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From Sorin Julean <>
Subject Re: Cassandra cluster HW spec (commit log directory vs data file directory)
Date Sun, 30 Oct 2011 22:34:20 GMT
Hey Chris,

 Thanks for sharing all  the info.
 I have few questions:
 1. What are you doing with so much memory :) ? How much of it do you
allocate for heap ?
 2. What your network speed ? Do you use trunks ? Do you have a dedicated
VLAN for gossip/store traffic ?


On Sun, Oct 30, 2011 at 5:00 AM, Chris Goffinet <>wrote:

> RE: RAID0 Recommendation
> Cassandra supports multiple data file directories. Because we do
> compactions, it's just much easier to deal with (1) data file directory
> that is stripped across all disks as 1 volume (RAID0). There are other ways
> to accomplish this though. At Twitter we use software raid (RAID0 & RAID10).
> We own the physical hardware and have found that even with hardware raid,
> software raid in Linux actually faster. The reason being is:
> We have found that using far-copies is much faster over near-copies. We
> set the i/o scheduler to noop at the moment. We might move back to CFQ with
> more tuning in the future.
> We use RAID10 for cases where we need better disk performance if we are
> hitting the disk often, sacrificing storage. We initially thought RAID0
> should be faster over RAID10 until we found out about the near vs far
> layouts.
> RE: Hardware
> This is going to depend on how well your automated infrastructure is, but
> we chose the path of finding the cheapest servers we could get from
> Dell/HP/etc. 8/12 cores, 72gb memory per node, 2TB/3TB, 2.5".
> We are in the process of making changes to our servers, I'll report back
> in when we have more details to share.
> I wouldn't recommend 75 CFs. It could work but just seems too complex.
> Another recommendation for clusters, always go big. You will be thankful
> in the future for this. Even if you can do this on 3-6 nodes, go much
> larger for future expansion. If you own your hardware and racks, I
> recommend making sure to size out the rack diversity and # of nodes per
> rack. Also take into account the replication factor when doing this. RF=3,
> should be min of 3 racks, and # of nodes per rack should be divisible by
> the replication factor. This has worked out pretty well for us. Our biggest
> problems today are adding 100s of nodes to existing clusters at once. I'm
> not sure how many other companies are having this problem, but it's
> certainly on our radar to improve, if you get to that point :)
> On Tue, Oct 25, 2011 at 5:23 AM, Alexandru Sicoe <>wrote:
>> Hi everyone,
>> I am currently in the process of writing a hardware proposal for a
>> Cassandra cluster for storing a lot of monitoring time series data. My
>> workload is write intensive and my data set is extremely varied in types of
>> variables and insertion rate for these variables (I will have to handle an
>> order of 2 million variables coming in, each at very different rates - the
>> majority of them will come at very low rates but there are many that will
>> come at higher rates constant rates and a few coming in with huge spikes in
>> rates). These variables correspond to all basic C++ types and arrays of
>> these types. The highest insertion rates are received for basic types, out
>> of which U32 variables seem to be the most prevalent (e.g. I recorded 2
>> million U32 vars were inserted in 8 mins of operation while 600.000 doubles
>> and 170.000 strings were inserted during the same time. Note this
>> measurement was only for a subset of the total data currently taken in).
>> At the moment I am partitioning the data in Cassandra in 75 CFs (each CF
>> corresponds to a logical partitioning of the set of variables mentioned
>> before - but this partitioning is not related with the amount of data or
>> is somewhat random). These 75 CFs account for ~1 million of the
>> variables I need to store. I have a 3 node Cassandra 0.8.5 cluster (each
>> node is a 4 real core with 4 GB RAM and split commit log directory and data
>> file directory between two RAID arrays with HDDs). I can handle the load in
>> this configuration but the average CPU usage of the Cassandra nodes is
>> slightly above 50%. As I will need to add 12 more CFs (corresponding to
>> another ~ 1 million variables) plus potentially other data later, it is
>> clear that I need better hardware (also for the retrieval part).
>> I am looking at Dell servers (Power Edge etc)
>> Questions:
>> 1. Is anyone using Dell HW for their Cassandra clusters? How do they
>> behave? Anybody care to share their configurations or tips for buying, what
>> to avoid etc?
>> 2. Obviously I am going to keep to the advice on the
>> and split the
>> commmitlog and data on separate disks. I was going to use SSD for commitlog
>> but then did some more research and found out that it doesn't make sense to
>> use SSDs for sequential appends because it won't have a performance
>> advantage with respect to rotational media. So I am going to use rotational
>> disk for the commit log and an SSD for data. Does this make sense?
>> 3. What's the best way to find out how big my commitlog disk and my data
>> disk has to be? The Cassandra hardware page says the Commitlog disk
>> shouldn't be big but still I need to choose a size!
>> 4. I also noticed RAID 0 configuration is recommended for the data file
>> directory. Can anyone explain why?
>> Sorry for the huge email.....
>> Cheers,
>> Alex

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