Power-fail safe writes

[Christophe Leroy]

On 04/02/2019 07:41 AM, Sebastian Brand wrote:
> Hello,
> 
> I recently had problems when updating a configuration file using sed 
> with in-place edit option (-i), shortly followed by a power fail, which 
> has the end result of an empty configuration file and a system that 
> won't boot.
> The reason behind this is me relying on sed and sed in-place editing not 
> using a power-fail safe write pattern (copy, edit copy, synchronize 
> copy, rename, synchronize folder).
> I have locally written a feature to enable "safe write" for sed, but 
> since this is a problem for many flash filesystems there might be more 
> people interested in this? (Perhaps especially people using Busybox for 
> embedded systems?) And this is probably not only relevant for sed?

All writes are affected I believe, so there is no point in modifying 
such or such command to perform sync writes, especially as it is 
counterperformant. There are power safe systems that don't require that.

On our boards, what we do is simply mounting flash filesystems in sync mode:

mount -t ubifs -o sync ubi0:rootfs /rootfs

Christophe


> 
> There is also a part 2 that arises when trying to "manually" do a safe 
> write: since the fsync applet uses the O_NOATIME flag it is impossible 
> for user1:group1 to synchronize a directory owned by user2:group1 (you 
> get an "Operation not permitted" error from fsync). Is this intended? 
> (Using gnu sync [8.26] and synchronizing on a specific folder works for 
> the same situation.)
> 
> Regards
> Sebastian Brand
> 
> 
> 
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