When using ESXi 5.1 while running from an usb drive a random datastore is selected by ESXi to store the host's logfiles. If you would detach this datastore the logfiles are lost, which is not really something you want. This article shows how to create a scratch partition and how to tell ESXi to use it. Note that you'll need a reboot and this is only necessary on ESXi hosts running without local storage.

The procedure for doing this is pretty straight forward:

• Create a datastore
  • Set the size to about 1-5 GB per host that will use it
• Attach the datastore to the host and create a directory in it
  • The VMware default for the directory name is .locker-hostname
• Configure scratch partition on host's advanced settings
  • Go to Configuration tab → Advanced Settings → ScratchConfig
    • Fill in the ScratchConfig.ConfiguredScratchLocation variable like this: /vmfs/volumes/<datastorename>/.locker-esxihostXXX
  • Note that after a reboot the friendly name will dissappear and change something like /vmfs/volumes/50d82ad1-872mrt40-a404-5cf3fabcf838/.locker-esxihostXXX
  • Als note that this friendly name is still used when browsing through the datastore using the Datastore Browser utility
• Reboot host
• Check that files have been created in the directory

On a medium to busy environment the amount of logging was about 2 GB after more than a year.

http://blogs.vmware.com/vsphere/2011/04/scratch-partition-best-practices-for-usbsd-booted-esxi.html
http://kb.vmware.com/kb/1033696