Ron wrote:
At 02:53 PM 8/20/2005, Jeremiah Jahn wrote:

On Fri, 2005-08-19 at 16:03 -0500, John A Meinel wrote:
> Jeremiah Jahn wrote:
> > On Fri, 2005-08-19 at 12:18 -0500, John A Meinel wrote:
> >
> > it's cached alright. I'm getting a read rate of about 150MB/sec. I would > > have thought is would be faster with my raid setup. I think I'm going to > > scrap the whole thing and get rid of LVM. I'll just do a straight ext3
> > system. Maybe that will help. Still trying to get suggestions for a
> > stripe size.
> >

Well, since you can get a read of the RAID at 150MB/s, that means that it is actual I/O speed. It may not be cached in RAM. Perhaps you could try the same test, only using say 1G, which should be cached.

> I don't think 150MB/s is out of the realm for a 14 drive array.
> How fast is time dd if=/dev/zero of=testfile bs=8192 count=1000000
time dd if=/dev/zero of=testfile bs=8192 count=1000000
1000000+0 records in
1000000+0 records out

real    1m24.248s
user    0m0.381s
sys     0m33.028s

> (That should create a 8GB file, which is too big to cache everything)
> And then how fast is:
> time dd if=testfile of=/dev/null bs=8192 count=1000000

time dd if=testfile of=/dev/null bs=8192 count=1000000
1000000+0 records in
1000000+0 records out

real    0m54.139s
user    0m0.326s
sys     0m8.916s

and on a second run:

real    0m55.667s
user    0m0.341s
sys     0m9.013s

> That should give you a semi-decent way of measuring how fast the RAID
> system is, since it should be too big to cache in ram.

about 150MB/Sec. Is there no better way to make this go faster...?

I'm actually curious about PCI bus saturation at this point. Old 32-bit 33MHz pci could only push 1Gbit = 100MB/s. Now, I'm guessing that this is a higher performance system. But I'm really surprised that your write speed is that close to your read speed. (100MB/s write, 150MB/s read).

Assuming these are U320 15Krpm 147GB HDs, a RAID 10 array of 14 of them doing raw sequential IO like this should be capable of at ~7*75MB/s= 525MB/s using Seagate Cheetah 15K.4's, ~7*79MB/s= 553MB/s if using Fujitsu MAU's, and ~7*86MB/s= 602MB/s if using Maxtor Atlas 15K II's to devices external to the RAID array.

I know I thought these were SATA drives, over 2 controllers. I could be completely wrong, though.

_IF_ the controller setup is high powered enough to keep that kind of IO rate up. This will require a controller or controllers providing dual channel U320 bandwidth externally and quad channel U320 bandwidth internally. IOW, it needs a controller or controllers talking 64b 133MHz PCI-X, reasonably fast DSP/CPU units, and probably a decent sized IO buffer as well.

AFAICT, the Dell PERC4 controllers use various flavors of the LSI Logic MegaRAID controllers. What I don't know is which exact one yours is, nor do I know if it (or any of the MegaRAID controllers) are high powered enough.

Talk to your HW supplier to make sure you have controllers adequate to your HD's.

...and yes, your average access time will be in the 5.5ms - 6ms range when doing a physical seek. Even with RAID, you want to minimize seeks and maximize sequential IO when accessing them.
Best to not go to HD at all ;-)

Well, certainly, if you can get more into RAM, you're always better off. For writing, a battery-backed write cache, and for reading lots of system RAM.

Hope this helps,
Ron Peacetree


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