moss: Prime vfs cache on boot via systemd service #548
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Note that, in addition to being slow for cold blits, ext4 only supports 65k hardlinks per file, and it has a limit of 4k for combined extended attribute data as well. Given we are looking at supporting SELinux, there is a very real chance that we are artificially (and knowingly) limiting ourselves by offering ext4 as a rootfs for Serpent OS. NB: serpent-os/recipes#548 is now up as a way to help mitigate the ext4 and f2fs cold blit slowness until we figure out a better solution. Signed-off-by: Rune Morling <[email protected]>
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Note that, in addition to being slow for cold blits, ext4 only supports 65k hardlinks per file, and it has a limit of 4k for combined extended attribute data as well. Given we are looking at supporting SELinux, there is a very real chance that we are artificially (and knowingly) limiting ourselves by offering ext4 as a rootfs for Serpent OS. NB: serpent-os/recipes#548 is now up as a way to help mitigate the ext4 and f2fs cold blit slowness until we figure out a better solution. Signed-off-by: Rune Morling <[email protected]>
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On an old 4790K w/a SATA-600 SSD with ext4 rootfs, I got the following numbers: Hot vfs cache install blit (setup+teardown + blit): 13s-ish Video below is dinky, but does the job. ext4-hot-and-cold-vfs-cache-blit-2025-01-13.webm |
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This should serve to make the first blit after boot run against a hot vfs cache, which should improve the performance of the first blit for Serpent OS systems installed to ext4 or f2fs roots in particular. Neither ext4 nor f2fs blits will be as fast as xfs blits (though, surprisingly enough, ext4 blits come very close w/ hot blits), so xfs currently remains the recommended root filesystem for Serpent OS. Additionally, include a oneshot .service for testing the time to prime w/ a cold vfs cache. Per the request of @ikeycode this service is gated behind a `ConditionKernelCommandLine=moss.debug` flag, ensuring that it will never be started by accident. Users who need this sort of functionality can add a drop-in `/etc/kernel/cmdline.d/moss.debug.cmdline` snippet to access it. This PR was also rebased onto the main branch, which now shows blit stats on actions creating a new fstx. Signed-off-by: Rune Morling <[email protected]>
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This should serve to make the first blit after boot run against a hot vfs cache, which should improve the performance of the first blit for Serpent OS systems installed to ext4 or f2fs roots in particular.
Neither ext4 nor f2fs blits will be as fast as xfs blits (though, surprisingly enough, ext4 blits come very close w/ hot blits), so xfs currently remains the recommended root filesystem for Serpent OS.
Additionally, include a oneshot .service for testing the time to prime w/ a cold vfs cache. Per the request of @ikeycode this service is gated behind a
ConditionKernelCommandLine=moss.debugflag, ensuring that it will never be started by accident.Users who need this sort of functionality can add a drop-in
/etc/kernel/cmdline.d/moss.debug.cmdlinesnippet to access it.This PR was also rebased onto the main branch, which now shows blit stats on actions creating a new fstx.
Status after cold boot on xfs: