Re: Scaling XLog insertion (was Re: Moving more work outside WALInsertLock)

On Wed, Mar 7, 2012 at 3:04 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
>> Just to keep things in perspective -- For a commit record to reach one
>> megabyte, it would have to be a transaction that drops over 43k tables.
>> Or have 64k smgr inval messages (for example, a TRUNCATE might send half
>> a dozen of these messages). Or have 262k subtransactions.  Or
>> combinations thereof.
>
>> Now admittedly, a page is only 8 kB, so for a commit record to be "many
>> pages long" (that is, >=3) it would require about 1500 smgr inval
>> messages, or, say, about 250 TRUNCATEs (of permanent tables with at
>> least one toastable field and at least one index).
>
> What about the locks (if running hot-standby)?

It's a list of active AccessExclusiveLocks. If that list is long you
can be sure not much else is happening on the server.

>> So they are undoubtely rare.  Not sure if as rare as Higgs bosons.
>
> Even if they're rare, having a major performance hiccup when one happens
> is not a side-effect I want to see from a patch whose only reason to
> exist is better performance.

I agree the effect you point out can exist, I just don't want to slow
down the main case as a result.

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