IBM® Informix® 12.10
xact_ramdirectory index parameter
By default, you build a bts index in a temporary sbspace. You can build a bts index faster in RAM than in a temporary sbspace.
Include the xact_ramdirectory="yes" in the bts index to build the index in memory. However, when building the index in memory uses too much memory, the build is switched to the temporary sbspace. The maximum amount of memory that is allowed for an index build is approximately one third of the value of that is specified by the xact_memory index parameter or the SHMTOTAL configuration parameter, whichever is more restrictive.