On Tue, May 07, 2019 at 11:12:07AM -0500, Probir Roy wrote: > > I am running Phoronix-fio benchmark on Linux kernel 4.18.0-rc5. I > observe the same nodes have been traversed on the extent status tree > in "ext4_es_lookup_extent" function when ext4 write begins. What's the > locality signature of "ext4_es_lookup_extent" in general? Is it > possible that same logical block being looked up repeatedly > (Temporal)? Is it possible that co-located logical blocks are searched > by ext4_es_lookup_extent (spatial)? Or is it totally random? I'm not sure what you are asking. The ext4_es_lookup_extent() is used as a fast map of an inode's logical block number to find the physical block number (e.g., the location on disk). It's a cache; lookups are fast, and is an in-memory lookup. Well, it's a little more than a cache, it also stores some information for delayed allocation buffered writes. If the workload is a random read or random write workload, then accesses to look up logical to physical block maps will be random. If the workload is mostly a sequential read or sequential write access, then the logical blocks looked up via ext4_es_lookup_extent() will largely be sequential. - Ted
