Online filesystem scrubbing and repair [LWN.net]

Online filesystem scrubbing and repair

Posted Apr 15, 2017 8:20 UTC (Sat) by Wol (subscriber, #4433)
In reply to: Online filesystem scrubbing and repair by nix
Parent article: Online filesystem scrubbing and repair

I don't get this. If I remember that article on SSD life a while back, it's only writes that age an SSD. So why would reading the entire device (or not caching the device, and reading everything every time) age the SSD?

(Oh, and I seem to remember the same article saying that even if you ran continuous read/write passes on an SSD with no break, it would still take several years to kill it.)

Cheers,
Wol

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Online filesystem scrubbing and repair

Posted Apr 19, 2017 15:12 UTC (Wed) by nix (subscriber, #2304) [Link]

Because bcache works by caching reads from an underlying block device on another device -- usually one caches rotational storage on a device with faster or near-zero seek time, like an SSD. It tries to detect contiguous reads from the underlying device and avoid caching them, but this is not 100% perfect and cannot kick in at once, but only after a few megabytes of reads have been cached, probably pointlessly -- it is better to note that the rotational block device reads are low priority when they are something like a pvmove (which really *is* low priority): these are assumed to be reads for which seek performance does not matter, and thus recent bcache will not cache them.

Online filesystem scrubbing and repair

Posted Apr 19, 2017 15:14 UTC (Wed) by nix (subscriber, #2304) [Link] (12 responses)

(Oh, and I seem to remember the same article saying that even if you ran continuous read/write passes on an SSD with no break, it would still take several years to kill it.)

That's definitely not true of most SSDs. Mine, a fairly expensive Intel one, is rated for several complete device writes per day, every day for five years. However, the device is only 480GiB and it writes at about 500MiB/s, so it only takes a thousand seconds to write to the entire device (though in practice I suspect this would slow drastically due to GC pauses etc). And for many of these SSDs, replacing them means pulling open the machine and messing about on the motherboard -- it's much more risky and worrisome than just swapping a disk would be.

Online filesystem scrubbing and repair

Posted Apr 19, 2017 17:17 UTC (Wed) by zlynx (guest, #2285) [Link] (11 responses)

Unless you mean *very* cheap machines with a soldered on eMMC device, modern small machines use M.2 SSDs. Slightly older ones use a 2.5" laptop drive bay.

Changing a M.2 card is much the same as changing a RAM stick. I suppose that is worrying to some people?

I think the people worried by it would be exactly as worried as they would be changing out *any* computer hardware.

Online filesystem scrubbing and repair

Posted Apr 24, 2017 16:39 UTC (Mon) by nix (subscriber, #2304) [Link] (10 responses)

Let's see, is pulling the side of my machine off and unplugging things from the motherboard worrying, given that I have static-shocked machines into destruction by touching the backplate accidentally while plugging in a monitor cable while the case was still on? Yes, yes it bloody well is. (I tried to change a DIMM once, too. The machine never booted again.)

Not everyone is cut out to deal with hardware, even if it doesn't need desoldering.

Online filesystem scrubbing and repair

Posted Apr 24, 2017 16:45 UTC (Mon) by zlynx (guest, #2285) [Link] (9 responses)

Have you tried an anti-static wrist strap plugged into a ground?

Some people have high levels of static electricity around them. Because of their skin moisture, their environment humidity levels, or the kinds of clothes and shoes they wear.

I don't remember ever zapping anything while I was wearing a strap. I've also got a grounded work pad, for putting the computer and parts on while working on it.

If you're a naturally staticky person these precautions may be needed. Assuming that you ever want to work on the insides of the PC. Some people know how but have no interest in it.

Online filesystem scrubbing and repair

Posted Apr 24, 2017 21:51 UTC (Mon) by nix (subscriber, #2304) [Link] (8 responses)

Every time I open a machine I try something different. I tried antistatic wrist straps. I tried rubber gloves. I tried rubber-soled shoes. I tried bare feet and tinfoil on the floor running out to something I hoped was grounded. I tried holding onto a hopefully-grounded lump of metal the entire time I was using it (really fun trying to work with one hand). Nothing worked. (But it may be that none of that was grounded. Honestly, except for the shielded and inaccessible earth pins in the power sockets, and I suppose the ground outside, I don't know what in a UK domestic property *is* grounded.)

I stopped trying to touch hardware about seven or eight years ago. I buy it and never open it again: sliding hotswap drives in is my limit. I'll pay someone who is not made of static electricity to do anything more. (And who doesn't have my terrible coordination and muscle tremors. Plus, you just *know* the machine will go wrong in the summer when I am made of hay fever. Massive sneezes are not compatible with hardware work. Or with software work, really, but you can fake it more easily with software.)

Online filesystem scrubbing and repair

Posted Apr 25, 2017 7:14 UTC (Tue) by farnz (subscriber, #17727) [Link] (2 responses)

FWIW, when I worked in a place with an on-site factory that handled ESD-sensitive items (BGA chips for rework), the standard for static handling was two big RS ESD-safe mats connected to the same earth, and Uvex ESD gloves. You stood in socks on one ESD-safe mat, you put the work item on the bench on the other, both shared an earth so that there was no charge gradient across you, you used the wriststrap to ensure that your body was at the same potential as the mat the work item was on (in case your socks were insulating), and you wore the ESD gloves to reduce the risk further. Kinda expensive for a home setup, though...

The other trick the factory workers all worked to was "cotton work clothes only" - apparently synthetics and wool were more likely to result in you getting zapped when you stepped on the mat than cotton was, and that wasn't a pleasant feeling.

Note that domestic rubber gloves aren't suitable - they tend to be static accumulators, as they don't have the conductive layer to dissipate static charges, and can make the zap worse. Same goes for non-ESD rated rubber-soled shoes - they can cause the static charge to build up where it might otherwise dissipate. It's easy to make your ESD problem worse if you're not using the right kit :-(

Online filesystem scrubbing and repair

Posted Apr 26, 2017 19:46 UTC (Wed) by nix (subscriber, #2304) [Link] (1 responses)

Fascinating! A real belt-and-braces approach, which makes sense when you're dealing with a lot of items that will cost real money if just one worker static-shocks them repeatedly.

I suspected the rubber gloves were a waste of time, but by that point I was fairly desperate :)

Online filesystem scrubbing and repair

Posted Apr 26, 2017 19:52 UTC (Wed) by farnz (subscriber, #17727) [Link]

The bigger reason to be this cautious was the supply constraint rather than the money - $2,000 chips that you can replace at the drop of a hat are annoying if shocked into an early grave, whereas chips where the next fab run isn't for another 6 months are far more worrying if you start blowing through your allotted supply - it's really problematic if you can't ship product for 3 months because some static took out a chunk of your IC supply.

Online filesystem scrubbing and repair

Posted Apr 25, 2017 13:32 UTC (Tue) by intgr (subscriber, #39733) [Link]

Did all of these incidents take place at the same location? If so, I suspect the grounding/earthing in that building is simply faulty or lacking.

Online filesystem scrubbing and repair

Posted Apr 25, 2017 14:29 UTC (Tue) by cladisch (✭ supporter ✭, #50193) [Link] (1 responses)

> … the shielded and inaccessible earth pins in the power sockets …

Schuko sockets have exposed ground pins. Moving to the Continent is a small price to pay for being properly grounded, isn't it?

Alternatively, to get access to the earth, just ~~stick a pin~~ use an earth bonding plug: [1] [2] [3] [4]

Online filesystem scrubbing and repair

Posted Apr 26, 2017 19:46 UTC (Wed) by nix (subscriber, #2304) [Link]

That looks very useful indeed (not just for electronics work, too). I had no idea any such thing existed.

Online filesystem scrubbing and repair

Posted Apr 25, 2017 19:57 UTC (Tue) by paulj (subscriber, #341) [Link] (1 responses)

Touch unpainted metal pipe-work going into a radiator. That is likely to work.

Online filesystem scrubbing and repair

Posted Apr 26, 2017 19:52 UTC (Wed) by nix (subscriber, #2304) [Link]

Some modern houses have pretty radiators with a metallic cover (no doubt ungrounded nad painted with electrically insulating paint, for the look of the thing) and with all pipework artfully concealed out of sight and out of reach: it goes into the wall directly behind the radiator, so you can't get at it without dismounting the whole thing.

It's really pretty: the radiators just hang on the wall, not visibly connected to anything practical like pipework, just a radiator and a thermostatic valve supported as if by magic. It also conceals anything that might be in any way electrically connected to earth and is also a complete bugger to maintain (I just turn the plastic-covered emergency stop valve and call a plumber, who curses a blue streak for some time while trying to figure out how the hell to get at any of the pipework).

The toilets are even worse: all the piping is actually hidden behind a false wall. Makes dealing with leaks, or even spotting them, a huge pile of fun.