Hi, > Can you explain further why it's necessary given that the kernel > explicitly blocks execution of close() to flush buffers to disk, > assuming the process is the last one with the device open? Am I > misinterpreting the code path I mentioned later in my email? In > practice, I can see this happening when I use dd. >From what I could gather empirically, close() does write dirty pages from the block device's page cache to disk but does not submit a "flush" request to the device driver. Depending on the driver and device, this may be necessary to ensure that data is persistently stored (e.g., if the device features an internal write-back cache). Performing a sync command with the block device as an argument, or reopening the device and invoking fsync() or fdatasync(), submits and awaits that request. The existence or non-existence of a device-internal write-back cache, and differences in the timing and policies of such a cache, may explain why in some cases the sync appears necessary and in others it doesn't. Alberto Faria
