[Stgt-devel] add aio backing store using eventfd

[FUJITA Tomonori]

I restored the aio backing store feature because Linux finally support
a resonable AIO event notification (eventfd). I tested the feature
with 2.6.24-rc8 but it also works with 2.6.23, I guess.

This backing store doesn't use the workaround in bs_rdwr to play with
pthread and pipe, which incurs large latency.

You can specify the backing store type when adding a logical
unit. Here is an example:

$ tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 -b /var/tmp/lun1 --bstype aio


Note that:

- Linux AIO works only with O_DIRECT. So this backing store mode
doesn't use page cache. You probably need fast disk to enjoy this
backing store. I guess that we still need to wait for Linux AIO to
support non O_DIRECT.

- The writeback cache is not supported yet. This always works in the
writethrough cache mode.