[mythtvnz] Database management

[Stephen Worthington]

On Sat, 24 Jul 2021 09:03:03 +1200, you wrote:

>
>> And presuming that your new system has an SSD, then you probably want
>> to TRIM the SSD more than once a week, as the MythTV database activity
>> can use up quite a bit of the erased SSD space.  The default settings
>> for Ubuntu's fstrim.service and fstrim.timer are to only run fstrim
>> once a week.  So you should do:
>>
>> sudo edit fstrim.timer
>>
>> and put something like this in the systemd override file:
>>
>> [Unit]
>> Description=Discard unused blocks once a day
>>
>> [Timer]
>> OnCalendar=
>> OnCalendar=daily
>> AccuracySec=10m
>>
>> Or you could add the "discard" option to your SSD partitions in fstab.
>> Or both, as I am now doing.
>
>Hi ,
>
>Both my 16.04 and 20.04 systems use SSDs for the OS - storage is on HDD.
>
>I've read conflicting opinions on the use of discard.

Yes, there are people out there who say that using discard will slow
down write operations to the SSD.  My experience says that is not the
case.  Their argument is that after any disk operation that decreases
the size of a file, the file system then will need to tell the SSD
with TRIM commands what storage has now been freed for re-use.  And
that will slow things down.  However, I believe that the kernel
authors would not have been so foolish as to have the system calls
wait for that to happen before they return.  Instead, I think the TRIM
operations will just be being queued up for the kernel to execute when
it has idle time, and it will not hold up the software making the
original file API call.  And in any case, SSDs these days have a queue
of TRIM operations they keep internally, so the TRIM command will
return immediately after the operation is queued by the SSD (just
about instantly).  I started using discard after upgrading to 20.04
and have not seen any performance changes, and now fstrim normally
reports no blocks discarded, so it is working.  It is possible that
older kernels may not have had such a good implementation of discard,
so in 16.04 there might be a performance impact.  And if you have a
very old SSD with a bad implementation of TRIM there could be an
impact - I suspect that is where the warnings about using discard come
from.

>'sudo edit fstrim.timer' in both my Ubuntu 21.04 desktop and 20.04 myth 
>TV box, results in: 'Error: no "edit" mailcap rules found for type 
>"cannot open `fstrim.timer' (No such file or directory)" ' What is the 
>name of the systemd override file you refer to? Cheers -Paul

Sorry, that was a typo.  It should be:

sudo systemctl edit fstrim.timer

Systemd override files are found in directories under
/etc/systemd/system.  The directory name for fstrim.timer would be
/etc/systemd/system/fstrim.timer.d - any .conf files in that directory
would be override files for fstrim.timer.  The directory and file will
be automatically created by "systemctl edit", or you can create them
manually.  I think that using systemctl edit means that systemd will
know about the file immediately, but you may need to tell it to look
with:

sudo systemctl daemon-reload

which you will certainly need to do if you create them manually.