Tracing syscalls, opening and reading an `xl.meta` looks like this:
```
openat(AT_FDCWD, "/mnt/drive1/ss8-old/testbucket/ObjSize4MiBThreads72/(554O51H/peTb(0iztdbTKw59.csv/xl.meta", O_RDONLY|O_NOATIME|O_CLOEXEC) = 34 <0.000>
fcntl(34, F_GETFL) = 0x48000 (flags O_RDONLY|O_LARGEFILE|O_NOATIME) <0.000>
fcntl(34, F_SETFL, O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_NOATIME) = 0 <0.000>
epoll_ctl(4, EPOLL_CTL_ADD, 34, {events=EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, data={u32=3172471557, u64=8145488475984499461}}) = -1 EPERM (Operation not permitted) <0.000>
fcntl(34, F_GETFL) = 0x48800 (flags O_RDONLY|O_NONBLOCK|O_LARGEFILE|O_NOATIME) <0.000>
fcntl(34, F_SETFL, O_RDONLY|O_LARGEFILE|O_NOATIME) = 0 <0.000>
fstat(34, {st_mode=S_IFREG|0644, st_size=354, ...}) = 0 <0.000>
read(34, "XL2 \1\0\3\0\306\0\0\1P\2\2\1\304$\225\304\20\0\0\0\0\0\0\0\0\0\0\0"..., 354) = 354 <0.000>
close(34) = 0 <0.000>
```
Everything until `fstat` is the `os.Open` call.
Looking at the code: https://github.com/golang/go/blob/master/src/os/file_unix.go#L212-L243
It seems for every file it "tries" to see if it is pollable. This causes `syscall.SetNonblock(fd, true)` to be called. This is the first `F_SETFL`.
It then calls `f.pfd.Init("file", true)`. This will attempt to set it as pollable using `epoll_ctl`. This will always fail for files. It therefore calls `syscall.SetNonblock(fd, false)` resulting in the second `F_SETFL`.
If we set the `O_NONBLOCK` call on the initial open, we should avoid the 4 `fcntl` syscalls per file.
I don't see any way to avoid the `epoll_ctl` call, since kind is either `kindOpenFile` or `kindNonBlock`, so "pollable" will always be true. However avoiding 4 of 6 syscalls still seems worth it.
This should not have any effect, since files will end up with "nonblock" anyway.
Instead of using O_SYNC, we are better off using O_DSYNC
instead since we are only ever interested in data to be
persisted to disk not the associated filesystem metadata.
For reads we ask customers to turn off noatime, but instead
we can proactively use O_NOATIME flag to avoid atime updates
upon reads.
Klaus Post
Harshavardhana
Steve Wills