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Storage Expansion - Part III

posted 4 Apr 2013, 05:31 by Flip Pipe   [ updated 4 Apr 2013, 14:54 ]
In last post, I've tell I will work with 3 USB flash drive in RAID 5 configuration. It will give something around 30 MB/s of writing throughput.

Meanwhile, the components to Raspberry Pi has arrive a I've been able to start doing some tests in it.

Disappointment... the first test with 3 pens in RAID 5 just gave me about 5MB/s... where are my 30MB/s???

Oh... I forgot do the fine tuning... lets increase the stripe_cache to about 8M and redo tests...

Luck... no, no luck at all... it goes down to less than 3MB/s.

Now I've realize I will need to do a bunch of tests again to see what are optimal values of the stripe_cache and what else can be fine tuned in order this works... at least I want 20MB/s.

Googling around, I've find, the value of chunk size when the raid is created also makes effected in the global performance.

So, I've done some tests, varying the chunk size and the stripe_cache. And this are the results:

As you can see, the fine tuning is really important to writing... with the same setup, you can go from ~3MB/s to ~12MB/s, 4x more.

The readings, well, the stripe_cache doesn't affect the performance, just the value of chunk size, so this graphic just show the evolution of the throughput changing the chunk size.

How can I improve the performance of writing... adding more disks? The hub I've used so far, didn't have external energy and when I connect the 4th pen it stop work. So I've to buy a powered hub... and I've found one with 7 usb ports :)

New hub, it will make difference in the previous results?

No, it doesn't make difference... and adding a 4th pen increase the performance. Good... should I be able to increase it to 20MB/s?

You are right... I've done more testing, with the same previous philosophy and this the results:

No, I can't reach 20MB/s... Probably with a powered hub with 7 ports and with 6 pens in hand I could reach the 20MB/s but will be one more Amp to provide to the system working with batteries, so, in the future i could do some tests with 6 pens, but for this project I will used just 4 and work with it.

All this tests have been done without a file system in the RAID, so, my last test was to see how much the performance will decrease with a file system... and this are the results:

More degradation... less 4MB/s... this is not good... more improvements must be done. The RAID software it is not static in kernel but in modules... and lots of it. So will try to improve it compiling the RAID directly in kernel.

For now, I will stop trying to improve this... At this point I've all I need to do the first stage of the project. I've gphoto comanding my camera. I've WebCamCon working. I've a wireless usb dongle doing as AP with my table connected to it. I've the (not good) expanded storage... Its time of put all thinks together.

Flip Pipe,
4 Apr 2013, 05:31