It's a common technique in SAN controllers, where writes go to a battery-backed cache, and are then written to the actual storage devices (using variations of R

It's a common technique in SAN controllers, where writes go to a battery-backed cache, and are then written to the actual storage devices (using variations of RAID) at the controller's leisure. The limited space in the immortal DRAM provides enough buffer to give the host near immediate responses (sub-sub-millisecond over fiber channel), while the data is safe if the controller crashes or loses power (the data is written as part of the recovery process). This organization also lets the controller gather up related writes and be more efficient about them, rather than writing each block in isolation. The battery-backed DRAM really is an implementation detail. Things will execute correctly if you don't have it, but it's usually a huge performance win. In this case we're using Nimble SANs (Nimble is now owned by HP and suffering customer service rot, oh well). The immortal DRAM is fairly small (8GB per controller head?). The storage arrays are petabyte scale, all flash, with many, many 16 and 32 Gbit fiber channel connections. Cost is a few million per SAN instance, of which you need several for real durability (and a replication scheme for remote storage, which I'm not going to discuss here).