Build servers offline due to failed SSD

James Linder jam at
Tue Mar 9 02:55:55 UTC 2021

> On 9 Mar 2021, at 5:53 am, Dave C via macports-users <macports-users at> wrote:
> Old technology drives use magnetism to hold bits. This works for decades, or so I’ve read. Usually the motor or bearings die before the magnetic medium fails.
> Solid State Drives use memory chips to hold bits. These “bit holders” can wear out after a few trillion transitions (changing from 1 to 0 and 0 to 1). I’d you’re using it in your laptop or PC, you’ll likely have no problems for many years. In an internet-connected server, you may exceed those maximum write cycles sooner rather than later.

Dave I was just reading up, interesting info …

SSDs work (as do eproms) by having an isolated ‘chamber’. You get electrons in or out of the chamber using quantum tunneling [it disapears here and teleports there] based on probability, higher with an electric field

Repeated used breaks down the insulation of the isolated ‘chamber’

SLC (the lowest capacity and most expensive) store 0 or 1 (volts or whatever unit) and are most tolerant of damage
0 is 0, 1, 2, 3 units, 1 = 6, 7 8 units with 4 more likely a 0 and 5 more likely a 1 (say)

DLC store 0, 1 2 ,3 units and are less tolerant of an extra, or a fewer u
DLC store 0, 1, 2, 3 … 13, 14, 15 units. Are the cheapest but most fragile ie 13 can leak away to 12, or gain from 13 to 14

But the link earlier is discuusion showed drives rated at 300 TBW going well past that to 1 or 2 PBW (Peta is 1000 times Tera)

In use a HD (specially with lots of them) is more likely to fail than SSD. Seagates old paper ‘ATA more than an interface’ says this:
Drive 1 seeks and the bump knocks others off track. They seek back knocking others off track. Process continues until a disk fails.
Their 10 year old assesment of reasons is even more relevant on todays drives.

SSDs generally give more warning that ’the end is nigh’

All considered I’d take SSD for work disks and HD for long term backup
Heck in my day (ouch) we’d teach 'tower of hanoi' backup stratedgy using tape. why not do likewise with HDs. Timemachine certainly make that easy.


> Dave
> - - - 
>>> On Sun, 7 Mar 2021, Michael A. Leonetti via macports-users wrote:
>>> I’d really love to know more about what you’re saying here. Up until I just read what you wrote, I thought SSDs were the savior of HDDs.
>> Real disk drives [tm] have their N/S magnetic poles lined up pretty much forever; SSDs rely upon capacitors storing their charge forever (hah!).
>> You need to have an electronics background to understand...
>> -- Dave (VK2KFU)

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