[Discuss] systemd reboot

[joe at polcari.com (Joe Polcari)]

Thanks for the sync update!

Sent from my iPhone

> On Mar 4, 2018, at 9:20 AM, Richard Pieri <richard.pieri at gmail.com> wrote:
> 
> The old "sync;sync;sync;halt" mantra is folklore from the days before we
> had a shutdown/reboot command which does this for us. The first sync
> flushes any dirty buffers, the second blocks waiting for the first to
> complete ensuring that there are no dirty buffers when the system goes
> down, and the third... makes us feel good (it has no technical benefit).
> 
> This doesn't work as expected today because most drives lie about
> committing writes to permanent storage. The second sync won't block
> unless the size of data in dirty kernel buffers exceeds the drives'
> write cache capacity and then it will block only long enough for that
> ratio to flip. If the system restarts, loses power, whatever, when the
> drives' on-board caches have not been committed then there will be data
> loss. The Linux kernel code which guarantees that writes are committed
> doesn't actually work because it relies on drives not lying about their
> cache commits.
> 
> In which case the explicit sync in the script doesn't do anything in
> terms of flushing data to disk. It does add a small delay between
> running update-grub and the reboot which, I guess, gives your drives
> enough time to commit their caches.
> 
> -- 
> Rich P.
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