Re: [tlug] Making better use of SSDs?

[Kalin KOZHUHAROV <me.kalin@example.com>]

Just my 2yen below:

On Mon, May 28, 2012 at 11:22 AM, Raymond Wan <rwan.kyoto@example.com> wrote:
> My employer is thinking of purchasing a server and wants to also buy
> an SSD along with it.  I can only think of two ways of making use of
> an SSD under Linux and I was wondering if anyone has better ideas.
>
> 1)  Well, make it into a drive.  This would only benefit programs that
> have a lot of I/O or programs like Unix's 'sort' which can allow you
> to specify which temporary directory to use.  I can just specify the
> SSD drive.
>
Well, if you ever sort things that cannot fit in memory, I guess it will work...
But what about using a database?

> 2)  Make it into a swap partition.  This would help if the amount of
> memory used by a program exceeds main memory and it starts paging to
> disk.  Paging to an SSD would surely be faster.  (Assuming it pages
> often...)
>
> The problem with #2 is that you don't see the benefit of the SSD until
> your program thrashes.  And, of course, the SSD still isn't main
> memory.  What's on the SSD will be swapped out static pages.
>
swap is generally not considered these days as a viable option, no
matter if it is HDD or SDD. If you are doing so much calculations
(exceeding RAM), use a cluster or other way of to distribute the load.
Going from RAM to swap means dropping the theoretical transfer rates
from 12GB/s to 0.5GB/s at best, so 24 times; in practice and with
subpar SSDs that will be more like 50-100 times. So, a task that runs
for an hour in full memory will take up 100 hours (4 days!) to
complete in swap with double the data. I am sure that you can find
better solutions for your problem if you spend some of those 4 days
trying to and then computing it in 1-2h (or even faster!).

Also, don't forget SSDs get easily trashed and need often TRIM (and fs
that supports it) to keep them running at full speed.

As long as you are in control of the software/OS (i.e. you are not
running proprietary unoptimized software in a corner case), you are
better spending some time thinking of how to distribute the execution
than purchasing SSDs. And start with RAM if you are going that way,
anyway.

Cheers,
Kalin.