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2530dc4679
This adds the implementation without using it in the code. The code still calls label_read() on each individual device to do scanning. Calling the new label_scan() will use async io if async io is enabled in config settings. If not enabled, or if async io fails, label_scan() will fall back to using synchronous io. If only some aio ops fail, the code will attempt to perform synchronous io on just the ios that failed. Uses linux native aio system calls, not the posix wrappers which are messier and may not have all the latest linux capabilities. Internally, the same functionality is used before: - iterate through each visible device on the system, provided from from dev-cache - call _find_label_header on the dev to find the sector containing the label_header - call _text_read to look at the pv_header and mda locations after the pv_header - for each mda location, read the mda_header and the vg metadata - add info/vginfo structs to lvmcache which associate the device name (info) with the VG name (vginfo) so that vg_read can know which devices to read for a given VG name The new label scanning issues a "large" read beginning at the start of the device, where large is configurable, but intended to cover all the labels/headers/metadata that is located at the start of the device. This large data buffer from each device is saved in a global list using a new 'label_read_data' struct. Currently, this buffer is only used to find the label_header from the first four sectors of the device. In subsequent commits, other functions that read other structs/metadata will first try to find that data in the saved label_read_data buffer. In most common cases, the data they need can simply be copied out of the existing buffer, and they can avoid issuing another disk read to get it.