Recently, the capital expenditure of flash-based Solid State Driver (SSDs) keeps declining and the storage capacity of SSDs keeps increasing. As a result, all-flash storage systems have started to become more economically viable for large shared storage installations in datacenters, where metrics like Total Cost of Ownership (TCO) are of paramount importance. On the other hand, flash devices suffer from write amplification, which, if unaccounted, can substantially increase the TCO of a storage system. In this paper, we first develop a TCO model for datacenter all-flash storage systems, and then plug a Write Amplification model (WAF) of NVMe SSDs we build based on empirical data into this TCO model. Our new WAF model accounts for workload characteristics like write rate and percentage of sequential writes. Furthermore, using both the TCO and WAF models as the optimization criterion, we design new flash resource management schemes (MIN TCO) to guide datacenter managers to make workload allocation decisions under the consideration of TCO for SSDs. Based on that, we also develop MIN TCO-RAID to support RAID SSDs and MIN TCO-O FFLINE to optimize the offline workload-disk deployment problem during the initialization phase. Experimental results show that MIN TCO can reduce the TCO and keep relatively high throughput and space utilization of the entire datacenter storage resources.