On Sat, Jul 13, 2019 at 12:33 AM Bruce Momjian <bruce@momjian.us> wrote:
>
> On Fri, Jul 12, 2019 at 02:15:02PM +0900, Masahiko Sawada wrote:
> > > We will use CBC AES128 mode for tables/indexes, and CTR AES128 for WAL.
> > > 8k pages will use the LSN as a nonce, which will be encrypted to
> > > generate the initialization vector (IV). We will not encrypt the first
> > > 16 bytes of each pages so the LSN can be used in this way. The WAL will
> > > use the WAL file segment number as the nonce and the IV will be created
> > > in the same way.
> > >
> > > wal_log_hints will be enabled automatically in encryption mode, like we
> > > do for checksum mode, so we never encrypt different 8k pages with the
> > > same IV.
> >
> > I guess that different two pages can have the same LSN when a heap
> > update modifies both a page for old tuple and another page for new
> > tuple.
> >
> > heapam.c:3707
> > recptr = log_heap_update(relation, buffer,
> > newbuf, &oldtup, heaptup,
> > old_key_tuple,
> > all_visible_cleared,
> > all_visible_cleared_new);
> > if (newbuf != buffer)
> > {
> > PageSetLSN(BufferGetPage(newbuf), recptr);
> > }
> > PageSetLSN(BufferGetPage(buffer), recptr);
> >
> > Wouldn't it a problem?
>
> I had the same question. If someone does:
>
> UPDATE tab SET col = col + 1
>
> then each row change gets its own LSN. You are asking if an update that
> just expires one row and adds it to a new page gets the same LSN. I
> don't know.
The following scripts can reproduce that different two pages have the same LSN.
=# create table test (a int);
CREATE TABLE
=# insert into test select generate_series(1, 226);
INSERT 0 226
=# update test set a = a where a = 1;
UPDATE 1
=# select lsn from page_header(get_raw_page('test', 0));
lsn
-----------
0/1690488
(1 row)
=# select lsn from page_header(get_raw_page('test', 1));
lsn
-----------
0/1690488
(1 row)
So I think it's better to use LSN and page number to create IV. If we
modify different tables by single WAL we also would need OID or
relfilenode but I don't think currently we have such operations.
Regards,
--
Masahiko Sawada
NIPPON TELEGRAPH AND TELEPHONE CORPORATION
NTT Open Source Software Center
