> а рассказывает чем readahead плохо ? и то что нужен баланс интересов..

- If you work on an appliance, consider disabling readahead collection in the shipped devices, but leave readahead replay enabled.
- Make use of EXT4_IOC_MOVE_EXT in systemd's readahead implementation. This allows reordering/defragmentation of the files needed for boot. According to the data from http://e4rat.sourceforge.net/ this might shorten the boot time to 40%. Implementation is not trivial, but given that we already support btrfs defragmentation and example code for this exists (e4rat as linked) should be fairly straightforward.
- Compress readahead pack files with XZ or so. Since boot these days tends to be clearly IO bound (and not CPU bound) it might make sense to reduce the IO load for the pack file by compressing it. Since we already have a dependency on XZ we'd recommend using XZ for this.
- Update the readahead logic to also precache directories (in addition to files).
- Add an option for service units to temporarily bump the CPU and IO priority of the startup code of important services. Note however, that we assume that this will not bring much and hence recommend looking into this only very late. Since boot-up tends to be IO bound, solutions such as readahead are probably more interesting than priorizing service startup IO. Also, this would probably always require a certain amount of manual configuration since determining automatically which services are important is hard (if not impossible), because we cannot track properly which services other services wait for.
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В общем, ходите по ссылкам, они полезные...