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XEON gets a new cable which should improve disk speed 29 December 2000
You might recall how I installed a different SCSI adapter and went from a
13 minute kernel build to a 9 minute kernel build. That's the The XEON lives! article. Shortly after posting that
article, I received a message from a reader suggesting that I try an LVD cable. That
should boost the disk speed.
I borrowed an LVD cable from the same person (llearch) that had loaned me the cable I was already using. That boosted the disks from this:
da1: 40.000MB/s transfers (20.000MHz, offset 31, 16bit), Tagged Queueing Enabled
da0: 80.000MB/s transfers (40.000MHz, offset 31, 16bit), Tagged Queueing Enabled
So I order the cable from CablesDirect (http://www.cablesdirect.co.nz/). I placed the order on Friday, expecting the goods to turn up the next business day (Wednesday). I was quite surprised, and of mixed feelings, when the courier knocked on the door Saturday morning at 7:30 AM.
When I initially tried the cable, I was disappointed to see 40/20 and not 80/40 as I expected. After some testing, I concluded it was because one of the drives was terminated (via a jumper on the drive). I removed that and reconnected everything. The drives then ran at 80/40.
kernel building changes
|After doing that, I expected a change in kernel building speed. I
saw none. It was exactly the same time (9 minutes, I wasn't counting the seconds).
Someone mentioned that kernel building is mostly CPU bound. So they suggested a test. I tried this one:
[dan@xeon:~] $ dd if=/dev/zero of=~/dead.file bs=1024k count=1000 1000+0 records in 1000+0 records out 1048576000 bytes transferred in 50.741196 secs (20665181 bytes/sec)
That's 20MB per second.
Then I swapped back to the old cable and tried that. I got
1048576000 bytes transferred in 53.954358 secs (19434501 bytes/sec) 10485760000 bytes transferred in 553.498003 secs (18944531 bytes/sec)
What? The same speed? Hmm. Then I looked at dmesg and found that both drives were at 80/40. Under the old cable?
Of course! The drive termination. It's changed. I removed it. That's why the old non-LVD cable ran at 40/20. Just like the LVD cable first did.
Well, I needed to buy a cable anyway. The one I had was borrowed and needed to be returned. I just wish I'd know this before. I would have bought a slight cheaper one. I think the LVD was overkill.
Fast roads, slow cars
|As a sharp eyed reader said: having a road built to handle cars
traveling 200mph doesn't matter if YOUR car is only capable of traveling 100mph.
My disks are rated at 29.4MB/second. That explains why the faster cable didn't make any difference. But it may make a difference if I'm using both drives very heavily.
Steve Wingate <email@example.com>