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.
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design considerations for VSAN

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