[mythtv-users] best use of limited RAM

[Mike Hodson]

Dear Marius,

What kind of SSD?  Sandforce, or other? Model would be most helpful.
What kind of NAND? Single-level? Multi-Level? Three-level?(almost assuredly
not 3-level due to size/age)

Could you paste a smartctl -a /dev/sdX ?

What are you attempting to cache? Reads or writes?

Reads, while potentially disruptive if you read the exact same sector
enough times, are almost certainly mitigated in the wear-leveling action of
the disk in question.

Writes never will 100% cache, unless you use something insane like XFS
which never seems to write data until sync() is called, if enough ram is
free.  Maybe theyve fixed that by now, but I know I've lost files I saved
when the system (after MANY hours) subsequently hard-locked due to kernel
weirdness with graphics drivers..

I've personally done a rather large amount of SSD usage over the past
..when was Sandforce 1 released? Many terabytes of writes per drive and not
a single disk has died on me, not even a tiny old 60GB one.

I've also worked for companies using consumer-grade (850Evo among others)
SSDs in enterprise webhosting scenarios. They rarely die, and almost always
give fair warning with SMART values before-hand.   Also, have you taken a
look at The Tech Report's "SSD Endurance Experiment" ?  its final
conclusion is that some disks can write PETABYTES before dying.
http://techreport.com/review/27909/the-ssd-endurance-experiment-theyre-all-dead

For MythTV use, the MySQL database would be the only major workload as long
as all video goes to the spinner; its definitely not anywhere near as bad
as shoving about 10-25 virtual private servers on each disk doing
$entity-knows what for the course of a few years. In RAID5/6 no less.

Periodic /var/log/messages and similar background OS daily workload is
negligible.

As for Swap?  why would your system need swap? Disable it entirely and let
things OOMkill themselves if something goes awry.

Or be like my desktop, and set 20GB of swap for a 16GB system (effectively
10GB for an 8GB system) to have room to hibernate, and potentially let some
random process chew up all 10 gigs before OOMkilling itself, and bogging
down the system for _minutes_ before the oomkiller responds properly.

I've only run into massive swap usage in 2 scenarios:
1, if I allow processes like Chrome to continue to spawn new tabs like
rabbits and never close them; sooner or later the swap finally fills, and
things OOMkill. but this is _gradual_ swapping.

2, I attempt to compile something large with -j16 as Sabayon is by default
setup to do, if you fail to tweak the out-of-the-box make.conf ... Ugh.

In almost all cases, just use the SSD as you would a normal disk; almost
industry-wide the wear-leveling algorithms were rock-solid as of about 4
years ago.

Hope this helps, and I would love to know the SMART data as well to make
proper predictions.

Mike


On Wed, Nov 22, 2017 at 7:41 AM, Marius Schrecker <marius.schrecker at lyse.net
> wrote:

> Hi,
>
>   I am repurposing an old, box that was previously used as a combined
> mythtv backend/frontend, Logitech Media Server and nfs fileserver as a
> mythtv backend only.
>
> The system has a maximum of 8GB RAM, a quite well used 120GB SSD (no signs
> of failure yet) and I just replaced the 3TB media storage spindle drive
> with a new 4TB unit.
>
> My main concern is offloading the SSD as much as possible to prolong its
> life.
>
> I have started with a base install of xubuntu, set up fstab wioth tempfs
> for /tmp, /var/tmp and /var/spool/mqueue and installed log2ram
> <https://github.com/azlux/log2ram> to take care of /var/log. Relatime is
> set by default on all partitions.
>
> The original plan was to increase the RAM to 16GB or even 32GB for maximum
> disk caching, but I see that my mobo doesn't support that, even if the
> physical dimm modules are available.
>
>   Does the group have any suggestions for tweaking to make my 8GB go as
> far as possible to ghelp minimise disk writes?
>
> I am in two minds about creating a swapfile on the spindle drive, maybe
> with zram and keeping swappiness relatively high. Would that encourage the
> system to increase the amount of RAM used for disk caching?
>
> BR.
>
> --Marius--
>
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