It seems that a lot of people haven’t picked up on this… With Virtual SAN in the past, or better said with vSphere, booting from SATADOM was not supported. This had to do with the default location of the scratch partition, the number of expected writes to the SATADOM device and simply the fact that we did not know how fast the device would wear out.
For those who don’t know, SATADOM devices are basically flash chips on a SATA module which usually directly goes on the motherboard. Great solution as it is as fast as SSD, as small as SD/USB which means you don’t lose a disk slot.
After many tests over the last year it was concluded that SATADOM can be fully supported for vSphere and Virtual SAN but that there are some requirements for the device itself:
- When you boot a Virtual SAN host from a SATADOM device, you must use:
- single-level cell (SLC) device
- The size of the boot device must be at least 16 GB.
Again, key reason for this is that all the trace logs and vSphere logs (etc) end up on this device and we don’t want it to wear out and cause all sorts of unexpected behaviour. As our documentation says: It is important that the SATADOM device meets the specifications outlined in this guide!
Anyway, now you know… more options when it comes to booting ESXi supported, which especially is handy when you want to use your disk slots for Virtual SAN!
Paul Waldman says
Good to know! What advantages does this have over a supported USB flash drive or SD card?
John Nicholson (@Lost_Signal) says
Paul, A SD or USB installation will trigger “embedded mode” on the install and not create crash dump, or log partitions. Now this is fine (you can redirect these to a nearby NFS share) but in some environments (ROBO especially) you may not have a handy location for this.
The problem with non-SLC SATADOM’s is that they look like a hard drive to the ESXi installer (over 5GB, and SATA) so they trigger the full install mode which would create these logging partitions and potentially burn out the flash.
Paul Waldman says
Do you know what the size limit is to go from usb to thin? I have noticed when installing to larger SD cards(8 or 16GB) the install type shows up as visor-thin instead of visor-usb.
Have you seen the prices for an SLC SATA DOM? My team has been assuming that using an affordable SATA DOM is a must and that as long as we move scratch to an nfs share it would be supported. Is this a bad assumption?
In the vSAN 6.2 design guide it reads “… the recommendation is to use an SLC class device for performance and more importantly endurance.” so it is not mandatory.
So non SLC SATADOM is no recommended but supported, or am I wrong?