Re: Could RAM possibly be just 3-4 times faster than bare hdd writes and reads? or, is the Linux kernel doing its 'magic' in the bg? or, ...
> Your test dataset is too small and you aren't flushing the cache before exiting dd, so you are largely seeing the time it takes to write to cache, not to disk.
> But that gives the RAID10 system 220 IOPs, still nowhere near the 100,000 IOPs of a single SSD.
> I suggest that you google a bit on how to do fileystem benchmarks first, then try it and report back if something is still odd.
> Your test dataset is too small and you aren't flushing the cache before exiting dd, so you are largely seeing the time it takes to write to cache, not to disk.
. . .
Oh, well, yes. I knew that I was "seeing" something that wasn't quite right.
Your answers grounded me on such issues.
Thank you und Entschuldigung!
lbrtchx
On 6/17/20, Anders Andersson <pipatron@gmail.com> wrote:
> On Wed, Jun 17, 2020 at 12:15 PM Albretch Mueller <lbrtchx@gmail.com>
> wrote:
>>
>> HDDs have their internal caching mechanism and I have heard that the
>> Linux kernel uses RAM very effitiently, but to my understanding RAM
>> being only 3-4 times faster doesn't make much sense, so I may be doing
>> or understanding something not entirely right.
>
> I suggest that you google a bit on how to do fileystem benchmarks
> first, then try it and report back if something is still odd. There
> are many ways but "dd" is not the way unless you really dig through
> the sync flags and understand what they do. I normally use "fio" but
> it's not very friendly (so it suits me).
>
> However, I just recently put a fast NVMe SSD in an older server with
> (lots) of DDR3 ECC RAM. The RAM bandwidth for one node/CPU is about
> 10-12 GB/s, and the SSD bandwidth is nearing 2 GB/s for most loads.
> That's getting close to your figures!
>
>
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