[Discuss] SSD caching

[Darren Duncan]

On 2015-03-27 4:42 PM, Alan W. Irwin wrote:
> So the question I have for both John and Darren is for users like me
> where HD disk space is essential, does Linux automatically take good
> advantage of the combination of large RAM, large HD disk cache, and
> very large SSD file caching capability for the HD or is that SSD file
> caching capability mostly just wasted by Linux?

I don't know the answer to your question as to what Linux will automatically do.

But my point is that from the perspective of simplicity and ease of customizing 
a system, having a permanent storage device whose sole purpose is caching 
because it is too small for regular work, is a bad idea for a new system.  If 
you need a large amount of cheap storage and only want 1 storage device, just 
have a HDD and no SSD, and get lots of RAM as the way of making things faster.

If you're going to have a SSD at all, it should be a minimum of say 128GB, or 
large enough to be a main ordinary storage device for regular day to day stuff. 
  If the economics aren't there to get an SSD large enough to be a primary disk, 
then don't get it at all.  Personally I think SSDs hit that place, hence I got a 
1TB SSD a year ago ( 
http://www.newegg.ca/Product/Product.aspx?Item=N82E16820148696 to be specific ), 
and they'll only get cheaper.  Later down the road the SSD will be cheap enough 
for you.

Incidentally, my understanding is that where SSDs shine speed-wise is when you 
have lots of random-access or small files, and if you're mainly doing sequential 
access of large files, such as video data, HDDs are still faster.  Also writes 
on SSDs are slower than reads, while on HDDs they're both the same speed I 
think.  SSDs have the benefit of no moving parts and lower power usage.

-- Darren Duncan