Things look quiet here. But I've been doing a lot of blogging at
dan.langille.org because I prefer WordPress now.
Not all my posts there are FreeBSD related.
I am in the midst of migrating The FreeBSD Diary over to WordPress
(and you can read about that here).
Once the migration is completed, I'll move the FreeBSD posts into the
new FreeBSD Diary website.
XEON gets a new cable which should improve disk speed29 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:
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 <steve@gte.net>
I decided I better elaborate, in case you use this info to update the article.
That 80MB/s cable allows the SCSI chain to reach a TOTAL bandwdith, or transfer rate, of
80MB/s. Example, you have two SCSI disks...let's assume each disk is capable of
transferring 25MB/s. If you were to transfer data on both disks simultaneously they
could combine to reach a transfer rate of 50MB/s. If you had three similiar disks
they could combine for a top rate of 75MB/s. If you were to have those two (or three)
disks on the 40MB/s cable for a RAID set for example, you'd be limited to a top rate of
40MB/s, since the cable becomes the bottleneck. In THAT case the 80MB/s cable DOES
help you, because the three disks combining to reach a theoretical 75MB/s is still less
than the 80MB/s the cable is capable of. So, two disks capable of transferring
20MB/s will do fine on a 40MB/s cable. Two disks capable of 30MB/s each (combining
to reach 60MB/s exceed the 40MB/s cable and therefore require one of two things. An
80MB/s U2 cable as you have bought OR a dual channel SCSI card with each drive on it's own
channel. That way each drive individually would have it's own cable allowing it to
use the full 40MB/s bandwidth for itself. This is where dual channel SCSI cards
come into play.
I fully realize this is an abbreviated description of the issue, but it'll suffice for
a laymen's terms explanation.
