[8.19] Force niofs for fdt tmp file read access when flushing stored fields #130312
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…lastic#129538) Due to the way how stored fields get flushed when index sorting is active, it is possible that we encounter significant page cache faults when memory is scarce. In order to mitigate some of the slowness around this, we're planning to no longer mmap the fdt temp file. Initially behind a feature flag, to check for unforeseen side effects. Typically using always mmap directory is better compared to noifs directory given there is a sufficient memory available to the OS for filesystem caching. However when that isn't the case, then indexing performance can vary a lot (often very slow). This is more true for files tmp files that stored fields create during flushing. These files exist for only a brief moment to sort stored fields in the order of the configured index sorting and are then removed. If these tmp files are mmapped there is risk to trash file system cache. This change only avoids using mmap for the fdt tmp file. This the file that actually contains the data and can large compared to other files that get flushed. The fdm (metadata) and fdi (stored field index) remain being mmapped.
martijnvg
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Backporting #129538 and #130308 to 8.19 branch.
Due to the way how stored fields get flushed when index sorting is active, it is possible that we encounter significant page cache faults when memory is scarce. In order to mitigate some of the slowness around this, we're planning to no longer mmap the fdt temp file. Initially behind a feature flag, to check for unforeseen side effects.
Typically using always mmap directory is better compared to noifs directory given there is a sufficient memory available to the OS for filesystem caching. However when that isn't the case, then indexing performance can vary a lot (often very slow). This is more true for files tmp files that stored fields create during flushing. These files exist for only a brief moment to sort stored fields in the order of the configured index sorting and are then removed. If these tmp files are mmapped there is risk to trash file system cache.
This change only avoids using mmap for the fdt tmp file. This the file that actually contains the data and can large compared to other files that get flushed. The fdm (metadata) and fdi (stored field index) remain being mmapped.