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SubjectRe: [PATCH] SMP race in ext2 - metadata corruption.
Rogier Wolff writes:
> Richard Gooch wrote:
> >
> > - next boot, init(8) checks the file, and if it exists, opens the root
> > FS block device and reads in each block listed in the file. The
> > effect is to warm the buffer cache extremely quickly. The head will
> > move in one direction, grabbing data as it flys by. I expect this
> > will take around 1 second
>
> FYI:
>
> Around 1992 or 1993, I rewrote Minix-fsck to do this instead of
> seeking all over the place.
>
> Cut the total time to fsck my filesystem from around 30 to 28
> seconds. (remember the days of small filesystems?)
>
> That's when I decided that this was NOT an interesting project: there
> was very little to be gained.
>
> The explanation is: A seek over a few tracks isn't much slower than a
> seek over hundreds of tracks. Almost any "skip" in linear access
> incurs the average 6ms rotational latency anyway.

Hm. I think the access patterns between boot-up and fsck are quite
different. An fsck has to seek to a large number of tracks. During
bootup, I think the number of tracks accessed is much lower, and there
is probably more data locality as well. Still, only one way to be
sure.
I haven't had time to look closely at this, but one thing that bothers
me is how to find out what is being accessed in the first place. A
C-library wrapper to intercept read(2) calls isn't any good, because a
lot of stuff is memory-mapped (in particular shared libraries). Anyone
have a clean way to do this?

Regards,

Richard....
Permanent: rgooch@atnf.csiro.au
Current: rgooch@ras.ucalgary.ca
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