Re: Soft-Raid 0 Performance | Transfer two Data-Streams (CPU+FPGA) to the same Soft-Raid

[NeilBrown <neilb@suse.com>]

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On Wed, Jan 18 2017, Eric Schwarz wrote:

> Hello mailing list,
>
> I have got two questions:
>
> 1.) I have set-up a softraid (raid level 0) with mdadm using two M.2 
> modules. For one module the throughput is ~350MB/s (no mdadm) for two 
> modules the throughput is ~500MB/s which is less than factor 1,5 of the 
> throughput of a single drive. The filesystem used is ext4. Is there 
> someone having some values for comparison? For me the throughput gain 
> seems to be too little. The test was done using a HP Z840 workstation.

Can you try a range of chunk sizes and see what difference it makes?
RAID0 simply remaps each request to the appropriate target device/offset,
imposing very little direct overhead.  There may be an indirect overhead
as a small chunk size causes large request to be chopped up into smaller
requests first.

>
> 2.) We want to configure a softraid (raid level 0) with mdadm which can 
> be used from within Linux but also it should be possible to write data 
> to the raid w/ DMA directly from the FPGA which is also connected to the 
> PCIe bus as a slave as well as the M.2 modules. How can that be achieved 
> using existing kernel infrastructure?

What sort of help do you want from the existing kernel infrastructure.
Presumably your FPGA must already understand RAID0 striping, so you just
need to tell the FPGA what devices, offsets, chunk sizes to use.
I suspect it would be easiest to have a user-space program query the
array and feed data to your FPGA, but as you provide no detail for how
the FPGA would be configured, I'm only guessing.

NeilBrown


>
> Many thanks for helpful replies
> Eric
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