This would be swap files, by default, the swap file is created in the same location as the virtual machine's configuration file. A swap file is created by the ESXi host when a virtual machine is powered on. The size of the swap file equals configured ram minus any memory reservations.
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vfk
Systems Manager / Technical Architect
VCP5-DCV, VCAP5-DCA, vExpert, ITILv3, CCNA, MCP