In the default autoexec.bat for DOS 6.22, you see these lines:
C:\DOS\SMARTDRV.EXE /X
@ECHO OFF
PROMPT $P$G
PATH C:\DOS
SET TEMP = C:\DOS
According to this source, /X disables write-behind caching. Write-behind caching is when the computer writes to the cache after a certain amount of time.
Why is write-behind cache disabled in DOS by default?
It's quite simple: write-behind caching means that after you have (for example) saved a file in a word processor, the saved file may live in the cache in RAM for a while before being written to the disk. If the computer crashes during that time, or is just switched off without using some command that flushes the cache to disk, then the file will be lost, or will still exist in its previous version. Worse still, if only some of the blocks of the file have been written, it will be a garbled mixture of the old and new versions.
Without caching (or with write-behind disabled) you can be sure that any saved file will have actually made it to the disk in a short time, so that waiting for a couple of seconds, or waiting for the disk light to stop flashing, before switching off the machine would ensure that the work was properly saved. With MS-DOS, it was normal to switch off the machine without any shutdown procedure.
As Stephen Kitt notes in another answer, SmartDrive would make sure the cache was flushed after exiting an application before the DOS prompt was shown again, making it safe to exit the program and then switch off. What wouldn't be safe would be to save a file and then switch the machine off without exiting the program. This behaviour of SmartDrive made it safer but a bit less effective, because a sequence of DOS commands (maybe from a batch file?) couldn't benefit from write caching between one command and the next.
The risk of crashes or unplanned shut-downs makes it safer to disable write-behind caching as the default setting, so that the cache is used only to hold copies of blocks on the disk that have been recently read or written. Even this use of a RAM cache is highly effective in speeding up average access times, because a typical workload would contain ten times as many block reads (for programs, input files, fonts, etc.) as block writes.
/X wasn’t specified. The acceleration came from caching reads, not writes.
Commented
May 30, 2021 at 15:12
SMARTDRV A+ for example; perhaps that was the case on the machine you remember!
Commented
May 30, 2021 at 20:35
The /X flag was specified by default in MS-DOS 6.22 at least. (See further down for the detailed history.)
What /X does is disable the write-back cache in all cases. The write-back cache is used to store data being written to a device, allowing programs to continue working while the data is written. This introduces the risk that, if power is lost or the computer crashes before the data is written, data will be lost, even though the program writing it and the user are under the impression that it’s safe. This is particularly relevant in DOS since there is no enforced shutdown procedure¹ and many users weren’t all that careful with how they powered their computers down (or rebooted them).
There are a number of subtleties which come into play here:
/X flag, so writing to a floppy and ejecting it is safe – once the drive light goes off at least;Thus, by default (with no flags), SMARTDrive was safe for users following recommended practice, and SMARTDRV did not have to be started with /X. Presumably Microsoft realised that that wasn’t sufficient, because even a small number of users not following recommended practice and losing data was too much, and thus added the /X switch to the default configuration in AUTOEXEC.BAT.
On a system running SMARTDrive, invoking SMARTDRV will show the current configuration, including cache information:
C:\>smartdrv
Microsoft SMARTDrive Disk Cache version 5.01
Copyright 1991,1993 Microsoft Corp.
Cache size: 2 097 152 bytes
Cache size while running Windows: 2 097 152 bytes
Disk Caching Status
drive read cache write cache buffering
--------------------------------------------
A: yes no no
B: yes no no
C: yes yes no
Write behind data will be committed before command prompt returns.
For help, type "Smartdrv /?".
The /X switch can be used to disable write-back caches even if SMARTDrive was initially loaded with the caches enabled.
To understand why the /X flag exists, and isn’t the default, some history is necessary. Microsoft provided a brief summary in Q127021: SMARTDrive Version History, and the salient points are as follows:
/X flag and ensuring that caches were flushed before the DOS prompt was displayed.So SMARTDRV, with no switches, always enables write caching at least on hard drives; one can only guess why the default wasn’t switched to /X, and instead the latter was added to the default AUTOEXEC.BAT invocation (Microsoft’s overriding drive for backwards compatibility presumably plays a role). Significantly, for a little while, waiting for the DOS prompt wasn’t safe.
(Later versions of 4DOS include explicit support for cache flushes, controlled by the SDFlush configuration setting, and the documentation claims that “Under PC DOS 6.1 and above, and MS-DOS 6.20 and above, COMMAND.COM instructs SMARTDRV to write all cached data to the disk before the command prompt is displayed.” I haven’t found any evidence of explicit support in COMMAND.COM, and I think that the general hook used in SMARTDrive 4.2 is how this was implemented in later versions too. This has been described somewhere but I haven’t found the reference yet.)
Note that all this only describes the cache provided by SMARTDrive itself, i.e. an area of RAM reserved to provide caches for the system’s drives. Drives can have their own caches, which SMARTDrive doesn’t control; back when SMARTDrive was still relevant, hard drives had tiny caches (the 1996 Quantum Fireballs had a whopping 128KiB of cache, which was large at the time; the 1998 IBM DeskStars increased that to 512KiB!) and those caches were write-through.
¹ Enforced meaning that the operating system complains vocally if it isn’t followed.
There is no “shutdown” process/command in DOS. You just cut the power at the end of your work. That was fine when PCs were simple, your storage options was either One Floppy or Two Floppies, and DOS never touched the disks when waiting at the command prompt. Things like Write Behind Caches make things more complicated and human habits die hard.
Write behind cache would lead to file corruption if you didn’t have the discipline to wait for the cache to flush before shutting down.(*)
In fact, I’ve known people who didn’t even bother closing the program: they would just finish entering data in say Lotus 1-2-3, type the Save command, wait for the disk light to stop and BANG hit the power switch. No amount of complaining on my part made a dent. I even saw the same habit with Windows, especially Windows 3.1 and earlier.
Now a bit of speculation: Microsoft must have added Write Behind cache to SmartDrive as ON by default in the original version. Once the complains came in about lost files and it became clear that Write Behind was too dangerous for the “uninitiated”, they must have felt they couldn’t change the default behavior without causing even more trouble, so they instead relied on Setup to add the /X parameter by default.
(*) I don’t remember if hard drives at the time were all smart enough to protect the disks during all power shutdown scenarios, so you might even risk media damage. I know early hard drives came with a “Park” command you were supposed to run to move the head to a safe location before shutting down (and yes I also know people who ignored it anyway). But that didn’t last very long and auto-parking drives became standard pretty quickly, so I presume it wasn’t a significant risk. However I suppose it’s possible even some auto-parking drives could have caused damage to the platters if power failed during writes.
