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From "Kevin O'dell" <kevin.od...@cloudera.com>
Subject Re: Acceptable CPU_WIO % ?
Date Fri, 08 Feb 2013 13:56:59 GMT
Azuryy,

  The main reason to recommend against RAID is that it is slow and it adds
redundancy that we already have in Hadoop.  RAID0 is another story as long
as all of the drives are healthy and you don't mind losing the whole volume
if you lose one drive.

JM,

  I would not even waste my time testing RAID5 or RAID6(unless it is just
for educational purposes :) ).  200+ IOPs consistently on one SATA drive is
pretty high, that would explain your high I/O wait time.  If your use case
allows for you to lose the whole node, there is not a good reason for you
to shy away from RAID0.  Please let us know how this plays out with your
environment.

On Thu, Feb 7, 2013 at 10:23 PM, Azuryy Yu <azuryyyu@gmail.com> wrote:

> JM,
>
> I don't have the context, but if you are using Hadoop/Hbase, so don't do
> RAID on your disk.
>
>
> On Fri, Feb 8, 2013 at 11:15 AM, Jean-Marc Spaggiari <
> jean-marc@spaggiari.org> wrote:
>
> > Ok. I see. For my usecase I prefer to loose the data and have faster
> > process. So I will go for RAID0 and keep the replication factor to
> > 3... If at some point I have 5 disks in the node, I will most probably
> > give a try to RAID5 and see the performances compared to the other
> > RAID/JBOD options.
> >
> > Is there a "rule", like, 1 HD per core? Or we can't really simplify that
> > much?
> >
> > So far I have that in the sar output:
> > 21:35:03          tps      rtps      wtps   bread/s   bwrtn/s
> > 21:45:03       218,85    215,97      2,88  45441,95    308,04
> > 21:55:02       209,73    206,67      3,06  43985,28    378,32
> > 22:05:04       215,03    211,71      3,33  44831,00    312,95
> > Average :      214,54    211,45      3,09  44753,09    333,07
> >
> > But I'm not sure what it means. I will wait for tomorrow to get more
> > results, but my job will be done over night, so I'm not sure the
> > average will be accurate...
> >
> > JM
> >
> >
> > 2013/2/7, Kevin O'dell <kevin.odell@cloudera.com>:
> > > JM,
> > >
> > >   I think you misunderstood me.  I am not advocating any form of RAID
> for
> > > Hadoop.  It is true that we already have redundancy built in with HDFS.
> >  So
> > > unless you were going to do something silly like sacrifice speed to run
> > > RAID1 or RAID5 and lower your replication to 2...just don't do it :)
> > >  Anyway, yes you probably should have 3 - 4 drives per node if not
> more.
> > >  At that point then the you will really see the benefit of JBOD over
> > RAID0
> > >
> > > Do you want to be able to lose a drive and keep the node up?  If yes,
> > then
> > > JBOD is for you.  Do you not care if you lose that node due to drive
> > > failure? You just need speed, then RAID0 may be the correct choice.
>  Sar
> > > will take some time to populate.  Give it about 24 hours and you should
> > be
> > > able to glean some interesting information.
> > >
> > > On Thu, Feb 7, 2013 at 9:50 PM, Jean-Marc Spaggiari
> > > <jean-marc@spaggiari.org
> > >> wrote:
> > >
> > >> Ok. I see with RAID0 might be better for me compare to JBOD. Also, why
> > >> do we want to use RAID1 or RAID5? We already have the redundancy done
> > >> by hadoop, is it not going to add another non-required level of
> > >> redundancy?
> > >>
> > >> Should I already think to have 3 or even 4 drives in each node?
> > >>
> > >> I tried sar -A and it's only giving me 2 lines.
> > >> root@node7:/home/hbase# sar -A
> > >> Linux 3.2.0-4-amd64 (node7)     2013-02-07      _x86_64_        (4
> CPU)
> > >>
> > >> 21:29:54          LINUX RESTART
> > >>
> > >> It was not enabled, so I just enabled it and restart sysstat, but
> > >> seems that it's still not populated.
> > >>
> > >> I have the diskstats plugin installed on ganglia, so I have a LOT of
> > >> disks information, but not this specific one.
> > >>
> > >> My write_bytes_per_sec is pretty low. Average is 232K for the last 2
> > >> hours. But my erad_bytes_per_sec is avera 22.83M for the same period.
> > >> The graph is looking like a comb.
> > >>
> > >> I just retried sar and some data is coming.. I will need to let it run
> > >> for few more minutes to get some more data ...
> > >>
> > >> JM
> > >>
> > >>
> > >> 2013/2/7, Kevin O'dell <kevin.odell@cloudera.com>:
> > >> > JM,
> > >> >
> > >> >   Okay, I think I see what was happening.  You currently only have
> one
> > >> > drive in the system that is showing High I/O wait correct?  You are
> > >> looking
> > >> > at bringing in a second drive to help distribute the load?  In your
> > >> testing
> > >> > with two drives you saw that RAID0 offerred superior performance vs
> > >> > JBOD.
> > >> >  Typically when we see RAID vs JBOD we are dealing with about 6 -
12
> > >> > drives.  Here are some of the pluses and minuses:
> > >> >
> > >> > RAID0 - faster performance since the data is striped, but you are
as
> > >> > fast
> > >> > as your slowest drive and one drive failure you lose the whole
> volume.
> > >> >
> > >> > JBOD - Better redundancy and faster than a RAID1, or a RAID5
> > >> > configuration(unsure about a RAID4), but you are slower than RAID0
> > >> >
> > >> > It sounds like since you only have 1 drive in the node right now,
> you
> > >> > wouldn't be gaining or losing any redundancy by going with RAID0.
>  For
> > >> what
> > >> > it is worth, I would agree that you are I/O bound.  If you run a sar
> > -A
> > >> > >
> > >> > /tmp/sar.out and you take a look at the drive utilization what is
> your
> > >> > TPS(IOPs) count that you are seeing?
> > >> >
> > >> > On Thu, Feb 7, 2013 at 9:00 PM, Jean-Marc Spaggiari
> > >> > <jean-marc@spaggiari.org
> > >> >> wrote:
> > >> >
> > >> >> Hi Kevin,
> > >> >>
> > >> >> I'm facing some issues on one of my nodes and I'm trying to find
a
> > way
> > >> >> to fix that. CPU is used about 10% by user, and 80% for WIO. So
I'm
> > >> >> looking for a way to improve that. The mother board can do RAIDx
> and
> > >> >> JBOD too. It's the server I used few weeks ago to run some disks
> > >> >> benchs.
> > >> >>
> > >> >> http://www.spaggiari.org/index.php/hbase/hard-drives-performances
> > >> >>
> > >> >> The conclusion was that RAID0 was 70% faster than JBOD. But JBOD
> was
> > >> >> faster than RAID1.
> > >> >>
> > >> >> I have a 2TB drive in this server and was thinking about just
> adding
> > >> >> another 2TB drive.
> > >> >>
> > >> >> What are the advantages of JBOD compared to RAID0? From the last
> > tests
> > >> >> I did, it was slower.
> > >> >>
> > >> >> Since I will have to re-format the disks anyway, I can re-run
the
> > >> >> tests just in case I did not configured something properly....
> > >> >>
> > >> >> JM
> > >> >>
> > >> >> 2013/2/7, Kevin O'dell <kevin.odell@cloudera.com>:
> > >> >> > Hey JM,
> > >> >> >
> > >> >> >   Why RAID0?  That has a lot of disadvantages to using a
JBOD
> > >> >> > configuration?  Wait I/O is a symptom, not a problem.  Are
you
> > >> actually
> > >> >> > experiencing a problem or are you treating for something
you
> think
> > >> >> > should
> > >> >> > be lower?
> > >> >> >
> > >> >> > On Thu, Feb 7, 2013 at 8:19 PM, Jean-Marc Spaggiari
> > >> >> > <jean-marc@spaggiari.org
> > >> >> >> wrote:
> > >> >> >
> > >> >> >> Hi,
> > >> >> >>
> > >> >> >> What is an acceptable CPU_WIO % while running an heavy
MR job?
> > >> >> >> Should
> > >> >> >> we also try to keep that under 10%? Or it's not realistic
and we
> > >> >> >> will
> > >> >> >> see more about 50%?
> > >> >> >>
> > >> >> >> One of my nodes is showing 70% :( It's WAY to much. I
will add
> > >> another
> > >> >> >> disk tomorrow and put them in RAID0, but I'm wondering
how low
> > >> >> >> shoud
> > >> I
> > >> >> >> go?
> > >> >> >>
> > >> >> >> JM
> > >> >> >>
> > >> >> >
> > >> >> >
> > >> >> >
> > >> >> > --
> > >> >> > Kevin O'Dell
> > >> >> > Customer Operations Engineer, Cloudera
> > >> >> >
> > >> >>
> > >> >
> > >> >
> > >> >
> > >> > --
> > >> > Kevin O'Dell
> > >> > Customer Operations Engineer, Cloudera
> > >> >
> > >>
> > >
> > >
> > >
> > > --
> > > Kevin O'Dell
> > > Customer Operations Engineer, Cloudera
> > >
> >
>



-- 
Kevin O'Dell
Customer Operations Engineer, Cloudera

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