Re: [PLUG] swappiness and ssd

[brent timothy saner <brent.saner@gmail.com>]

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(responses inline)

On 07/11/2015 12:08 PM, Keith C. Perry wrote:
(SNIP)
> Doesn't a swappiness of 80 mean you are swapping
> /more/ than when the system is set to 60?

https://en.wikipedia.org/wiki/Swappiness explains a fair bit of the
concept behind swappiness, but generally the higher the number, the more
the system is going to try to use swap space rather than physical memory.

for more in-depth,
http://askubuntu.com/questions/184217/why-most-people-recommend-to-reduce-swappiness-to-10-20
has some good information (which is surprising for askubuntu.com, but i
digress...)

the swappiness number is a percentage (*roughly*) of your unused RAM.
i.e. swappiness = 30, then start swapping when the physical memory is
70% consumed.

Now would be a good time to mention you should actually use
vm.swappiness = 1 rather than vm.swappiness = 0 if you're using a kernel
>= 3.5.x - some intelligent optimizations have been done to make 1
behave more like what 0 was previously used for.

> Also, if that is the case, does that imply you have to run fstrim more?

nah, running fstrim regularly (assuming it's on a disk that doesn't
perform its own collection routines, and some do) will be enough.

see https://wiki.archlinux.org/index.php/Solid_State_Drives for some
great info on this.


plus write lifetimes on SSD's have gotten WAY better as well.


> I'm an XFS user and thankfully realtime discarding is available if I am
> ok with the performance penalty and I don't want to use batch mode.
>  Something to remember for my next server build which will  use an SSD
> for the system drive.

i use RAID-1 SSD for / on my box with ext4 (created with tune2fs -o
trim), and it's definitely a boon (i mount /root, /home, and /opt on
platters as that's where the bulk of my data-at-rest goes).


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