[mythtv-users] backend storage performance needs?

[Jan Ceuleers]

On 21/05/2020 05:11, Stephen Worthington wrote:
> And I was deliberately using weasel words as I have not been following
> drive technology well enough to know for sure that drives will
> guarantee being able to write their cache to journal space before
> losing the power to do that.  But that certainly seems to be what they
> are aiming for.  And I do not know at all whether drives receive and
> act on fsync, or whether they choose to ignore that signal and keep on
> doing their own caching.  Or maybe they have an option you can set to
> tell them which way to do things.  Enterprise drives are more
> configurable than desktop drives.

I suspect that on journaling filesystems fsync only guarantees that the
data has been committed to the journal (which is sufficient for ensuring
data integrity).

Unless you disable barriers.

http://fibrevillage.com/storage/565-what-s-barriers-how-to-enable-disable-it-on-linux

As I wrote before I run with disabled barriers. I can do that because my
server is powered through a UPS which gives it a few minutes of autonomy
in case of a power cut. In order to mitigate the risk that the server
might not shut down in time (which is a very small risk indeed) I enable
write barriers from apcupsd's onbattery script thusly:

while fs=`findmnt -l -t ext4 -O nobarrier --output=target --noheadings`
do
   mount -o remount,barrier $fs
done

and disable them again if power returns before shutdown by means of the
following in the offbattery script:

while fs=`findmnt -s -l -t ext4 -O nobarrier --output=target --noheadings`
do
   mount -o remount,barrier=0 $fs
done

Cheers, Jan