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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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