Kernels 4.10 and later (since commit a528d35e8b) have an enhanced statx() interface that can specify a bitmask to fetch specific attributes from a file (e.g. st_uid, st_gid, st_mode, and st_btime == file creation time), rather than fetching all of the normal stat() attributes (such as st_size and st_blocks). It also has a AT_STATX_DONT_SYNC mode which allows the kernel to return "lazy" stats without flushing all of the client data and fetching an accurate result from the server.
This matches (not coincidentally) very well with the Lustre attribute mechanism, and could be advantageous for fetching attributes from ls --color (needs st_mode) without having to do a glimpse to fetch the size (which is just discarded anyway).
The Lustre patch https://review.whamcloud.com/27715 "
LU-9558 llite: user enhanced getattr functionality in newer kernels" added very basic support for this interface, but it only went so far as to handle the kernel API changes, but doesn't actually handle the different attribute flags appropriately (it currently fetches and returns all normal attributes - STATX_BASIC_STATS). This interface should be wired up properly so that we can take advantage of the efficiencies available.
- only fetch MDS attributes if STATX_SIZE and STATX_BLOCKS are not requested, and avoid OSS glimpse RPCs completely
- hook this into statahead to avoid async glimpse locks (AGL) if OST information not needed
- enhance the MDS RPC interface to return the file creation time stored in both ldiskfs and ZFS already, and enable STATX_BTIME
- in conjunction with Lazy Size-on-MDT (LSOM,
LU-9538) return the LSOM size if AT_STATX_DONT_SYNC is used