Some block storage device standards are working on a “logical depop" function which allows a system to decommission a defective physical element, e.g. a disk head or an SSD die or channel, reformat the device and continue using it with a reduced capacity. Such feature can allow reduced operation costs (delayed device replacement) but has the drawback of data loss (data under the remaining valid physical elements) and device downtime during re-formatting.
Online logical depop is another proposed new feature allowing retaining the device valid data and eliminating the need for a re-format. The basic idea is to introduce new commands for the host to discover the ranges of LBAs impacted by a defective element. Using this information, the host can take actions when an element failure event is suspected or reported by the device: deallocate the LBAs served by the defective element and operate the device in a thin-provisioned mode, amputate the LBAs or truncate the device LBA range to restore operation as a fully provisioned device with a lower capacity.
The goal of this BoF is to discuss the usefulness of such features and gather feedback from different application and system point of views for drafting a standard minimizing the impact of this feature on existing systems as well as enabling enhanced functionality for file systems, device mapper drivers (including logical volume manager) and application level distributed storage systems (key value stores, object stores).
