On Thu, Aug 23, 2018 at 7:47 PM Bryan Henderson <[email protected]>
wrote:
> I've been reading MDS log code, and I have a question: why does it "probe
> for
> the end of the log" after reading the log header when starting up?
>
> As I understand it, the log header says the log had been written up to
> Location X ("write_pos") the last time the log was committed, but the
> end-probe code determines whether there is stuff physically in the log
> (based
> on Rados object size) beyond X and if so, ignores the header and uses the
> physical end of the log instead.
>
> Wouldn't stuff after where the header says writing left off be unreliable?
> Maybe incompletely or incorrectly written?
>
> I'm looking at this because I have an MDS that will not start because there
> is junk (zeroes) in that space after where the log header says the log
> ends,
> so replay of the log fails there.
No, the log end in the header is a hint. This is because we can’t
atomically wrote to two objects (the header and the last log object) at the
same time, so we do atomic appends to the end of the log and flush out the
journal header lazily.
I believe zeroes at the end of the log are deliberate, as we “pre-zero” to
avoid some rare edge cases when MDSes restart and the log might have had
writes to later objects complete successfully while earlier ones were
blocked. If your MDS is not restarting it is probably because of the
non-zero data.
-Greg
>
> --
> Bryan Henderson San Jose, California
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