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Extend iSCSI storage to 4 nodes

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Tony_
Occasional Advisor

Extend iSCSI storage to 4 nodes

Hello

 

I'm working for a small company with approx 500 pcs. Our virtual environments storage consist of 2x HP StoreVirtual 4330 configured with Network-Raid-10 (2 way-mirror) . We are running approx 20 vms today from this storage.

 

We are going to extend the storage this year, and because I'm new to SAN storage (this is the only san I've admninistered) I need suggestions what is the best option for us. All equipment will be placed in the same serverrack, but this may change in the future. The extended storage will be used mainly for vms running different services, databases etc, but also for filestorage. I hope someone can answer my questions below.

 

The plan is to buy 2 more of the HP StoreVirtual 4330 model OR 2 HP StoreVirtual 4130

 

1.  Are we required to buy the same model if we want so use the new nodes in the same cluster? (if we buy 4130 the disk size is different than the 4330 model).

 

2. Suggestions regarding Network-Raid level when we have 4-systems? (I've read that we shouldn't use Raid 5 or 6 when we use databases and so on on this storage)

 

3. Is the best option to add the new nodes to the same cluster or to configure a new 2-node cluster?

 

Thanks in advance!

 

Regards

Tony

 

 

P.S. This thread has been moved from Storage Area Networks (SAN) (Small and Medium Business) to HP StoreVirtual Storage / LeftHand. - Hp Forum Moderator

10 REPLIES
oikjn
Honored Contributor

Re: Extend iSCSI storage to 4 nodes

if you want to simply increase your storage size as a single block, you should buy nodes that are exactly the same as the existing ones you have and add them to the existing cluster.  If you want to add its own new storage space to isolate performance, then you can get any nodes you want and add them to their own new cluster.  Think of nodes like disks in a raid group... the cluster will only be as fast as the slowest node and only have as much space to contribute as the smallest node.  the last thing in the world you want to do is have a cluster with 15k disks in one node and 7.2k disks in another.

 

 

You can add future nodes one at a time and have an odd number of nodes in the cluster, so unless you need to jump up two nodes right away or are getting a deal for it, you don't have to buy the nodes in pairs.

 

as for raid, there are two raid levels to keep in mind here, "network raid" and "HDD raid", the nodes come standard formatted with their HDDs in HDD raid5.  Generally you don't need to change this and can leave it as is.  If you really care about performance and don't need much space, you can convert it to Raid10, but it usually isn't needed.  As for network raid, anything in production should be NR10.  Avoid NR5 for anything other than static archive data (not active packups, but write once and hold forever things like your .iso CD library), and avoid NR0 for anything you care about just like you shouldn't use Raid0 for anything you care about either.

 

Network Raid 10 basically mirrors your data so that at all times it is stored on two nodes.  The great thing about that is it allows you to reboot individual nodes and not lose data availability or really have a heart attack when any individual node has a problem.  The downside is that NR10 is just like raid10 in that it takes up 2x your storage space.

Tony_
Occasional Advisor

Re: Extend iSCSI storage to 4 nodes

Hi

Thank you for your answer. Really useful information!

 

I've read about Netowork-Raid levels and understand that NRaid-10 works different than "internal" raid-10 with discs.

(I thought an even number of storagenodes was required, but I understand that's just the case with RAID-10 between discs, not between storagenodes, right?)

 

OK, so two possibilities here I think:

 

1. Extend the exisiting cluster/Netowork Raid-10 to 4 nodes (I have budget to buy two)

or

2. Build a new cluster between the two new nodes

 

Whats the best option? As I understood this both scenarios will provide the same ammount of netto storagespace?

 

I mean if I extend to 4 nodes in the same cluster only one node of four can be down, but if I create a new cluster one node per cluster can become unavailable... So isn't it safer to have two clusters then?

 

What about performance between schenario 1 and 2?

I'm using VIP Load-balancing now when only 2-nodes, and I suppose a client will get all data from one of them, but if  I have 4 nodes, will data then be  read from several nodes every time then?

 

Do I need another FOM if i select option 2 or can the same one as in the current cluster be used for both clusters?

 

Thank you in advance!

oikjn
Honored Contributor

Re: Extend iSCSI storage to 4 nodes

correct, network raid 10 is not exactly the same as disk raid 10.  both do a mirror+stripe scheme, but network raid is more advanced in that it will make sure your data is on two different nodes, but it doesn't have to be the same two different nodes, so if your data is "ABC", node one might hold data AB, node two might hold BC, and node three might hold AC.

 

As for what is better... a 4-node cluster or two 2-node clusters, the answer is "it depends".  If the nodes you are planning to add are identical AND you don't require performance isolation between some of your LUNs, you are better off creating a single 4-node cluster because of a number of reasons: (1) all LUNs will benefit slightly by a performance increase by striping data across more nodes, (2) you will have one larger pool of space to split into LUNs so you can be more efficient with thin provisioning.  If you are using a different model node which is faster or has a different storage capacity than your existing nodes then you need to create a seporate cluster for those nodes for the same reason you wouldn't put a 128GB SSD into a raid1 group with a 2TB 5400RPM HDD.  Also, if you are really concerened that you have some LUNs that are going to require so much performance that they will negitively affect the rest of your LUNs, the only way to 100% isolate the performance is to maintian two clusters since each cluster is affectively a pool of IO+storage.

 

As for the FOM, it doesn't matter either way, FOMs are for maintining the managment group infrastructure and not the cluster structure.  You may notice a FOM isn't a member of a cluster and you can't join it to one.  You only need one FOM per management group.  With four nodes, you can chose to either run four managers + one FOM, or you can run only three managers.  generally the more managers the better (I think up to 5).  The critical thing is you need to have an odd number of managers+FOM so there can be a clear majority to avoid a "split-brain" situation.  What is better in your case would only be something you could know without more details provided.  Just consider all the ways a manager could lose contact with the others and then plan for what would happen in every situation and you will be able to tell if running 5 managers (4+FOM) holds any benefit for management group availability than running 3 managers and no FOM.  When planning, think about when one or multiple nodes might crash, lose power, unplug one or more NICs, have a switch die, catch fire... you name it.

Tony_
Occasional Advisor

Re: Extend iSCSI storage to 4 nodes

Thank you again, really good explanation!
I hope this helps someone else to.

 

I think I will extend the current cluster to 4 nodes when the two new nodes will be of the same model.

 

Just a few more things I'm wondering about, I hope someone have time to answer :)

 

1. What happens to data if two nodes goes offline at the same time in this 4 node cluster (N Raid 10)?

    I understand that data will be unavailable during the time 2 of them are down, but if lets say the reason was motherboad or power failure (something else than multiple HDD failures) will the data be available again when the two nodes are online, or is there a risk for data corruption or anything like that?

 

2. Network RAID 10 is fine with 4 nodes, but if I need to extend the cluster again in the future, approx when should I consider another RAID level  for increased safety? (for example Network RAID 10+1)

 

3. I understand that the array will only use the ammount of space as the smallest node is even if there's a node with more space in the array, but lets say I need to replace theese 4 nodes in the future when they are old, is it possible then to change to another model (lager storegespace) and to just extend the usable space to the hosts when all 4 nodes has been replaced?

 

Reg

Tony

Bart_Heungens
Honored Contributor

Re: Extend iSCSI storage to 4 nodes

With NR10 (network raid 10) 1 node can fail in a cluster.

 

However in some situations 2 nodes can fail. For instance in a 4node cluster nodes 1 and 3 or 2 and 4 (odd or equal) can fail at the same time.

This is the reason, in a 2 site situation, that a site can fail while the other remains online. Here the rule is that the odd nodes must be in 1 site and the equal nodes in the other...

 

So in this setup it will be the same in a 10 node cluster. 5 nodes can fail, being or the odd or the equal nodes...

 

I will post soon (I will try this week) an indepth explanation with pictures on my blog, if you see the picture you will see it will make sense...

 

So NR10+1 gives you more availability. In this case 2 nodes can fail, even 1 in both sites... Buth you have less net capacity (33% instead of 50% with NR10)

 

Kr,

Bart

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oikjn
Honored Contributor
Solution

Re: Extend iSCSI storage to 4 nodes

The answer to #1 is "it depends". Its just like any hardware Raid10 in that respect... you can lose one node and be 100% positive that you will still have data availability due to the mirror. After that, if you lose a 2nd node, you will still maintain data availability as long as that node isn't the one node with that mirror data on it. The manual goes into this somewhere, but for a 4-node cluster with nodes A,B,C, & D, A &C are mirrors as are B & D. In that scenario, you could lose node A and then either Node B or node D without any availability problem, but if you lost node A & C, then you would lose data. That answer to #2 is also "it depends." IMO, there is no need for NR10+x until you get into multi-site clusters, but of course if you have a LUN that is so mission critical that NR10 doesn't let you sleep well at night, then you can always increase a LUN's NR level. The only rules I would stick to is not to use NR0 for anything you care the the slightest about (just like raid0), and only use NR5 for static archive data such as .iso storage vaults because the write penalty on NR5 is too high. Beyond that, you will need to break your idea of network raid at the NODE level... network raid is defined at the LUN level, so you have have clusters with LUNs that are a mix of many different network raid levels (though in practice, 99.9% will/should be NR10). As for #3. The space usable on each node will be equal to the space of the smallest node, so you could have 3x 10TB nodes and one 3TB node in the same cluster, but each 10TB node would only contribute 3TB to the cluster and the other 7TB (or 21TB when you think about it at the cluster level) would be 100% wasted capacity and wasted power/cooling. The cluster won't stop you from doing exactly that, and there could be a situation where it might not matter or be OK as a temporary thing, but its always a waste and it just depends on if you are OK with that waste or not. Its just like any hardware raid group and you should treat it like such.
Tony_
Occasional Advisor

Re: Extend iSCSI storage to 4 nodes

Thank you both for your replies, I have a much better understanding of this now!

 

(I'm still intrested to look into the images mentioned about if/when published)

Bart_Heungens
Honored Contributor

Re: Extend iSCSI storage to 4 nodes

Hi,

 

There you go!

http://www.bitcon.be/?p=2537

 

 

Kr,

Bart

--------------------------------------------------------------------------------
If my post was useful, clik on my KUDOS! "White Star" !
My blog: http://blog.bitcon.be
oikjn
Honored Contributor

Re: Extend iSCSI storage to 4 nodes

nice link bart!

Tony_
Occasional Advisor

Re: Extend iSCSI storage to 4 nodes

Thank you for the link Bart!