[dm-devel] Balanced Striping

[Leonardo Santos]

I'm trying to make a experimental balance striping between SSD and HDD
based on some performance parameter (latency or throughput) gotten on the
fly.

I realized that when I increase the number of operations for SSD (e.g. 3x1,
SSDxHDD) it doesn't give me the better of both.

My HDD read sequentially at ~100MB/s and my SSD at ~400MB/s. My hope is
throughput to be the sum of SSD and HDD, but this its not true for my tests.

For instance, when I put operations in 1x1:
SSD = ~100MB/s
HDD = ~100MB/s

When I put 3x1:
SSD = ~150MB/s
HDD = ~50MB/s (why its not 100MB/s?)

For me its not parallel, but only distributes the operations in a
proportional way.

Questions:
- Why isn't throughput equal to HDD + SSD in normal mode (1x1)?
- Why HDD reduces proportionally?
- Its not parallel? Where can I change it to make this parallel?

I using this in my research and changed dm-stripe.c.

Thanks a lot!

*Leonardo Antônio dos Santos*
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