Re: unlogged tables vs. GIST

Hi Heikki,

On Mon, Jan 28, 2013 at 2:34 PM, Heikki Linnakangas <hlinnakangas(at)vmware(dot)com
> wrote:

> On 23.01.2013 17:30, Robert Haas wrote:
>
>> On Wed, Jan 23, 2013 at 4:04 AM, Jeevan Chalke
>> <jeevan(dot)chalke(at)enterprisedb(dot)**com <jeevan(dot)chalke(at)enterprisedb(dot)com>>
>> wrote:
>>
>>> I guess my earlier patch, which was directly incrementing
>>>
>>> ControlFile->unloggedLSN counter was the concern as it will take
>>> ControlFileLock several times.
>>>
>>> In this version of patch I did what Robert has suggested. At start of the
>>> postmaster, copying unloggedLSn value to XLogCtl, a shared memory struct.
>>> And
>>> in all access to unloggedLSN, using this shared variable using a
>>> SpinLock.
>>> And since we want to keep this counter persistent across clean shutdown,
>>> storing it in ControlFile before updating it.
>>>
>>> With this approach, we are updating ControlFile only when we shutdown the
>>> server, rest of the time we are having a shared memory counter. That
>>> means
>>> we
>>> are not touching pg_control every other millisecond or so. Also since we
>>> are
>>> not caring about crashes, XLogging this counter like OID counter is not
>>> required as such.
>>>
>>
>> On a quick read-through this looks reasonable to me, but others may
>> have different opinions, and I haven't reviewed in detail.
>>
> > ...
>
>> [a couple of good points]
>>
>
> In addition to those things Robert pointed out:
>
> /*
>> + * Temporary GiST indexes are not WAL-logged, but we need LSNs to detect
>> + * concurrent page splits anyway. GetXLogRecPtrForUnloggedRel() provides
>> a fake
>> + * sequence of LSNs for that purpose. Each call generates an LSN that is
>> + * greater than any previous value returned by this function in the same
>> + * session using static counter
>> + * Similarily unlogged GiST indexes are also not WAL-logged. But we need
>> a
>> + * persistent counter across clean shutdown. Use counter from
>> ControlFile which
>> + * is copied in XLogCtl.unloggedLSN to accomplish that
>> + * If relation is UNLOGGED, return persistent counter from XLogCtl else
>> return
>> + * session wide temporary counter
>> + */
>> +XLogRecPtr
>> +GetXLogRecPtrForUnloggedRel(**Relation rel)
>>
>
> From a modularity point of view, it's not good that you xlog.c needs to
> know about Relation struct. Perhaps move the logic to check the relation is
> unlogged or not to a function in gistutil.c, and only have a small
> GetUnloggedLSN() function in xlog.c
>

I kept a function as is, but instead sending Relation as a parameter, it
now takes boolean, isUnlogged. Added a MACRO for that.

>
> I'd suggest adding a separate spinlock to protect unloggedLSN. I'm not
> sure if there's much contention on XLogCtl->info_lck today, but
> nevertheless it'd be better to keep this separate, also from a modularity
> point of view.
>

Hmm. OK.
Added ulsn_lck for this.

>
> @@ -7034,6 +7078,8 @@ CreateCheckPoint(int flags)
>> LWLockAcquire(ControlFileLock, LW_EXCLUSIVE);
>> ControlFile->state = DB_SHUTDOWNING;
>> ControlFile->time = (pg_time_t) time(NULL);
>> + /* Store unloggedLSN value as we want it persistent
>> across shutdown */
>> + ControlFile->unloggedLSN = XLogCtl->unloggedLSN;
>> UpdateControlFile();
>> LWLockRelease(ControlFileLock)**;
>> }
>>
>
> This needs to acquire the spinlock to read unloggedLSN.
>

OK. Done.

>
> Do we need to do anything to unloggedLSN in pg_resetxlog?
>

I guess NO.

Also, added Robert's comment in bufmgr.c
I am still unsure about the spinlock around buf flags as we are just
examining it.

Please let me know if I missed any.

Thanks

>
> - Heikki
>

--
Jeevan B Chalke
Senior Software Engineer, R&D
EnterpriseDB Corporation
The Enterprise PostgreSQL Company

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