The IFS Restoration feature applies to systems that require an ultra-low power mode in which the main CPU is completely turned off while the SDRAM is left in a self-refresh state. This provides the ability to create a low-power mode with power consumption ranging from 1-3 mA (depending on the power draw of the SDRAM in self-refresh) with an accelerated wake-up/reboot.
IFS Restoration makes use of the fact that the image filesystem (IFS) is already in RAM on wake-up from this state and avoids the slow process of copying the image from FLASH to RAM. Note that when booting for the first time (SDRAM turned off), there are no time savings—savings are only on subsequent boots:
- On initial power-up, the system performs a full boot.
- On subsequent boots in which the SDRAM was left in self-refresh, the system performs an accelerated boot via IFS restoration. A full boot is still performed; only the copying of the image from flash to RAM is avoided.
- The boot time saved depends on the size of the image in flash; the larger the image, the more time is saved.
- The CPU and OS context aren't saved or restored. Although this would be faster, it requires device-driver-, CPU-, and board-specific support. The IFS Restoration feature is generic and doesn't require platform-specific development (other than the code to put the SDRAM to self-refresh).
The following diagram shows the difference between a normal boot and an accelerated boot using IFS Restoration:
There are two mechanisms to achieve the IFS restoration:
- Kernel restoration
- The OS is booted using a single image including QNX Neutrino startup, kernel, and the IFS (containing libraries, drivers, and applications). On wake-up, all of the startup and only part of the kernel (the data section of the procnto binary) are copied to RAM. The remainder of the IFS is reused without having to copy.
- Secondary IFS restoration
- The OS is booted using two images: a primary IFS containing only the
QNX Neutrino startup and kernel, and a secondary IFS containing all other
libraries, drivers, and applications.
The secondary IFS is automatically mounted.
On wake-up, the entire primary IFS is copied, while the secondary IFS can be reused without having to copy. Note that additional binaries can be added to the primary IFS, but these will be copied along with the rest of the primary IFS.
The advantage of kernel restoration is that it can be enabled and used with very little intervention by the user. Secondary IFS restoration may be desired for projects that wish to use multiple image filesystems, but requires more management by the user, such as maintaining multiple buildfiles and specifying the secondary IFS size and location.
It's possible to enable both kernel and secondary IFS restoration at the same time (i.e., the startup and kernel from the primary IFS are restored, and the secondary IFS is reused and automatically mounted).
IFS Restoration was designed to support the following features:
- compressed and uncompressed images
Note: Compressed secondary image filesystems aren't currently supported.
- IFS checksum — both primary and secondary IFSs can be verified using an optimized checksum algorithm (checksum can be enabled or disabled)
- compatibility with minidrivers
- automatic secondary IFS source and destination address selection