Device → device copy offload?

Posted Apr 3, 2025 7:25 UTC (Thu) by DemiMarie (subscriber, #164188)
In reply to: offload to ssd? neat by willy
Parent article: Updates on storage standards

What about offloading device to device copies? The device already needs to be able to do such copies for GC.

Device → device copy offload?

Posted Apr 3, 2025 13:31 UTC (Thu) by willy (subscriber, #9762) [Link]

I assume you mean intra-device copying (as opposed to one device sending data to another device, which is functionality that exists).

Funnily, it's a completely different operation from the device's point of view. The GC operation copies the data block intact and updates the FTL so that lookups of LBA 45678 now point to the new location on flash. An offloaded copy needs to read in the data block, decrypt, update the tags, encrypt, write it out and update the LBA. That's because both the encryption and tag verification use the LBA as the seed, not the location on the flash.

This is why I was never able to get the REMAP command into NVMe. It looks cheap from the host point of view, but it's very expensive for the drive. It saves PCIe bandwidth, but that's not generally the limiting factor.