Disabling use of anonymous fds in open-behind

[Raghavendra Gowdappa <rgowdapp@xxxxxxxxxx>]

All,

This is an option in open-behind, which lets fuse native mounts to use anonymous fds. The reasoning being since anonymous fds are stateless, overhead of open is avoided and hence better performance. However, bugs filed [1][2] seemed to indicate contrary results.

Also, using anonymous fds affects other xlators which rely on per fd state [3].

So, this brings to the point do anonymous-fds actually improve performance on native fuse mounts? If not, we can disable them. May be they are useful for light weight metadata operations like fstat, but the workload should only be limited to them. Note that anonymous fds are used by open-behind by only two fops - readv and fstat. But, [1] has shown that they actually regress performance for sequential reads.

[1] https://bugzilla.redhat.com/show_bug.cgi?id=1419807
[2] https://bugzilla.redhat.com/1489513, "read-ahead underperrforms expectations"
      open-behind without patch (MiB/s) with patch (MiB/s)
          on          132.87                133.51
          off         139.70                139.77

[3] https://bugzilla.redhat.com/show_bug.cgi?id=1084508

PS: Anonymous fds are stateless fds, where a client like native fuse mount doesn't do an explicit open. Instead, bricks do the open on-demand during fops which need an fd (like readv, fstat etc).

regards,

Raghavendra

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