From: | Magnus Hagander <magnus(at)hagander(dot)net> |
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To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Bruce Momjian <bruce(at)momjian(dot)us>, pgsql-hackers(at)postgresql(dot)org |
Subject: | Re: Fairly serious bug induced by latest guc enum changes |
Date: | 2008-07-01 18:57:38 |
Message-ID: | 486A7E22.7050405@hagander.net |
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Lists: | pgsql-hackers |
Tom Lane wrote:
> Magnus Hagander <magnus(at)hagander(dot)net> writes:
>> Tom Lane wrote:
>>> Hmm ... or at least more or less fixed. Seems like there's no provision
>>> to close and reopen the file if enableFsync changes. Not sure if that's
>>> worth worrying about.
>
>> We didn't have that before either, did we?
>
> No, so I think it's a pre-existing bug.
Ok, at least I'm reading the code right.
>> We close it when the sync bit
>> changes, but not just if we change say between fsync() and fdatasync().
>> Is there any actual reason we'd want to close it?
>
> The point is that if you turn the fsync GUC on or off while using a wal
> sync mode that requires supplying an option flag to open(), then really
> you ought to close the WAL file and re-open it with the new correct
> option flags. The fact that we're not doing that implies that the
> effects of a change in fsync might not fully take effect until the next
> WAL segment is started. Whether this is worth fixing isn't real clear.
What scenario does it actually happen in, though? Doesn't the check:
if (get_sync_bit(sync_method) != get_sync_bit(new_sync_method))
take care of that? If the sync bit changed, we close the file?
Or are you talking about changing the variable "fsync"? If so, doesn't
"fsync=off" also change the behavior of other parts of the code, so it's
not just WAL, which means it'd be pretty unsafe *anyway* unless you
actually "sync" the disks, and not just fsync?
//Magnus
//Magnus
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