[Gluster-users] 4 node cluster (best performance + redundancy setup?)

[Ashish Pandey]

----- Original Message -----

From: "K. de Jong" <kees.dejong+lst at neobits.nl> 
To: gluster-users at gluster.org 
Sent: Thursday, August 13, 2020 11:43:03 AM 
Subject: [Gluster-users] 4 node cluster (best performance + redundancy setup?) 

I posted something in the subreddit [1], but I saw the suggestion 
elsewhere that the mailinglist is more active. 

I've been reading the docs. And from this [2] overview the distributed 
replicated [3] and dispersed + redundancy [4] sound the most 
interesting. 

Each node (Raspberry Pi 4, 2x 8GB and 2x 4GB version) has a 4TB HD 
disk attached via a docking station. I'm still waiting for the 4th 
Raspberry Pi, so I can't really experiment with the intended setup. But 
the setup of 2 replicas and 1 arbiter was quite disappointing. I got 
between 6MB/s and 60 MB/s, depending on the test (I did a broad range 
of tests with bonnie++ and simply dd). Without GlusterFS a simple dd of 
a 1GB file is about 100+ MB/s. 100MB/s is okay for this cluster. 

My goal is the following: 
* Run a HA environment with Pacemaker (services like Nextcloud, 
Dovecot, Apache). 
* One node should be able to fail without downtime. 
* Performance and storage efficiency should be reasonable with the 
given hardware. So with that I mean, when everything is a replica then 
storage is stuck at 4TB. And I would prefer to have some more than that 
limitation, but with redundancy. 

However, when reading the docs about disperse, I see some interesting 
points. A big pro is "providing space-efficient protection against disk 
or server failures". But the following is interesting as well: "The 
total number of bricks must be greater than 2 * redundancy". So, I want 
the cluster to be available when one node fails. And be able to 
recreate the data on a new disk, on that forth node. I also read about 
the RMW efficiency, I guess 2 sets of 2 is the only thing that will 
work with that performance and disk efficiency in mind. Because 1 
redundancy would mess up the RMW cycle. 

My questions: 
* With 4 nodes; is it possible to use disperse and redundancy? And is a 
redundancy count of 2 the best (and only) choice when dealing with 4 
disks? 

With 4 nodes, yes it is possible to use disperse volume. 
Redundancy count 2 is not the best but most often used as far as my interaction with users. 
disperse volume with 4 bricks is also possible but it might not be a best configuration. 
I would suggest to have 6 bricks and 4 +2 configuration 
where 4 - Data bricks 
and 2 - Redundant bricks, in other way maximum number of brick which can go bad while you can still use disperse volume. 

If you have number of disks on 4 nodes, you can create the 4 +2 disperse volume in different way while maintaining the requirenment of EC (disperse volume) 


* The example does show a 4 node disperse command, but has as output 
`There isn't an optimal redundancy value for this configuration. Do you 
want to create the volume with redundancy 1 ? (y/n)`. I'm not sure if 
it's okay to simply select 'y' as an answer. The output is a bit vague, 
because it says it's not optimal, so it will be just slow, but will 
work I guess? 


It will not be optimal from the point of view of calculation which we make. 
You want to have a best configuration where yu can have maximum redundancy (failure tolerance) and also maximum storage capacity. 
In that regards, it will not be an optimal solution. Performance can also be a factor. 

* The RMW (Read-Modify-Write) cycle is probably what's meant. 512 * 
(#Bricks - redundancy) would be in this case for me 512 * (4-1) = 1536 
byes, which doesn't seem optimal, because it's a weird number, it's not 
a power of 2 (512, 1024, 2048, etc.). Choosing a replica of 2 would 
translate to 1024, which would seem more "okay". But I don't know for 
sure. 

Yes, you are right. 

* Or am I better off by simply creating 2 pairs of replicas (so no 
disperse)? So in that sense I would have 8TB available, and one node 
can fail. This would provide some read performance benefits. 
* What would be a good way to integrate this with Pacemaker? With that 
I mean, should I manage the gluster resource with Pacemaker? Or simply 
try to mount the glusterfs, if it's not available, then depending 
resources can't start anyway. So in other words, let glusterfs handle 
failover itself. 


gluster can handle fail over on replica or disperse level as per its implementation. 
Even if you want to go for replica, it does not replica 2 does not look like a best option, you should 
go for replica 3 or arbiter volume to have best fault tolerance. 
However, that will cost you a big storage capacity. 


Any advice/tips? 




[1] 
https://www.reddit.com/r/gluster/comments/i8ifdd/4_node_cluster_best_performance_redundancy_setup/ 
[2] 
https://docs.gluster.org/en/latest/Administrator%20Guide/Setting%20Up%20Volumes/ 
[3] 
https://docs.gluster.org/en/latest/Administrator%20Guide/Setting%20Up%20Volumes/#creating-distributed-replicated-volumes 
[4] 
https://docs.gluster.org/en/latest/Administrator%20Guide/Setting%20Up%20Volumes/#creating-distributed-dispersed-volumes 

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