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    SubjectRe: fsck, raid reconstruction & bad bad 2.4.3
    On Sun, 15 Apr 2001, Jonathan Lundell wrote:
    >At 9:23 PM -0500 2001-04-15, Jesse Pollard wrote:
    >> >b) wait with fsck until rebuild is fixed
    >>Depends on your definition of "fixed". The most I can see to fix is
    >>reduce the amount of continued update in favor of updating those blocks
    >>being read (by fsck or anything else). This really ought to be a runtime
    >>configuration option. If it is set to 0, then no automatic repair would
    >>be done.
    >The original post was referring to RAID 1; there's no repair necessary at
    >the RAID level to give fsck the correct data. Seems to me the basic problem
    >here is that the RAID re-sync is supposed to be throttling back to allow other
    >activity to run, but that in the case of fsck the other activity is still
    >slower by a large factor (compared to no RAID re-sync).

    If I've got the numbering right;
    0 - concatenated stripes => no sync required
    1 - mirrored => resync required
    a: which drive has the correct info?
    b: having determined that, read the correct block,
    it must now be written to the mirror.
    all others => resync required (rebuild possible bad block)

    >Is this a pathological case because of the way fsck does business, or does
    >the RAID re-sync affect any disk-bound process that severely?

    My experience has been with hardware raid, and even then there has been
    a 1-5% decrease in I/O during resync (not accurately measured - fsck took
    longer, and then only when the channel is maxed out -- otherwise the 1-5% is
    not visible; filesystem was 3 IRIX efs, spread across two raid luns).

    fsck is particularly bad, since nearly every read instigates a write to
    the mirror drive. Fsck can then modify the block and write it back, causing
    two more writes; for a total of 3 writes for a read (worst case).

    It does mean that when fsck finishes, MOST of the re-sync will be finished.
    All of the metadata will be synced, and only file data blocks will remain.

    Jesse I Pollard, II

    Any opinions expressed are solely my own.
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