Hi, Damien and I would like to propose a couple of topics centering around zoned block devices: 1) Zoned block devices require that writes to a zone are sequential. If the writes are dispatched to the device out of order, the drive rejects the write with a write failure. So far it has been the responsibility the deadline I/O scheduler to serialize writes to zones to avoid intra-zone write command reordering. This I/O scheduler based approach has worked so far for HDDs, but we can do better for multi-queue devices. NVMe has support for multiple queues, and one could dedicate a single queue to writes alone. Furthermore, the queue is processed in-order, enabling the host to serialize writes on the queue, instead of issuing them one by one. We like to gather feedback on this approach (new HCTX_TYPE_WRITE). 2) Adoption of Zone Append in file-systems and user-space applications. A Zone Append command, together with Zoned Namespaces, is being defined in the NVMe workgroup. The new command allows one to automatically direct writes to a zone write pointer position, similarly to writing to a file open with O_APPEND. With this write append command, the drive returns where data was written in the zone. Providing two benefits: (A) It moves the fine-grained logical block allocation in file-systems to the device side. A file-system continues to do coarse-grained logical block allocation, but the specific LBAs where data is written and reported from the device. Thus improving file-system performance. The current target is XFS but we would like to hear the feasibility of it being used in other file-systems. (B) It lets host issue multiple outstanding write I/Os to a zone, without having to maintain I/O order. Thus, improving the performance of the drive, but also reducing the need for zone locking on the host side. Is there other use-cases for this, and will an interface like this be valuable in the kernel? If the interface is successful, we would expect the interface to move to ATA/SCSI for standardization as well. Thanks, Matias
