mmlspdisk which Lists information for one or more GPFS Native RAID pdisks. To check faulty disks, do the commands

# mmlspdisk all --not-ok
mmlspdisk: [I] No disks were found.

OR

# mmlspdisk all --replace
mmlspdisk: [I] No disks were found.

We have encountered a situation where a defunct disk was accepting IO request and did not return any failure in time. As a result, these IO requests hanged there till time out (default 10 seconds). Typically, Spectrum Scale/GPFS will fail to read or write a disk, the failure is written in log and we have to shift IO to other available disks which should be quick.

Normally such operations should return in 20 milliseconds or less. When we have IO timeout, this request has wasted us
10 seconds / 20 milliseconds = 500 times of time. Even if Spectrum Scale/GPFS is able to choose a fast disk in the second attempt, we are much slower than normal.

Due to the utilization of striping technology, a bad/slow disks always affects IO of many files, much more than the situation without striping. IO on the same file involves more than several disks, and the IO has to wait for the slowest request to return. So a bad/slow disk may have considerable influence on Spectrum Scale/GPFS performance.

Before restarting the NSD Nodes or Quorum Manager Nodes or other critical nodes, do check the following first to ensure the file system is in the right order before restarting.

1. Make sure all three quorum nodes are active.

# mmgetstate -N quorumnodes

*If any machine is not active, do *not* proceed

2. Make sure file system is mounted on machines

# mmlsmount gpfs0

If the file system is not mounted somewhere, we should try to resolve it first.

There was a good and varied topics being discussed at the Spectrum Scale

If your cluster has symptoms of overload and GPFS kept reporting “overloaded” in GPFS logs like the ones below, you might get long waiters and sometimes deadlocks.

Wed Apr 11 15:53:44.232 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 15:55:24.488 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 15:57:04.743 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 15:58:44.998 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 16:00:25.253 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 16:28:45.601 2018: [I] Sending 'overloaded' status to the entire cluster
Wed Apr 11 16:33:56.817 2018: [N] sdrServ: Received deadlock notification from

Increase scatterBuffersize to a Number that match IB Fabric
One of the first tuning will be to tune the scatterBufferSize. According to the wiki, FDR10 can be tuned to 131072 and FDR14 can be tuned to 262144

The default value of 32768 may perform OK. If the CPU utilization on the NSD IO servers is observed to be high and client IO performance is lower than expected, increasing the value of scatterBufferSize on the clients may improve performance.

# mmchconfig scatterBufferSize=131072

There are other parameters which can be tuned. But the scatterBufferSize worked immediately for me.
verbsRdmaSend
verbsRdmasPerConnection
verbsRdmasPerNode

Disable  verbsRdmaSend=no

# mmchconfig verbsRdmaSend=no -N nsd1,nsd2

Verify settings has taken place

# mmfsadm dump config | grep verbsRdmasPerNode

Increase verbsRdmasPerNode to 514 for NSD Nodes

# mmchonfig verbsRdmasPerNode=514 -N nsd1,nsd2

References:

1. Best Practices RDMA Tuning