Timeline for Why do operating systems have an option to shut down?
Current License: CC BY-SA 3.0
12 events
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| Mar 11, 2016 at 16:08 | comment | added | supercat | @DewiMorgan: While write caching issues are important, I don't think systems would have enabled write-back caching if that would be the sole factor making it unsafe to power down the system when it was simply "idle". | |
| Mar 11, 2016 at 16:03 | comment | added | supercat | @DewiMorgan: My point was that older systems would only start writing to disk in response to application request, which would generally occur in response to direct user action; all one had to do to make sure the system didn't start writing to disk before shutting down was refrain from actions (like hitting "save") that would trigger a disk write. If there are background tasks that can write to disk, however, that's no longer true--a point I didn't see mentioned in any other answers. | |
| Mar 11, 2016 at 15:17 | comment | added | Dewi Morgan | @Supercat Ah, I finally understand: your point is "if there's nothing being written to disk when the disk is powered off, then it's safe"... even though you don't mention this important proviso in your answer. And yes, that's true... but it's also blatantly obvious, a banal truism of no value. Also, that writes only happened when someone hit save wasn't a safe assumption even on a machine with floppies and no hard drives. | |
| Mar 11, 2016 at 1:40 | comment | added | supercat | @DewiMorgan: If one hits "save", all of the writes will complete before control returns to the application. If the user waits until the application indicates that the save is complete before killing power, why would anything get corrupted? | |
| Mar 10, 2016 at 18:26 | comment | added | Dewi Morgan | @Supercat: How does that prevent powering off from corrupting/desynching a written file, and the FAT, where power is turned off between the write-through to the file, and the write-through to the FAT? And how does it prevent a four-block file from being corrupted if only the first two blocks have been written before power-off? Write-through is good, trading performance for reliability to avoid power-interruption issues when writing a single block. But it's not a disk corruption panacea. | |
| Mar 10, 2016 at 5:40 | comment | added | supercat | @DewiMorgan: Write-through caching implies that when a write is requested, it will be performed immediately, but if a subsequent request is made to read the data the system will use a cached copy of the data that was just written, as opposed to reloading the data from the media. | |
| Mar 10, 2016 at 5:31 | comment | added | Dewi Morgan | @Supercat As I understand it, write-through caching protects a single block of data from corruption during a write. It does not protect two different blocks (FAT and file contents) from falling out of synch if the power is cut between writing one and the other. | |
| Mar 10, 2016 at 4:43 | comment | added | TOOGAM |
supercat: Nah, MS-DOS didn't do such caching unless you ran software to do that, such as the bundled SmartDrv. How you would notify that the system was to shut down is to use SmartDrv/C. (I believe the default behavior, regarding write caching, was different between MS-DOS 5.0 and 6.22... I don't offhand remember if it was the 5.0->6.0 upgrade or a later upgrade that made the change). If memory serves me right, one of the later upgrades (starting from 6 or later) caused SmartDrv to automatically do that before letting Command.Com show the prompt again, so safe to power off when at prompt
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| Mar 10, 2016 at 2:41 | comment | added | supercat | @DewiMorgan: I know some old hard drives had head-park utilities, but those were pretty much obsolete well before Windows 95 replaced DOS. | |
| Mar 10, 2016 at 2:13 | comment | added | supercat | @DewiMorgan: In the DOS days there were utilities available for write-back caching, but every standalone version of DOS I can remember used write-through caching as its normal behavior. What would one do otherwise to notify the system one wanted to shut down? | |
| Mar 10, 2016 at 1:55 | comment | added | Dewi Morgan | Even in the DOS days, it was a really bad idea. If a file had been updated but the FAT had not been updated, then you'd get a corrupted file. | |
| Mar 9, 2016 at 23:43 | history | answered | supercat | CC BY-SA 3.0 |