More and more enterprise servers storage systems are migrating toward flash based drives (Solid State Drives) thanks to their attractive characteristics. They are lightweight, power efficient and supposed to outperform traditional disks. The two main constraints of flash memories are: 1) the limited number of achievable write operations beyond which a given cell can no more retain data, and 2) the erase-before-write rule decreasing the write performance. A RAM cache can help to reduce this problem; they are mainly used to increase performance and lifetime by absorbing flash write operations. RAM caches being very costly, their dimensioning is critical. In this paper, we explore some OLTP I/O workload characteristics with regards to flash memory cache systems structure and configuration. We try, throughout I/O workload analysis to reveal some important elements to take into account to allow a good dimensioning of those embedded caches.