If your ESX host has lost connection to the vCenter server, but you can still directly access the host's management, and can find nothing else wrong with the vCenter server or its network connection to the host, you may want to the free space on your ESX host.
Logs and other temporary files may have filled up one or more ramdisk partitions (e.g. when you don't have a persistent scratch partition, it gets mounted on /tmp). This can cause any of the following issues:
You can check free space for all ramdisks through the ESXi Shell or remotely through SSH:
You can ignore output about the tardisks, and look directly at the ramdisk partitions. You should get an output similar to this:
Ramdisk Size Used Available Use% Mounted on root 32M 520K 31M 1% -- etc 28M 184K 27M 0% -- tmp 192M 4K 191M 0% -- hostdstats 303M 3M 299M 1% --
If you see that one of these has filled up (i.e. no or almost no available space left), you need to make some room.
Usually the tmp ramdisk gets filled up by logs from certain VIBs. Look in the /tmp folder, and check whether any logs are taking up a lot of space. The usual culprits are those generated by SANs or faulty/buggy drives, which don't log to the default location, but log to the /tmp partition instead.
You can easily clear logs from the shell by emptying them, like so:
If you require the contents of the log files, you can copy them off the server first, either to a datastore, or through SCP to another host or SSH-enabled file server.
Normally, it is good and common practice to set up logging to a persistent datastore. This resolves the problem about your scratch partition filling up with logs. However, not all VIBs play fair. Some ignore the default location for logging, and create log files on the /tmp partition. As this partition is also used for installation and configuration of VIBs (among other things), a full /tmp can cause issues like those stated earlier in this article.