Re: Archive log compression keeping physical log available in the crash recovery

Sorry for the late responce;

Gzip can reduce the archive log size about one fourth. My point is
that it can still be large enough. Removing physical log record (by
replacing them with logical log record) from archive log will achieve
will shrink the size of the archive log to one twentieth, in the case of
pgbehcn test about ten hours (3,600,000 transactions) with database size
about 2GB. In the case of gzip, maybe becuase of higher CPU load,
total throughput for gzip is less than just copying WAL to archive. In
our case, throughput seems to be slightly higher than just copying
(preserving physical log) or gzip. I'll gather the meaturement result
and try to post.

The size of archive log seems not affected by the size of the database,
but just by the number of transactions. In the case of
full_page_writes=on and full_page_compress=on, compressed archive log
size seems to be dependent only on the number of transactions and
transaction characteristics.

Our evaluation result is as follows:
Database size: 2GB
WAL size (after 10hours pgbench run): 48.3GB
gzipped size: 8.8GB
removal of the physical log: 2.36GB
fullpage_writes=off log size: 2.42GB

The reason why archive log size of our case is slightly smaller than
full_page_writes=off is because we remove not only the physical logs
but also each page header and the dummy part at the tail of each log
segment.

Further, we can apply gzip to this archive (2.36GB). Final size is
0.75GB, less than one sixtieth of the original WAL.

Overall duration to gzip from WAL (48.3GB to 8.8GB) was about 4000sec,
and our compression to 2.36GB needed about 1010sec, slightly less than
just cat command (1386sec). When gzip is combined with our compression
(48.3GB to 0.75GB), total duration was about 1330sec.

This shows that phyiscal log removal is good selection for the following
case:

1) Need same crash recovery possibility as full_page_writes=on, and
2) Need to shrink the size of archive log for loger period to store.

Of course, if we care crash recovery in PITR slave, we still need
physical log records in archive log. In this case, because archive log
is not intended to be kept long, its size will not be an issue.

I'm planning to do archive log size evalutation with other benchmarks
such as DBT-2 as well.

Materials for this has already been thrown to HACKERS and PATCHES. I
hope you try this.

Jim Nasby wrote:
> I thought the drive behind full_page_writes = off was to reduce the
> amount of data being written to pg_xlog, not to shrink the size of a
> PITR log archive.
>
> ISTM that if you want to shrink a PITR log archive you'd be able to get
> good results by (b|g)zip'ing the WAL files in the archive. I quick test
> on my laptop shows over a 4x reduction in size. Presumably that'd be
> even larger if you increased the size of WAL segments.
>
> On Jan 29, 2007, at 2:15 AM, Koichi Suzuki wrote:
>
>> This is a proposal for archive log compression keeping physical log in
>> WAL.
>>
>> In PotgreSQL 8.2, full-page_writes option came back to cut out physical
>> log both from WAL and archive log. To deal with the partial write
>> during the online backup, physical log is written only during the online
>> backup.
>>
>> Although this dramatically reduces the log size, it can risk the crash
>> recovery. If any page is inconsisitent because of the fault, crash
>> recovery doesn't work because full page images are necessary to recover
>> the page in such case. For critical use, especially in commercial use,
>> we don't like to risk the crash recovery chance, while reducing the
>> archive log size will be crucial too for larger databases. WAL size
>> itself may be less critical, because they're reused cyclickly.
>>
>> Here, I have a simple idea to reduce archive log size while keeping
>> physical log in xlog:
>>
>> 1. Create new GUC: full_page_compress,
>>
>> 2. Turn on both the full_page_writes and full_page_compress: physical
>> log will be written to WAL at the first write to a page after the
>> checkpoint, just as conventional full_page_writes ON.
>>
>> 3. Unless physical log is written during the online backup, this can be
>> removed from the archive log. One bit in XLR_BKP_BLOCK_MASK
>> (XLR_BKP_REMOVABLE) is available to indicate this (out of four, only
>> three of them are in use) and this mark can be set in XLogInsert().
>> With the both full_page_writes and full_page_compress on, both logical
>> log and physical log will also be written to WAL with XLR_BKP_REMOVABLE
>> flag on. Having both physical and logical log in a same WAL is not
>> harmful in the crash recovery. In the crash recovery, physical log is
>> used if it's available. Logical log is used in the archive recovery, as
>> the corresponding physical log will be removed.
>>
>> 4. The archive command (separate binary), removes physical logs if
>> XLR_BKP_REMOVABLE flag is on. Physical logs will be replaced by a
>> minumum information of very small size, which is used to restore the
>> physical log to keep other log records's LSN consistent.
>>
>> 5. The restore command (separate binary) restores removed physical log
>> using the dummy record and restores LSN of other log records.
>>
>> 6. We need to rewrite redo functions so that they ignore the dummy
>> record inserted in 5. The amount of code modification will be very
>> small.
>>
>> As a result, size of the archive log becomes as small as the case with
>> full_page_writes off, while the physical log is still available in the
>> crash recovery, maintaining the crash recovery chance.
>>
>> Comments, questions and any input is welcome.
>>
>> -----
>> Koichi Suzuki, NTT Open Source Center
>>
>> --Koichi Suzuki
>>
>> ---------------------------(end of broadcast)---------------------------
>> TIP 6: explain analyze is your friend
>>
>
> --
> Jim Nasby jim(at)nasby(dot)net
> EnterpriseDB http://enterprisedb.com 512.569.9461 (cell)
>
>
>

--
Koichi Suzuki