| From: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
|---|---|
| To: | Alvaro Herrera <alvherre(at)commandprompt(dot)com> |
| Cc: | Heikki Linnakangas <heikki(dot)linnakangas(at)enterprisedb(dot)com>, PostgreSQL-development <pgsql-hackers(at)postgresql(dot)org> |
| Subject: | Re: WAL format |
| Date: | 2009-12-07 20:48:23 |
| Message-ID: | 24297.1260218903@sss.pgh.pa.us |
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| Lists: | pgsql-hackers |
Alvaro Herrera <alvherre(at)commandprompt(dot)com> writes:
> Heikki Linnakangas wrote:
>> - at the end of WAL segment, when there's not enough space to write the
>> next WAL record, always write an XLOG SWITCH record to fill the rest of
>> the segment.
> What happens if a record is larger than a WAL segment? For example,
> what if I insert a 16 MB+ datum into a varlena field?
That case doesn't pose a problem --- the datum would be toasted into
individual tuples that are certainly no larger than a page. However
we do have cases where a WAL record can get arbitrarily large; in
particular a commit record with many subtransactions and/or many
disk files to delete. These cases do get exercised in the field
too --- I can recall at least one related bug report.
regards, tom lane
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