Re: [HACKERS] Idea for better handling of cntxDirty
Wouldn't it work for cntxDirty to be set not by LockBuffer, but by XLogInsert for each buffer that is included in its argument list? I thought to add separate call to mark context dirty but above should work if all callers to XLogInsert always pass all modified buffers - please check. Vadim ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
Re: [HACKERS] Idea for better handling of cntxDirty
Mikheev, Vadim [EMAIL PROTECTED] writes: Wouldn't it work for cntxDirty to be set not by LockBuffer, but by XLogInsert for each buffer that is included in its argument list? I thought to add separate call to mark context dirty but above should work if all callers to XLogInsert always pass all modified buffers - please check. AFAICT it is safe. There are some places (in sequences and btree) where not all the modified buffers are explicitly listed in XLogInsert's arguments, but redo of those types of WAL records will always reinit the affected pages anyway. So we don't need to worry about forcing checkpoint to write the pages early. In general I don't think this adds any fragility to the system. A WAL record that is not set up to restore all buffers modified by the logged operation would be broken by definition, no? regards, tom lane ---(end of broadcast)--- TIP 2: you can get off all lists at once with the unregister command (send unregister YourEmailAddressHere to [EMAIL PROTECTED])
[HACKERS] Idea for better handling of cntxDirty
Vadim,
In LockBuffer() you wrote
else if (mode == BUFFER_LOCK_EXCLUSIVE)
{
LWLockAcquire(buf-cntx_lock, LW_EXCLUSIVE);
/*
* This is not the best place to set cntxDirty flag (eg indices do
* not always change buffer they lock in excl mode). But please
* remember that it's critical to set cntxDirty *before* logging
* changes with XLogInsert() - see comments in BufferSync().
*/
buf-cntxDirty = true;
}
The comments in BufferSynx are
/*
* We can check bufHdr-cntxDirty here *without* holding any lock
* on buffer context as long as we set this flag in access methods
* *before* logging changes with XLogInsert(): if someone will set
* cntxDirty just after our check we don't worry because of our
* checkpoint.redo points before log record for upcoming changes
* and so we are not required to write such dirty buffer.
*/
Wouldn't it work for cntxDirty to be set not by LockBuffer, but by
XLogInsert for each buffer that is included in its argument list?
This would avoid setting the flag for pages that are not modified
after being locked. XLogInsert would of course set the flag before
doing the actual WAL insertion, so it seems to me that the condition
we want is met, and we still have only a single place that needs to
remember to set the flag.
regards, tom lane
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