v2: Mainly fixes the fs_devices::readmirror declaration type from atomic_t to u8. (Thanks Josef). v1: As of now we use only %pid method to read stripped mirrored data. So application's process id determines the stripe id to be read. This type of routing typically helps in a system with many small independent applications tying to read random data. On the other hand the %pid based read IO distribution policy is inefficient if there is a single application trying to read large data as because the overall disk bandwidth would remains under utilized. One type of readmirror policy isn't good enough and other choices are routing the IO based on device's waitqueue or manual when we have a read-preferred device or a readmirror policy based on the target storage caching. So this patch-set introduces a framework where we could add more readmirror policies. This policy is a filesystem wide policy as of now, and though the readmirror policy at the subvolume level is a novel approach as it provides maximum flexibility in the data center, but as of now its not practical to implement such a granularity as you can't really ensure reflinked extents will be read from the stripe of its desire and so there will be more limitations and it can be assessed separately. The approach in this patch-set is sys interface with in-memory policy. And does not add any new readmirror type in this set, which can be add once we are ok with the framework. Also the default policy remains %pid. Previous works: ---------------------------------------------------------------------- There were few RFCs [1] before, mainly to figure out storage (or in memory only) for the readmirror policy and the interface needed. [1] https://www.mail-archive.com/linux-btrfs@xxxxxxxxxxxxxxx/msg86368.html https://lore.kernel.org/linux-btrfs/20190826090438.7044-1-anand.jain@xxxxxxxxxx/ https://lore.kernel.org/linux-btrfs/5fcf9c23-89b5-b167-1f80-a0f4ac107d0b@xxxxxxxxxx/ https://patchwork.kernel.org/cover/10859213/ Mount -o: In the first trial it was attempted to use the mount -o option to carry the readmirror policy, this is good for debugging which can make sure even the mount thread metadata tree blocks are read from the disk desired. It was very effective in testing radi1/raid10 write-holes. Extended attribute: As extended attribute is associated with the inode, to implement this there is bit of extended attribute abuse or else makes it mandatory to mount the rootid 5. Its messy unless readmirror policy is applied at the subvol level which is not possible as of now. An item type: The proposed patch was to create an item to hold the readmirror policy, it makes sense when compared to the abusive extended attribute approach but introduces a new item and so no backward compatibility. ----------------------------------------------------------------------- Anand Jain (3): btrfs: add readmirror type framework btrfs: sysfs, add readmirror kobject btrfs: sysfs, create by_pid readmirror attribute fs/btrfs/sysfs.c | 74 ++++++++++++++++++++++++++++++++++++++++++++++++++++++ fs/btrfs/volumes.c | 16 +++++++++++- fs/btrfs/volumes.h | 9 +++++++ 3 files changed, 98 insertions(+), 1 deletion(-) -- 1.8.3.1
