need to move to 2 to reduce WIO.
>> What will be better with JBOD against dfs.data.dir? I have done some
>> tests JBOD vs LVM and did not find any pros for JBOD so far.
>> JM
>> 2013/2/10, Michael Katzenellenbogen <[EMAIL PROTECTED]>
>>  One thought comes to mind: disk failure. In the event a disk goes bad,
>> then with RAID0, you just lost your entire array. With JBOD, you lost
>> one disk.
>> -Michael
>> On Feb 10, 2013, at 8:58 PM, Jean-Marc Spaggiari<[EMAIL PROTECTED]>
Michael Katzenellenbogen 2013-02-11, 16:02
Marcos Ortiz 2013-02-11, 02:39