design considerations for VSAN

Here are a few facts that sould be concidered by planing and deploying VSAN

• flash space is divided into 30% read cache, 70% write  cache
• multiple disk groups on a host
  an option to put more SSDs into the system
  use it to reduce failure domain: just the disks in a group get lost when a part of it fails
• flash size
  it is recommended to put 10% flash of allocated space (FTT (failures to tolerate) not considered) into the cluster
• the cluster needs (2n+1) hosts to allow a FTT of n
• a failure of a single disk = failure of a disk group
• Number of Disk Stripes per Object
  Definition: “number of physical disks across which each replica of a storage object is distributed”. Results in better read performance
• Flash Read Cache Reservation
  not recommended to set, the system should allocate the cache on demand.
• Object Space Reservation
  by default VMs on VSAN are thin provisioned, with this setting, you can reserve space for objects
  
• Force Provisioning
  enrolls an object even if there are not enough resources to fulfill the storage policy settings. When the resources are available, the settings will be fulfilled.
• Maintenance/failure
  if a host is longer than 30 minutes away from cluster, a rebuilt will start.
• changes to policies, already applied on objects can be synced manually (reappliy)
• You cant do an snapshot for VMs on VSAN with more than 255GB of memory! This is because the limit of 256 GB of the object VM Home Namespace.
• Reinstall ESXi Host does NOT wipe the data!
• Migrate VSAN from one vCenter to another, just create a new VSAN Cluster on the new vCenter and add the hosts to it.
• A non-working device can either be ABSENT or DEGRADED. VSAN waits 60 minutes before its rebuilt ABSENT devices, but rebuilt DEGRADED devices immediately.