I have a general question about iSCSI that I will ask within the context of my setup.
I have a FreeNAS host running the following hardware:
C2750D4I 8-core atom motherboard
16GB ECC RAM
4x4TB WD RED
The 4x4TB is configured to provide an 8TB volume, and under that volume there are various NFS and CIFS shares, as well as an iSCSI.
I have an ESXi 6 host which uses the iSCSI as it's primary datastore for the disk images of the VMs, as well as a collection of linux physical hosts which mount the NFS shares, and windows laptops which mount the CIFS.
I see what looks to me as strange behavior on the iSCSI when data is written to the disk of any of the VMs (either linux or windows). As the data is written, there is a corresponding amount of data read (but less because it is being read across 4 disks). The data is coming from a dedicated disk in another machine on the network and not originating on the NFS shares, or anywhere on the FreeNAS host.
So - in short - my question is why is an iSCSI write generating so much read traffic on the disks? Is this normal? This behaviors seems to cause the system to write at only 400Mb/s, while it can read over iSCSI at 800Mb/s.
For NFS, read and write speeds are about equal at around 800Mb/s.
It might be important to note that we saw this same behavior when the NFS/iSCSI traffic was all on the same network interface, and it continues to behave the same when with the iSCSI traffic split off onto a dedicated NIC with a connection directly between the ESXi host and the FreeNAS host.
I have a FreeNAS host running the following hardware:
C2750D4I 8-core atom motherboard
16GB ECC RAM
4x4TB WD RED
The 4x4TB is configured to provide an 8TB volume, and under that volume there are various NFS and CIFS shares, as well as an iSCSI.
I have an ESXi 6 host which uses the iSCSI as it's primary datastore for the disk images of the VMs, as well as a collection of linux physical hosts which mount the NFS shares, and windows laptops which mount the CIFS.
I see what looks to me as strange behavior on the iSCSI when data is written to the disk of any of the VMs (either linux or windows). As the data is written, there is a corresponding amount of data read (but less because it is being read across 4 disks). The data is coming from a dedicated disk in another machine on the network and not originating on the NFS shares, or anywhere on the FreeNAS host.
So - in short - my question is why is an iSCSI write generating so much read traffic on the disks? Is this normal? This behaviors seems to cause the system to write at only 400Mb/s, while it can read over iSCSI at 800Mb/s.
For NFS, read and write speeds are about equal at around 800Mb/s.
It might be important to note that we saw this same behavior when the NFS/iSCSI traffic was all on the same network interface, and it continues to behave the same when with the iSCSI traffic split off onto a dedicated NIC with a connection directly between the ESXi host and the FreeNAS host.