[TUHS] The evolution of Unix facilities and architecture

[Warner Losh]

On Thu, May 11, 2017 at 6:21 PM, Larry McVoy <lm at mcvoy.com> wrote:
> Yeah, I get ordered writes, I taught a CS course at Stanford and I made
> my students learn all about them.  I'm a UFS guy, so far as I know I'm
> the last guy to push UFS/FFS forward (which is sort of sad).
>
> The Linux stuff is better.  It just is.  And we should all respect that,
> I know we sit around and love on ancient Unix, and believe me, I love
> that stuff it changed the world, but we should respect people who have
> moved it past what Unix did.  And I think Linux moved the file system
> past what Unix did.

The Linux stuff was incrementally better in that it didn't fry the
filesystem, but instead would flush large amounts of changes if there
was no synchronous write before the power failed. This was progress.
However, it got its speed by doing async writes even of the metadata,
and that lead to the famous story where Linus was able to recover an
entire kernel he'd just accidentally rm -rf'd by hitting reset on his
machine before the syncer ran. Better than a scrambled filesystem?
Sure. But consistent and robust? Not so much. Plus, driver bugs could
still wreck havoc.

Soon after things like soft-updates came along in the BSD world which
solved the scrambled filesystem problems (for the most part), then SU
Journalling to make it robust. On the Linux side, ext2fs begat ext3
and then ext4fs came along and added journalling. ZFS came into being.
As did btrfs and many others. Some on Linux, some on Solaris, some on
BSD. The competition between them all has helped to make them all
better.

I've been running a v7 derivative (Venix) on my Rainbow lately.
There's issues with the motherboard that causes many insta-panics (it
turns off the interrupts due to something bad on the mobo --- a new
mobo doesn't do that). I've yet to lose anything, but the performance
isn't that great. So it's not an 'all the time' sort of thing.

Warner

> --lm
>
> On Thu, May 11, 2017 at 07:48:27PM -0400, Ron Natalie wrote:
>> Ordered writes go back to the original BSD fast file system, no?   I  seem
>> to recall that when we switched from our V6/V7 disks,
>> the filesystem got a lot more stable in crashes.
>>
>> -----Original Message-----
>> From: TUHS [mailto:tuhs-bounces at minnie.tuhs.org] On Behalf Of Dave Horsfall
>> Sent: Thursday, May 11, 2017 7:47 PM
>> To: The Eunuchs Hysterical Society
>> Subject: Re: [TUHS] The evolution of Unix facilities and architecture
>>
>> On Thu, 11 May 2017, Larry McVoy wrote:
>>
>> [...]
>>
>> > Try the same thing with Linux.  The file system will come back,
>> > starting with, I believe, ext2.
>>
>> That's a journalled FS, isn't it?  In which case the transactions get
>> replayed.
>>
>> > My belief is that Linux orders writes such that while you may lose
>> > data (as in, a process created a file, the OS said it was OK, but that
>> > file will not be in the file system after a crash), but the rest of
>> > the file system will be consistent.  I think it's as if you powered
>> > off the machine a few seconds earlier than you actually did, some
>> > stuff is in flight and until they can write stuff out in the proper
>> > order you may lose data on a hard reset.
>>
>> And FreeBSD (at least) has been doing ordered writes for quite some time.
>>
>> --
>> Dave Horsfall DTM (VK2KFU)  "Those who don't understand security will
>> suffer."
>
> --
> ---
> Larry McVoy                  lm at mcvoy.com             http://www.mcvoy.com/lm