[Discuss] SSD caching

[Alan W. Irwin]

On 2015-03-27 14:32-0700 Darren Duncan wrote:

> On 2015-03-27 1:56 PM, John Blomfield wrote:
>> I guess there is not a lot of interest in this topic on this list;

Actually, John, there has been several responses so have you missed
those in your INBOX?

>> maybe 
>> because
>> with the cost of SSD's dropping there is a trend for laptops to be built 
>> solely
>> with an SSD and no Hard Drive and therefore no need for an SSD cache? This 
>> trend
>> tends perhaps to make SSD caching to speed up HD performance redundant?
>
> I agree.
>
> Personally I made the switch starting 16 months ago from all-HDD to all-SSD. 
> That is, prior to that, when I or my family got a new computer it had just an 
> HDD in it, and afterwards, the computer had just an SSD in it.
>
> I believe that SSDs are large enough for a small enough cost and have enough 
> benefits that one is best to use it as their primary storage device on a new 
> computer, and not just as a cache.  Getting HDDs today is then just when you 
> want supplemental storage in large capacity, but not as the primary storage.
>
> These days, SSDs of 256GB are very cheap, and larger ones are close to cheap, 
> and that size is large enough for all the typical day to day application or 
> document storage one needs.  If one does a lot of photos or videos and that 
> isn't large enough (for photos it still often is), larger SSDs are still 
> relatively inexpensive, or secondary HDDs can be used.
>
> The days of having SSDs just as caches is over, buying a new machine that 
> works this way is unreasonable.

Just to add some actual cost numbers to the discussion, the top 3
"special offer" HD's at
<http://www.ncix.com/category/hard-drives-8a-109.htm> cost ~$50 per
TB.  The equivalent numbers for SSD's (from
<http://www.ncix.com/category/solid-state-drives-ssd-f6-1275.htm) are
$150 per 0.25 TB or 12 times the cost of the HD per unit storage.  As
implied also by others in this thread, for small disk usage that cost
ratio doesn't really matter that much, but if you start needing more
disk space than say 200 GB (my case), you should probably look very
carefully at the HD option.

Other cost numbers from NCIX are that DDR4 memory currently costs
something like $300 for 16GB so it is an order of magnitude more
expensive than SSD memory.  I have also just done a quick google
check for search terms like <SSD speed> and <DDR4 speed>, and it
appears the two are very close to the same speed (at least if
a SATA 3 controller is used for the SSD).

In sum, I think it is always a good idea when buying a new system to
buy the maximum possible RAM for it because Linux can always use that
for some productive purpose (to reduce paging and/or do lots of file
caching) to make the effective speed of the system larger.  And
similarly, when buying hard drives I would tend to buy those with a
larger hardware memory cache.  That combination has worked well for my
present system (bought years ago before SSD's were prevalent).  I have
no present plans to buy, but when I do that in the future I may just
stick to that combo (i.e., ignore SSD's altogether), but the research
I have just done indicates that adding a substantial SSD file caching
capability might make sense for my (astronomical research computer)
case where I tend to have lots of large files being used
simultaneously.

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?

Alan
__________________________
Alan W. Irwin

Astronomical research affiliation with Department of Physics and Astronomy,
University of Victoria (astrowww.phys.uvic.ca).

Programming affiliations with the FreeEOS equation-of-state
implementation for stellar interiors (freeeos.sf.net); the Time
Ephemerides project (timeephem.sf.net); PLplot scientific plotting
software package (plplot.sf.net); the libLASi project
(unifont.org/lasi); the Loads of Linux Links project (loll.sf.net);
and the Linux Brochure Project (lbproject.sf.net).
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Linux-powered Science
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