Most editions of Informix Dynamic Server limit the amount of shared memory that can be allocated:
Compare the Informix Version 12 editions
This primarily constrains how big the buffer pools (disk cache) can be, while still leaving enough for other essential memory pools. However, modern machines will often have much more RAM than this, which could be put to better use.
This article will provide the complete process to use spare memory as RAM disk file systems. Their contents are volatile, so they can only be used for temporary tables (DBSPACETEMP) and sort/merge files (PSORT_DBTEMP). The commands to create them on all supported Linux and UNIX flavours will be given, along with other relevant environment and configuration parameter settings.
Temporary data is then never written to disk, dramatically improving run times of larger queries and preventing interference with OLTP sessions by reducing buffer turnover. In an actual case, disk writes were reduced by 96%, eliminating SAN contention with other applications.
Even using Ultimate Edition which has no shared memory limit, index builds in temp dbspaces are not only slower but can crash the instance on some versions if they run out of space. This can be avoided by setting PSORT_DBTEMP to use file systems instead, and run time can be made shorter still if these are RAM disks.
Note that RAM disk should not be confused with solid state drives (SSD), which are persistent and a better technology for logical and physical logs when combined with RAID 1 or 10.