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20. Bits and Pieces

This is basically a section where I stuff all the bits I have not yet decided where should go, yet that I feel is worth knowing about. It is a kind of transient area.

20.1 Swap Partition: to Use or Not to Use

In many cases you do not need a swap partition, for instance if you have plenty of RAM, say, more than 64 MB, and you are the sole user of the machine. In this case you can experiment running without a swap partition and check the system logs to see if you ran out of virtual memory at any point.

Removing swap partitions have two advantages:

In the end, having a swap partition is like having a heated toilet: you do not use it very often, but you sure appreciate it when you require it.

20.2 Mount Point and /mnt

In an earlier version of this document I proposed to put all permanently mounted partitions under /mnt. That, however, is not such a good idea as this itself can be used as a mount point, which leads to all mounted partitions becoming unavailable. Instead I will propose mounting straight from root using a meaningful name like /mnt.descriptive-name.

Lately I have become aware that some Linux distributions use mount points at subdirectories under /mnt, such as /mnt/floppy and /mnt/cdrom, which just shows how confused the whole issue is. Hopefully FHS should clarify this.

20.3 Power and Heating

Not many years ago a machine with the equivalent power of a modern PC required 3-phase power and cooling, usually by air conditioning the machine room, some times also by water cooling. Technology has progressed very quickly giving not only high speed but also low power components. Still, there is a definite limit to the technology, something one should keep in mind as the system is expanded with yet another disk drive or PCI card. When the power supply is running at full rated power, keep in mind that all this energy is going somewhere, mostly into heat. Unless this is dissipated using fans you will get a serious heating inside the cabinet followed by a reduced reliability and also life time of the electronics. Manufacturers state minimum cooling requirements for their drives, usually in terms of cubic feet per minute (CFM). You are well advised to take this serious.

Keep air flow passages open, clean out dust and check the temperature of your system running. If it is too hot to touch it is probably running too hot.

If possible use sequential spin up for the drives. It is during spin up, when the drive platters accelerate up to normal speed, that a drive consumes maximum power and if all drives start up simultaneously you could go beyond the rated power maximum of your power supply.

20.4 Deja

This is an Internet system that no doubt most of you are familiar with. It searches and serves Usenet News articles from 1995 and to the latest postings and also offers a web based reading and posting interface. There is a lot more, check out Deja for more information. It changed name from Dejanews.

What perhaps is less known, is that they use about 120 Linux SMP computers many of which use the md module to manage between 4 and 24 Gig of disk space (over 1200 Gig altogether) for this service. The system is continuously growing but at the time of writing they use mostly dual Pentium Pro 200MHz and Pentium II 300 MHz systems with 256 MB RAM or more.

A production database machine normally has 1 disk for the operating system and between 4 and 6 disks managed by the md module where the articles are archived. The drives are connected to BusLogic Model BT-946C and BT-958 PCI SCSI adapters, usually one to a machine.

For the production systems (which are up 365 days a year) the downtime attributable to disk errors is less than 0.25 % (that is a quarter of 1%, not 25%).

Just in case: this is not an advertisement, it is stated as an example of how much is required for what is a major Internet service.

20.5 Crash Recovery

Occationally hard disks crash. A crash causing data scrambling can often be at least partially recovered from and there are already HOWTOs describing this.

In case of hardware failure things are far more serious, and you have two options: either send the drive to a professional data recovery company, or try recovering yourself. The latter is of course high risk and can cause more damage.

If a disk stops rotating or fails to spin up, the number one advice is first to turn off the system as fast as safely possible.

Next you could try disconnecting the drives and power up the machine, just to check power with a multimeter that power is present. Quite often connectors can get unseated and cause all sorts of problems.

If you decide to risk trying it yourself you could check all connectors and then reapply power and see if the drive spins up and responds. If it still is dead turn off power quickly, preferrably before the operating system boots. Make sure that delayed spinup is not deceiving you here.

If you decide to progress even further (and take higher risks) you could remove the drive, give it a firm tap on the side so that the disk moves a little with respect to the casing. This can help in unsticking the head from the surface, allowing the platter to move freely as the motor power is not sufficient to unstick a stuck head on its own.

Also if a drive has been turned off for a while after running for long periods of time, or if it has overheated, the lubricant can harden of drain out of the bearings. In this case warming the drive slowly and gently up to normal operating temperature will possibly recover the lubrication problems.

If after this the drive still does not respond the last possible and the highest risk suggestion is to replace the circuit board of the drive with a board from am identical model drive.

Often the contents of a drive is worth far more than the media itself, so do consider professional help. These companies have advanced equipment and know-how obtained from the manufacturers on how to recover a damaged drive, far beyond that of a hobbyist.


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