Add WAL compression item to TODO.detail.

doc/TODO.detail/wal

@@ -2903,3 +2903,104 @@ I regret I do not currently have time to pursue further.
 
 Best Regards, Simon Riggs
 
+From pgsql-hackers-owner+M65147=pgman=candle.pha.pa.us@postgresql.org Fri Mar 11 12:35:29 2005
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+Message-ID: <4231E416.4030900@cybertec.at>
+Date: Fri, 11 Mar 2005 19:31:50 +0100
+From: =?ISO-8859-1?Q?Hans-J=FCrgen_Sch=F6nig?= <postgres@cybertec.at>
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+To: Simon Riggs <simon@2ndquadrant.com>
+cc: Tom Lane <tgl@sss.pgh.pa.us>,
+   Mark Cave-Ayland <m.cave-ayland@webbased.co.uk>,
+   pgsql-hackers@postgresql.org
+Subject: Re: [HACKERS] Cost of XLogInsert CRC calculations
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+Status: OR
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+
+> One of the things I was thinking about was whether we could use up those
+> cycles more effectively. If we were to include a compression routine
+> before we calculated the CRC that would 
+> - reduce the size of the blocks to be written, hence reduce size of xlog
+> - reduce the following CRC calculation
+> 
+> I was thinking about using a simple run-length encoding to massively
+> shrink half-empty blocks with lots of zero padding, but we've already
+> got code to LZW the data down also.
+> 
+> Best Regards, Simon Riggs
+> 
+> 
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+
+
+Simon,
+
+I think having a compression routine in there could make real sense.
+We have done some major I/O testing involving compression for a large 
+customer some time ago. We have seen that compressing / decompressing on 
+the fly is in MOST cases much faster than uncompressed I/O (try a simple 
+"cat file | ..." vs." zcat file.gz | ...") - the zcat version will be 
+faster on all platforms we have tried (Linux, AIX, Sun on some SAN 
+system, etc. ...).
+Also, when building up a large database within one transaction the xlog 
+will eat a lot of storage - this can be quite annoying when you have to 
+deal with a lot of data).
+Are there any technical reasons which would prevent somebody from 
+implementing compression?
+
+	Best regards,
+
+		Hans
+
+-- 
+Cybertec Geschwinde u Schoenig
+Schoengrabern 134, A-2020 Hollabrunn, Austria
+Tel: +43/660/816 40 77
+www.cybertec.at, www.postgresql.at
+
+
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