[gentoo-user] Re: aligning SSD partitions

[Nicolas Sebrecht <nsebrecht@piing.fr>]

The 06/09/12, Dale wrote:

> I do get it.  I CLEARED #1 and #2, there is no usage of #3 and #4 is not
> large enough here to matter.  So, it is left with #5. 
> 
> See the point?  The test was a NORMAL emerge with portages work
> directory on tmpfs and a NORMAL emerge with portages work directory on
> disk and compare the results.  The test resulted in little if any
> difference. 
> 
> If I ran the test and did not clear the cache, then I would expect
> skewed results because after the first emerge, some files would be
> cached in ram and the drive would not be used.  If you clear the cache,
> then it has to take the same steps regardless of whether it was run
> first, second or third time. 

What you want to measure is the difference of times required by emerge
whether you use a real disk or tmpfs as backend.

What you would expect is a difference because a disk is much slower than
RAM.

What you see is no difference. You won't conclude that disk is as fast
as RAM, right? Can you explain why you don't see much difference? No.

Here is the explanation: if you have enough RAM, the emerge rapidity
will NOT rely on the disk rapidity whatever storage backend you use. It
will only rely on the RAM rapidity because of the kernel cache.

Now, pretending that whatever backend you use (real disk or tmpfs) never
changes the emerge time is WRONG because of the persistence strategy
used by the kernel for the kernel cache.

When having lot of RAM like you have, the persistence strategy of the
kernel cache is NEVER raised in the process.

This is exactly what your tests demonstrate demonstrate: if you have
enough RAM, the persistence strategy of kernel cache is not raised, so
everything happens in RAM, so the emerge times do not differ.

-- 
Nicolas Sebrecht