Date: Tue, 27 Aug 1996 12:40:59 +0200 (MET DST) From: Philippe Regnauld <regnauld@tetard.glou.eu.org> To: joerg_wunsch@uriah.heep.sax.de Cc: hackers@freebsd.org (hackers) Subject: Re: -current kills harddrives Message-ID: <199608271040.MAA09631@tetard.glou.eu.org> In-Reply-To: <199608270659.IAA23681@uriah.heep.sax.de> from J Wunsch at "Aug 27, 96 08:59:30 am"
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J Wunsch écrit / writes:
> As Philippe Regnauld wrote:
>
> > BTW, I'm writing up a "Hardware maintenance" section -- could be
> > useful in the handbook (proper ventilation, do's and dont's), or
> > does this seem too way out of line with FreeBSD ?
>
> The extended cooling requirements of 7200 rpm drives should of course
> be mentioned there.
They are :-) Comments and critics welcome.
->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--->8--
Hardware maintenance guidelines
FreeBSD is a high performance operating systems, and as such, it is
often used as the driving force behind high performance servers.
Unfortunately, experience shows that selecting the right OS and the
right components (See "PC Hardware Compatibility") is sometimes not
enough; there are several technical issues involved that will
(hopefully) be covered here, such as correct cabling, proper insulation
and ventilation, thermal constraints, etc... These are mostly tips and
hints acquired from personal experience in working environments -- they
are not in any case absolute rules (i.e.: your mileage may vary).
* COMPONENT ASSEMBLY *
Casings
If you have ever assembled a PC yourself, you may have noticed some
aberrations in the design and/or basic construction of the casing.
Remember that the majority of these were designed with cheapness in
mind, not fault tolerance. To start with, try and avoid all forms of
"small tower" designs, as they have a tendency to double as ovens --
this is mainly due to the amount of cabling generally present in your
average PC (see Cabling). High towers are recommendable for three
reasons:
o they offer room for more devices and disks in particular;
o they provide better airflow as more space is available;
o there usually is room for additional fan units
Depending on the make and model of the casing, they may or may not
offer facilities that are often useful :
o ventilation slots on the underside of the cabinet
o clip-on facade or hinged front door
provides easy access to the disk units
o screwed in motherboard mount-plate
avoids excess manipulation of the motherboard (i.e.:
unclipping nylon columns, sliding the board out).
Some manufacturers do make these kinds of casings, though they are
usually not cheap (count at least $150). For real easy access to
cabling, disks, bus cards, I can only recommend rack-mount units, with
its reliability (redundant power supplies, UPS support)... and price!
Also, always make sure that the cables of the power supply and
led/switch connectors are long enough to reach the mainboard (I'm
serious about this, it happens often !), and that there are enough
mainboard support holes, so that half of it doesn't hang unsupported.
Expansion cards
There are not many restrictions concerning expansion card installation
and/or manipulation other than the usual static precautions and
placement constraints due to available cabling. Still, some points
should be respected as a safe measure, though they should not be
required with quality power supplies and mainboards.
Note: for those of you using 486s with VLB (good luck...), remember
that most, if not all, mainboards that offer 3 VLB slots only allow 2
of those to do bus mastering. This may not seem like a big issue, but
I have seen peak disk I/O rise from 2 Mb/sec to 3 Mb/sec by simply
moving the I/O card to another VLB slot.
Placement of the expansion cards, whether they be ISA, EISA, PCI or VLB
should try and follow these recommendations:
o avoid placing video cards too close to the power supply. It might
cause static or other kind of video disturbances with badly shielded
power supplies, or sensitive RAMDACs on cheap cards. If it does
occur, try moving the card further 'down', away from the power
supply. If this is not possible (PCI cards), get a bigger casing, or
switch video cards;
o same advice for audio cards: they are known to be sensitive to
electromagnetic disturbances, which may cause humming and electric
noises in the speaker output. Once again, try to move the sound card
as far away from the power supply, cables permitting;
In general, if a problem occurs that seems to be of a hardware nature
(stray IRQs, erratic and/or intermittent response of a device or plain
refusal to boot), try and switch the cards around. Sadly, this has
worked in many cases for me with cheap mainboards or expansion cards.
Cabling
- Power cabling -
When setting up your system, try to envision future needs you may have:
it is highly recommended that you buy a power supply with enough
connectors in the first place, rather than use "Y" power splitters.
These are often of bad manufacture and can cause hard disk locks, bus
resets and other mishaps through inconsistent of faulty power supply.
If you do have to use them, ALWAYS check if the colors match (yellow to
yellow, red to red, etc...): it has once happened that 5V (red) was
cross-wired to 12V (yellow) on such an adapter :-(
- Drive cabling -
There are simply two cases for disk configurations: non-SCSI and SCSI
configurations :-)
It is not that SCSI is a sensitive or otherwise unreliable subsystem,
but it is a high performance one, and should be treated with the
corresponding awareness.
Regarding IDE disks :
IDE disks sometimes seem to behave strangely (no probing at boot,
slave drive is not seen) depending on which drive is master or slave,
and which one of these is first on the cable (though officially this
should not have any influence on the way the disks operate). The
best here is to try different configurations and see the results.
See the section "What is SCSI" for the description of the SCSI
subsystem, bus types and cabling recommendations. Keep in mind the
following points :
o always terminate both ends of the bus with the right terminator --
this means SCSI-I (big centronics-type connector) terminators for
SCSI-I busses (also known as passive terminators), and SCSI-II (small
connector) for busses containing SCSI-II devices (active
terminators);
o internal termination by the drives/devices themselves (i.e.: jumper
activated) should be avoided -- use clip-on terminators that fit on
the ribbon cable like any device;
o use the right cabling for external devices : SCSI-I cables and
SCSI-II cables do not have the same electrical specifications. Also
respect maximum lengths
- Ribbon cables -
Always try and use cables that are long enough, especially for disk
units: it is better to have spare cable rather than subject a barely
long enough one to stress by twisting or folding it. Also, long enough
cables will let you push them out of the way a bit, which is important
for correct ventilation and air flow.
* PLACING STORAGE DEVICES *
This is probably the most important point here. It is crucial that you
let storage devices (disks, CD Rom units, tape drives) have enough air
for them to function correctly. Without ventilation, you can expect
your 7200RPM drives to last a couple of hours at the most, if they do
not have built-in thermal switches (recent ones do).
There are a few things to know about ventilation:
o ventilation is not simply exposing your unit to open air -- that may
help, but it will in most cases only heat up the air around it;
o air acts as a fluid, and it must circulate to be efficient. A closed
cabinet with two fans blowing outward is a good way to achieve this;
o having the fans simply blow INTO the cabinet will not achieve
cooling: this will not help the hot air escaping. The idea is to
have the air be pushed out of the cabinet. Maximum effect is
obtained with two fans, one extracting and one intaking, if and only
if those are running at the same speed: if the intaking fan is
rotating faster, the resulting flow will be less efficient
(turbulence). Also, intaking fans push a LOT of dust into the
cabinet.
Proper ventilation is guaranteed by respecting the following points:
o do not place disks that you know are running very hot (more than
55°C) next to each other -- this will only make disks close to each
other overheat much faster;
o leave some space between the units (1/2" to 1") -- this allows better
air flow;
o leaving 5"1/4 front bays opened up creates a "leak" in the air
circuit, which could result in less efficient cooling;
o try to achieve good thermal dissipation. This means:
- mount the disks directly on metal whenever you can -- the metal
frame inside the cabinet usually provides average to good thermal
dissipation
- avoid disk drawers and other "easy extraction" mecanisms. Since
they are made of plastic, they trap heat (if your disk died suddenly,
you'll still probably have to restore a backup, and saving 2 minutes
in opening the casing won't make that much difference)
= = =
If you follow these guidelines, you will already avoid many of the
problems of building and keeping running a PC based server or
workstation -- a process which Jordan <jkh@FreeBSD.ORG> rightly
describes as 'Tapdancing through a minefield'. Most PC hardware is
indeed not among the best in quality, but through careful selection AND
correct placement, it is possible to approach the performance and
reliability of much more expensive dedicated hardware.
--
-[ Philippe Regnauld / regnauld@eu.org / +55.4N +11.3E @ Sol3 / +45 31241690 ]-
-[ "To kärve or nøt to kärve, that is the qvestion..." -- My sister ]-
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