Re: HDD vs SSD without explanation

On 16/01/18 13:18, Fernando Hevia wrote:

>
>
>
> The 6 Gb/s interface is capable of a maximum throughput of around 600
> Mb/s. None of your drives can achieve that so I don't think you are
> limited to the interface speed. The 12 Gb/s interface speed advantage
> kicks in when there are several drives installed and it won't make a
> diference in a single drive or even a two drive system.
>
> But don't take my word for it. Test your drives throughput with the
> command Justin suggested so you know exactly what each drive is
> capable of:
>
> Can you reproduce the speed difference using dd ?
> time sudo dd if=/dev/sdX of=/dev/null bs=1M count=32K
> skip=$((128*$RANDOM/32)) # set bs to optimal_io_size
>
>
> While common sense says SSD drive should outperform the mechanical
> one, your test scenario (large volume sequential reads) evens out the
> field a lot. Still I would have expected somewhat similar results in
> the outcome, so yes, it is weird that the SAS drive doubles the SSD
> performance. That is why I think there must be something else going on
> during your tests on the SSD server. It can also be that the SSD isn't
> working properly or you are running an suboptimal OS+server+controller
> configuration for the drive.
>

I would second the analysis above - unless you see your read MB/s
slammed up against 580-600MB/s contunuously then the interface speed is
not the issue. We have some similar servers that we replaced 12x SAS
with 1x SATA 6 GBit/s (Intel DC S3710) SSD...and the latter way
outperforms the original 12 SAS drives.

I suspect the problem is the particular SSD you have - I have
benchmarked the 256GB EVO variant and was underwhelmed by the
performance. These (budget) triple cell nand SSD seem to have highly
variable read and write performance (the write is all about when the SLC
nand cache gets full)...read I'm not so sure of - but it could be crappy
chipset/firmware combination. In short I'd recommend *not* using that
particular SSD for a database workload. I'd recommend one of the Intel
Datacenter DC range (FWIW I'm not affiliated with Intel in any way...but
their DC stuff works well).

regards

Mark