On Fri, Feb 23, 2024 at 01:03:01AM +0530, Kanchan Joshi wrote: > With respect to the current state of Meta/Block-integrity, there are > some missing pieces. > I can improve some of it. But not sure if I am up to speed on the > history behind the status quo. > > Hence, this proposal to discuss the pieces. > > Maybe people would like to discuss other points too, but I have the > following: > > - Generic user interface that user-space can use to exchange meta. A > new io_uring opcode IORING_OP_READ/WRITE_META - seems feasible for > direct IO. Buffered IO seems non-trivial as a relatively smaller meta > needs to be written into/read from the page cache. The related > metadata must also be written during the writeback (of data). > > > - Is there interest in filesystem leveraging the integrity capabilities > that almost every enterprise SSD has. > Filesystems lacking checksumming abilities can still ask the SSD to do > it and be more robust. > And for BTRFS - there may be value in offloading the checksum to SSD. > Either to save the host CPU or to get more usable space (by not > writing the checksum tree). The mount option 'nodatasum' can turn off > the data checksumming, but more needs to be done to make the offload > work. As I understand it, btrfs's checksums are on a variable extent size, but offloading it to the SSD would do it per block, so it's forcing a new on-disk format. It would be cool to use it, though: you could atomically update data and checksums without stable pages. > NVMe SSD can do the offload when the host sends the PRACT bit. But in > the driver, this is tied to global integrity disablement using > CONFIG_BLK_DEV_INTEGRITY. > So, the idea is to introduce a bio flag REQ_INTEGRITY_OFFLOAD > that the filesystem can send. The block-integrity and NVMe driver do > the rest to make the offload work. > > - Currently, block integrity uses guard and ref tags but not application > tags. > As per Martin's paper [*]: > > "Work is in progress to implement support for the data > integrity extensions in btrfs, enabling the filesystem > to use the application tag."
