Joy Division

Some Idiotic Questions, Things I Should've Known

Recommended Posts

Hi all,

I'm posting these questions bearing in mind that "he who asks is dumb for five minutes, he who doesn't stays dumb for the rest of his life".

I haven't considered myself a noob for more than a few years, but at times in life you tread on new ground and discover your own ignorance when it comes to certain topics. Maybe some of these could be incorporated into a new or existing FAQ entry, or maybe not, but I'll let you be the judge of that.

Let's get on with it:

1) I've always been very careful when installing hardware. Powering everything off before attaching a new device for example, except when hot-plugging is explicitly supported (USB devices, a simple RJ-45 network cable) or the change is trivial and most of the time works without reboot (DIN or PS/2 keyboard and mouse, serial,...). This is common practice for all of us.

When it comes to SCSI however, I'm not sure anymore. A couple weeks ago an IBM technician came by at work for some trouble with our tape library, and he happily connected and disconnected LVD SCSI cables with both the server and tape library powered on. I can't imagine that this was a safe thing to do? Termination issues, data corruption etc.

2) We have quite a lot of fiber optic hardware at work: Cisco switches, a Compaq RA-8000 SAN and an IBM FAStT700. Sometimes when making changes you need to know which cable out of a bunch of fiber optic cables is the one you just attached to the switch in the server room. Pointing the connector to a white sheet of paper doesn't help, I can never distinguish a red dot whatsoever. The only remaining option is looking briefly into the laser (hence pointing it into your eyes) to see the red light.

My superior says he's been doing it for years and it's not harmful at all, but I don't trust it. I've googled around for some information on this, and it seems that my boss is correct, but I'd like your input nevertheless.

3) (This one's embarassing.) A few weeks ago a slashdot post was made on the topic of PC screws, i.e. which screw goes where? I wasn't happy to find that I've been installing hard drives with case screws instead of hard drive screws... You can take the test yourself here.

I never had too much trouble installing hard drives like this, as the thread pattern is almost the same, but some force was always required (messing up the thread, making it impossible to use the right screws later on, I can't tighten them anymore).

Although using screws which are a little bit oversized probably results in some stress to the hard drive chassis, I can't say I had drives die on me because of it. My only drive failures in almost 9 years were 2 IBM 75GXP's, so these don't count. Anyway, I thought that perhaps it would be a good idea to have a picture of the right screws somewhere on SR...

Not that I think people are losing sleep over it, but as I've attached my motherboard using the HD screws (not standardized apparently?), I'm probably not the only one out there.

Don't laugh at me ;),

JD

Share this post


Link to post
Share on other sites
Don't laugh at me ;),

JD

I would not...

Every answer has it's question and vice verca.

Dunno which ends is the best but I tend to favour

the answer end...

Of your Q's I find 3) quite awakening altho I

always try to use HDD screws. But from now on

I'll always do that just in precaution.

Tnx

/casa

Share this post


Link to post
Share on other sites

1) SCSI supports hot swap. no problem

2) Never look into fiber optic when lasers are used. You can however look into fiber when LEDs are used.

You wont always see the light, but it will burn your retinas and hurt your vision over time. Don't do it. Use the sheet of paper or a shadow or something other than your eye.

3) I got them all correct.

In my experience case screws and HDD screws are almost always the same coarse thread. Floppy and CD drives almost always use the same fine thread.

As long as the threading is the same betweent he drive and the screw then there is no problem. HDD screws often have a smaller head so they work better with drive rails compared to case screws.

Share this post


Link to post
Share on other sites
2) Never look into fiber optic when lasers are used. You can however look into fiber when LEDs are used.

You wont always see the light, but it will burn your retinas and hurt your vision over time. Don't do it.  Use the sheet of paper or a shadow or something other than your eye.

Well, the thing is, I've tried the sheet of paper, but I can't see a thing! I even turned off the light in the server room, but I still couldn't tell if the light was on or not :(

Guess my eyes are screwed already :angry:

Should it be clearly visible or is it really hard to see?

JD

Share this post


Link to post
Share on other sites

It depends on the type of fiber you're using and the type of equipment.

If these fiber runs are short and use 62.5 or 50 micron multi mode fiber then i'm almost entirely sure you're using LEDs. If you're using single mode 9 micron then I'm almost sure you're using lasers.

The LEDs use visible light that apears red.. but could be infrared. I think the laser transmitters are usually out of the visible spectrum and as such are invisible.

I dont have a whole lot of experience here, but better safe than sorry.

Share this post


Link to post
Share on other sites
If these fiber runs are short and use 62.5 or 50 micron multi mode fiber then i'm almost entirely sure you're using LEDs. If you're using single mode 9 micron then I'm almost sure you're using lasers.

As far as I know it's all multimode fiber, so with a little luck I shouldn't be to worried.

The Cisco GBICs are of the WS-G5484 type, details are on the Cisco website but LEDs vs laser diodes aren't mentioned...

The SAN cabling is also multimode. I hope all of them are using the cheaper LEDs. :)

I'm gonna give it another try to distinguish the red dot tomorrow...

Thanks a lot for your input,

JD

Share this post


Link to post
Share on other sites

SCSI does support hot swap, but officially, only in one particular configuration, SCA.

SCA type connectors are specifically designed for hot-swap. If you look carefully, you'll see that the power and ground (and a few other signals) on an SCA connector have longer "fingers" to guarantee they engage with their mates *before* the SCSI Bus signaling connections mate. This ensures that an SCA drive will act "bus-friendly" when it comes on the bus.

The P68 connector (or any of it's high-density D-connector bretheren) is not designed or intended for hot plugging and should not be used for such unless you really know what you're doing. That being said, tou can (and indeed, I often do) plug in a P68 type connector while your system is live. If you're not doing IO at the time, and you're careful to make sure the connector is lined up properly, the odds of a problem are all but zero. P68 connectors do not include provision for power (SCA does). If you're hot plugging P68 drives, you should connect/remove the Power cable after/before you connect/remove the SCSI cable.

I would NOT, however, make/break a P68 cable connection on a system performing IO on that same bus. Unless you short something out by misaligning the connector, you're unlikely to do any "hard" damage, but data loss or a system crash is a definite possibility.

As for the wrong Screws... there is a hidden risk that most people don't recognize. As you observed in your original note, the wrong screws seem to work, but they ruin the threaded holes so that next time, the right screws don't work. The problem is that the HDD base casting is Aluminum. The screws are generally Nickle or Zinc plated Stainless Steel. The screws are much harder material than the base casting, and the larger, incorrect screws cut the aluminum when you drive them in (removing material from the drilled/tapped holes to make them accomodate the larger, incorrect screws). That re-tapping of the holes by the larger screws makes all sorts of little metal shavings that fall into your PC, or end up on the HDD PCBA itself, with obvious implications to system reliability.

NEVER use the incorrect screws to mount drives. The thread pitch, screw diameter, and screw lenths are specified by HDD manufacturers for a reason (that being, to keep your drive or system from failing due to aluminum particulate induced short circuits).

Also, while the LASER wattage used in most optical switches is low and probably won't do any permanent damage, looking into the Fibre is probably not a good habit to get into. Some long-haul optical systems use LASER wattages high enough to do damage, and if you're used to habitually looking at the end of the Fibre to see if it's lit up, you might draw the "short straw" one day and find out (too late to do anything about it) that you're staring down enough optical energy to hurt.

Share this post


Link to post
Share on other sites
If you look carefully, you'll see that the power and ground (and a few other signals) on an SCA connector have longer "fingers" to guarantee they engage with their mates *before* the SCSI Bus signaling connections mate.  This ensures that an SCA drive will act "bus-friendly" when it comes on the bus.
P68 connectors do not include provision for power (SCA does).  If you're hot plugging P68 drives, you should connect/remove the Power cable after/before you connect/remove the SCSI cable.

Erm... I'm confused. If I break your last sentence into its two respective meanings, I should connect the power cable after connecting the SCSI cable. Isn't it backwards, turning on the power first so the device acts bus friendly when connecting it? Think I'll get some sleep and look at it again in the morning...

That re-tapping of the holes by the larger screws makes all sorts of little metal shavings that fall into your PC, or end up on the HDD PCBA itself, with obvious implications to system reliability.

NEVER use the incorrect screws to mount drives.  The thread pitch, screw diameter, and screw lenths are specified by HDD manufacturers for a reason (that being, to keep your drive or system from failing due to aluminum particulate induced short circuits).

Indeed, I noticed this sometimes when screwing drives into drive cages (outside of the case). I swear I will never use the wrong screws again :-)

Thank you for all your input,

JD

Share this post


Link to post
Share on other sites
I would NOT, however, make/break a P68 cable connection on a system performing IO on that same bus.  Unless you short something out by misaligning the connector, you're unlikely to do any "hard" damage, but data loss or a system crash is a definite possibility.

For this reason I would never do it, period. Yes, you can get away with it, esp. if there is no OS drive or whatever on the channel, but if you plug&unplug SCSI often (sometimes 100s of times in a week) and you are not in Centronics world sooner or later you will mis-seat a cable. This will nearly always ruin the cable, may ruin the connector on the SCSI device, and if powered can cause rapid nastiness. OTOH, in a well equipped shop if the power is OFF this is not a huge deal - toss the cable in the NFG cable pile, scowl at going through $1000s in cables every year (SCSI, VGA, RGBHS, RGB, XLR, etc.), check the connector, be more careful....

In fact, it is sometimes possible to mis-align a 68 pin SCSI cable and not realize it until powered up, which is why I always double-check.

A tape drive on its own card may be an exception to this, but why chance it?

As to the screws, my eyes are too aged these days to be able to always tell, so I always line up some "usual suspects" and lightly test by hand straight into the drive. If the screw goes in easily like that, then I know I am good. OTOH, on one or two occasions I have deliberately used screws that were too big due to a need to hang a drive in a particular, unusual way (kiosk or whatever). Big screws sometimes seem like the only way to really mash a drive in, though this is probably not recommended for long-term use.

Share this post


Link to post
Share on other sites

In my experience, the worst that usually happens if plugging/unplugging a 68-pin SCSI cable is the SCSI bus may hang, requiring a reset. This usually means power cycling or rebooting the server. If the tape library is the only thing attached, the end result of this is that you can't test/use the library until after the reboot.

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now