mongodb optlog

Additionally, I'm not sure which O/S you are running. However, for 64-bit Linux, Solaris, and FreeBSD systems, MongoDB will allocate 5% of the available free disk space to the oplog. If this amount is smaller than a gigabyte, then MongoDB will allocate 1 gigabyte of space. For 64-bit OS X systems, MongoDB allocates 183 megabytes of space to the oplog and for 32-bit systems, MongoDB allocates about 48 megabytes of space to the oplog.