Notes on disk flush commands and why they are important.

Michel Talon talon at
Sun Jan 10 11:08:23 PST 2010

Matthew Dillon wrote:

>     * A large number of your installations will be running systems without
>       a UPS or without shutdown signaling mechanics.  The enterprise
>       systems will not, but these operating systems are not designed JUST
>       for enterprise use.  How about the home client or server?  What         
>       about turnkey systems trying to minimize costs? 

I think you are perfectly right. I am quite sure that the vast majority of
systems running Linux or FreeBSD are built cheaply with sata disks and so
on, and don't run on UPS. Moreover there are many other reasons for stuck
systems besides power failures, such as kernels panics etc., X failures, ...
which can only be resolved by hitting the power button. The most severe
softupdate failures i have seen are of the last category. Hence i think
that adding a journal which doesn't issue physical flush commands to disk
when necessary is of very limited usefulness. If i remember well, one of
the features of gjournal was that such syncs were managed by gjournal, at
least for the disks which obey them, thus greatly improving the security
of the filesystem.

Michel Talon

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