For most SSDS, we can see that they have onboard DRAM, which is mainly used to store this mapping table. SSD based on sandforce master is exception, which do not support on-board DRAM. So where does its mapping exist? When SSDs work, most of their mappings are stored in FLASH, and some are stored on on-chip RAM.
When HOST needs to read some data, for SSD with DRAM, it just look up the mapping table in the DRAM to get the physical address and then access FLASH to get HOST data. During this time, Flash is only accessed once. While for SSD with Sandforce, it first check whether mapping table Host page corresponding to is in RAM. If it is there, Flash is read by mapping table directly. If not, it first read the mapping table from FLASH, and then read the Host data based on that mapping. This means that SSDs with Sandforce need to read FLASH twice to get HOST data out compared with DRAM, resulting in the underlying effective bandwidth halved. So we can see the random read performance of Sandforce-based SSDs is not ideal.
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