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What Sustained Disk Transfer Rates Do You Get?
Mr. Jackson asks: "What kind of disk transfer rates (MB/s) do people get in the real world when moving around large (100s MB) files? Either every machine in our building is mis-configured, or our notions about what we were getting are way off. I've tested half a dozen machines, mostly Win2k, some Linux, by just copying a large file and timing it with a watch. 8 MB/s seems to be about average for inter-disk copies. RAID 1 (stripped) got as high as 12 MB/s after fiddling with cache settings. RAID 5 was as low as 2 MB/s. We all thought the numbers should have been around 30 MB/s."
Sequential reads on my drives can go from 25-65MB/sec (maxtor and cheetah), if the file is heavily fragmented I've seen it drop as low as 5-10MB/sec. Not so bad on the 15Krpm cheetah because the access rate helps, but on the lowely 5400rpm maxtor fragmentation destroys read/write speeds.
"I don't know that atheists should be considered citizens, nor should they be considered patriots." George HW Bush
From your average on board IDE controller without any special configuration, the numbers you're seeing look about correct. The fastest you can really expect to get with any consistancy is like 15mb/sec, and that's with tuned interfaces AND tuned I/O. With a high quality IDE controller, or a reasonable SCSI controller, and fast discs (10,000RPM) you can get 50-75% better then that. The fastest I/O I've seen in linux was with 2 gigabit Fibre Channel, and an array of 15 striped 15,0000 RPM disks. I managed about 120MB/sec, and that was only with certain block sizes. The average was still in the 60MB/sec range.
Bottom line, with a 7000 RPM IDE disk, and a regular cp command using a 4kB or so block size, you're probably not going to get better then 10MB/second. Disks are just too slow.
I don't need a lecture about how to configure my system to use DMA, I write I/O device drivers for a living, and I'm fairly sure I know how to use them.
Those specs you give are great, but the ST318452LW is a 15,000 RPM SCSI disk, not a 7000 RPM ATA-133 disk. Throw a filesystem on there, and do I/O in 4kB or smaller chunks, and you'll see 35MB/sec, which is exactly in line with the numbers I gave in my post. Sure you can get 61MB/sec average with that disk if the only thing you ever do is something like "dd if=/dev/sda of=/dev/null bs=4096k", but that's not a real world type use of a disk, is it?
Now, take your 160MB/sec interface, and make it 133MB/second, make the spin rate of your disk half of that, and decrease your bit density, and you'll take another 60% off that speed. We're back down in the 10-15MB/second range that I was mentioning. This isn't rocket science.
We just build a 2 TB fileserver using two 3Ware 7850 controllers, with eight 160 G Maxtor drives per controller. Each controller has RAID 5 across all it's drives. We split each RAID 5 partition into "inner" and "outer" partitions, and striped inner-to-inner and outer-to-outer using software RAID 0. Bonnie++ benchmarks show the "outer" array is getting > 241 MB/sec sustained read, and > 81 MB/sec sustained write.
Click here for the Bonnie++ results
RAID Info
It took me a while to figure out, but the numbers ("0123456710530+1") in the upper right hand corner are links to different RAID level explainations.
It even explained RAID 2, which I haven't seen before.