How to Tag/Enable a device as SSD in ESXi.

ESXi 5.0 supports SSD disk. What does it mean?
You  might already rememeber some hints on SSD support prediction in the past on this blog.Yes, ESX now officially support SSD disks.With this support user now can plugin SSD into ESX ( vsphere5.0 onwards), and configure ESX to use the same for caching or storing VM swap files.To utilize the SSD disk for holding caching purpose , you need to create a datastore on the device and then you can select the %-age space to use it for esxi caching from 1-100%. You can observe the ESXi performance better then its predecessor.

That said here is the catch.Some times even though the disk is SSD ESXi can not recognize the disk as SSD. There is a reason for that, if the disk attached to raid controller , few raid controllers donot  give ESX  the access to the mode page ,to determine whether the device behind the RAID is  SSD or not. In such cases where device does not show up as SSD we have a provision  to Tag/Enable the disk as SSD. We can as certain claimrules in ESX for the disk which we are sure to be a SSD. You can also mark a magetic disk as SSD using this method for testing.

This can be done using the PSA claim rules,

#  esxcli nmp satp addrule --satp VMW_SATP_ALUA_CX -t vendor -V DGC -M "RAID 0" --option="enable_ssd"

--satp = SATP which claims the device  
Note: you can get the device satp by 
"esxcli storage nmp device list"
vendor -- vendor of the device
model  -- Model of the device
Instead of using vendor/model there are different options available in esxi 5.x to add SATP Claimrules.
You can use device,transport....etc.

Why is the device status on ESX shown "degraded"?

Today I would like to touch upon a specific  field in command output "esxcli storage core device list" related to "Status" of the device.

~ # esxcli storage core device list
naa.90090160c1e01de150060160c1e01de1
   Display Name: DGC iSCSI Disk (naa.90090160c1e01de150060160c1e01de1)
   Has Settable Display Name: true
   Size: 0
   Device Type: Direct-Access
   Multipath Plugin: NMP
   Devfs Path: /vmfs/devices/disks/naa.90090160c1e01de150060160c1e01de1
   Vendor: EFG
   Model: ABD
   Revision: 0326
   SCSI Level: 4
   Is Pseudo: true
   Status: degraded
   Is RDM Capable: true
   Is Local: false
   Is Removable: false

This field for the device is specifically reserved for indicating the path status of the device.When the device has more than one path to the target( storage array), the path status is "on". When all the paths to the target are down ( either off / dead ) the device status is "dead". If  there is only one path to the target then the status is "degraded".

Also note that when the device is in PDL( Permanent Device Loss , This scenarios arises when a device is  unmapped from the storage array while  ESX was using the device) the status of the device is "not connected".

If ESX fails to recognize the state of the device ( if all above mentioned scenarios are not applicable) then the device status is "unknown".