Hi Christoph, On Mon, 23 May 2022, Christoph Hellwig wrote: > ... wait. > > Can someone explain what this is all about? Devices with power fail > protection will advertise that (using VWC flag in NVMe for example) and > we will never send flushes. So anything that explicitly disables flushed > will generally cause data corruption. Adriano was getting 1.5ms sync-write ioping's to an NVMe through bcache (instead of the expected ~70us), so perhaps the NVMe flushes were killing performance if every write was also forcing an erase cycle. The suggestion was to disable flushes in bcache as a troubleshooting step to see if that solved the problem, but with the warning that it could be unsafe. Questions: 1. If a user knows their disks have a non-volatile cache then is it safe to drop flushes? 2. If not, then under what circumstances is it unsafe with a non-volatile cache? 3. Since the block layer wont send flushes when the hardware reports that the cache is non-volatile, then how do you query the device to make sure it is reporting correctly? For NVMe you can get VWC as: nvme id-ctrl -H /dev/nvme0 |grep -A1 vwc ...but how do you query a block device (like a RAID LUN) to make sure it is reporting a non-volatile cache correctly? -- Eric Wheeler > >
