Asynchronous usage of sg

The Linux SCSI Generic (sg) HOWTO
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# Chapter 12. Asynchronous usage of sg

It is recommended that synchronous sg-based applications use the new SG_IO ioctl() command. Existing applications (which are mainly synchronous) can continue to use the older sg_header based interface which is still supported.

Asynchronous usage allows multiple SCSI commands to be queued up to the device. If the device supports command queuing then there can be a major performance gain. Even if the device doesn't support command queuing (or is temporarily busy) then queuing up commands in the mid level or the host driver can be a minor performance win (since there will be a lower latency to transmit the next command when the device becomes free).

Asynchronous usage usually starts with setting the O_NONBLOCK flag on open() [or thereafter by using the fcntl(fd, SETFD, old_flags | O_NONBLOCK) system call]. A similar effect can be obtained without using O_NONBLOCK when POSIX threads are used. There are several strategies that can then be followed:

1. set O_NONBLOCK and use a poll() loop

2. set O_NONBLOCK and use SIGPOLL signal to alert app when readable

3. use POSIX threads and a single sg file descriptor

4. use POSIX threads and multiple sg file descriptors to same device

The O_NONBLOCK flag also permits open(), write() and read() [but not the ioctl(SG_IO)] to access a SCSI device even though it has been marked offline. SCSI devices are marked offline when they are detected and don't respond to the initial SCSI commands as expected, or, some SCSI error condition is detected on that device and the mid level error recovery logic is unable to "resurrect" the device. A SCSI device that is being reset (and still settling) could be accessed during this period by using the O_NONBLOCK flag; this could lead to unexpected behaviour so the sg user should take care.

In Linux SIGIO and SIGPOLL are the same signal. If POSIX real time signals are used (e.g. when SA_SIGINFO is used with sigaction() and fcntl(fd, F_SETSIG, SIGRTMIN + <n>) ) then the file descriptor with which the signal is associated is available to the signal handler. The associated file descriptor is in the si_fd member of the siginfo_t structure. The poll() system call that is often used after a signal is received can thus be bypassed.

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