Split install across SSD and HDD - optimum layout?

[Ralf Mardorf]

On Sat, 2020-04-04 at 09:05 +0100, Colin Law wrote:
> I am building a new system with an SSD for speed and a hard disc for
> capacity and am contemplating the best layout across the discs, so
> would be interested in any suggestions.
> I am thinking of putting the system on the SSD and /home on the HDD.
> Should I put /var on the HDD too as it has the logs on it?

Hi,

I wouldn't care about log files. The system should be on the SSD, but
also data for daily usage, current projects and home, since all the app
configurations are in home. While log files might be way larger than app
configuration files, I suspect that sheduling as well as the path of the
kernel routines matters here. Actually my PC has got 5 SSDs and no HDD,
I'm using external HDDs for archiving less often needed data and for
backups. IMO there's no reason to pay for expensive SSDs and a
combination of SSD and HDD isn't optimal. I decided to go with cheap
SSDs and no HDD at all.

When using a combination of SSD and HDD, then it only makes sense, if
all work related files, IOW the software, it's configurations and data
are available by the SSD or tmpfs.

As for log files I guess that the bottleneck more likely is the path of 
the kernel routines, than IO to/from a file. Apart from this not all log
files are located in /var, some are in $HOME, e.g.
.xsession-errors and this, as almost all other log files, too, should be
more or less empty.

[rocketmouse at archlinux ~]$ echo $(uptime | cut -f1 -dl ; cat .xsession-errors | wc -l; echo "lines")
11:17:23 up 1 day, 3:46, 1 user, 5 lines

If it shouldn't be more or less empty something is fishy, maybe just the
packager was to lazy to build by taking care of build flags, maybe
something is seriously broken, maybe... IOW if IO of log files is a
bottleneck, then something else is already fishy.

FWIW the cheap Toshiba-TL100 and Toshiba-TR-200 SSDs I'm using are
reliable and the vendor supports Linux with its ocz-ssd-utility, so I
don't need to care about the smartctl data base and even can update the
firmware of the SSDs during a Linux session, while the Linux session
does use those SSDs.

Regards,
Ralf