On 04/01/2011 16:41, Jan Kasprzak wrote:
John Robinson wrote:
[...]
Yes, I am aware of this. A single disk is able to do about 147 MB/s according to hdparm -t. However (a big "however"), my usage pattern rarely issues big/sequential requests, and for more random load the total throughput generated by all disks will be much lower and the disks themselves become the bottleneck.
Sure, but doing a resync does require huge sequential reads and writes.
I have just been suriprised that for initial RAID-10 resync the bottleneck is in the (single) CPU. : but because you're throttled by the PCIe x4 interface, you're only : getting about half of what your discs could do. I have not talked about PCIe x4, but SAS 4-way multichannel.
My bad. Same effect in this situation though.
Anyway, my SAS controller is connected by PCIe 2.0 x8, which equals to (if I read Wikipedia correctly :-) 32 Gbit/s, i.e. 2 GByte/s. So PCIe is not a bottleneck here. SAS is, and I am aware of that.
Which is why the the md kernel threads appear to be using 100% of CPU, they're blocked waiting for I/O. (And possibly RAM, per my other reply to this thread.)
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