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Re: Sata speed difference on G-RAIDS?

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Steve Modica
Re: Sata speed difference on G-RAIDS?
on Oct 27, 2012 at 3:31:56 pm

[Thomas Morter-Laing] "when you run "iosnoop -D" (I had to run mine as sudo to allow it to work), how the heck am I supposed to make sense of the mountains of writing which happens after?"

DELTA UID PID D BLOCK SIZE COMM PATHNAME
218 0 83 W 434557744 4096 mds ??/68CF91E8/tmp.spotlight.state

This little snippet is the header that's printed along with one line of output. I trimmed the file path down a little to fit.

"DELTA" is the time it took to complete the IO. This is measured from the disk driver strategy routine to the driver interrupt routine (a very good indicator of the hardware speed). So this IO took 218 usec. The IO was issued by root (UID 0) and by process id 83 (mds daemon). It was a write command ("w") and it was 4096 bytes long.

If you start up an actual playout (or an IO benchmark) you can watch a little of this to get some idea if the disks are performing as you would expect.

For example, you would like to see all the IOs be very quick and about the same duration (give or take). If you see some jump way up, you might have a problem. It's easy to dump iosnoop's output into a file (iosnoop -D > filename) and then sort it later with "sort" to see if some IOs were really long.

iosnoop is part of the dtrace toolkit. It's eztremely useful and it's amazing that Apple ported it into MaC OS. That was huge.

Steve

Steve Modica
CTO, Small Tree Communications


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