Re: Fwd: Re: [HACKERS] For Review: Allow WAL information to recover

Attached is a patch the author had trouble posting that adds a -r
(recover) option to pg_resetxlog. I will put this in the 8.2 queue
along with the original message.

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yuanjia lee wrote:

I had added an option -r to pg_resetxlog to enable

the tool can rebuild the corrupted pg_control file
from the old xlog files.

here is the patch. Sorry I had tried to attached

it to the mail, but it failed, I dont know why, here
is the

link:http://www.geocities.com/yuanjia_pg/pg_resetxlog.diff.txt

There are also some changes in the logic of other

options.

Option -n: only print out the control values in

the existing pg_control file, if the file is
corrupted , inform the use to rebuild it first only.

Option -f: if pg_control file is fine, then reset

the xlog file; if pg_control is corrupted , then try
to rebuild the control file from old xlog file, if
it fails, then just guessing the value, then reset
the xlog file.

The algorithm of restoring the pg_control value

from old xlog file:

1. Retrieve all of the active xlog files from

xlog directory into a list by increasing order,
according their timeline, log id, segment id. (Tom
had informed me that we can not know which segment
file is latest just by the name itself, so before
adding the segment file to the list, it should be
checked that it is an active segment file.)

2. Search the list to find the oldest xlog file of

the latest time line. (Although it is better to let
the user to select the time line which is used for
rebuild the xlog file, but I think there is not so
necessary. I had tried to use only the last file in
the latest time line, but I found that in many
cases, there are possible that the last checkpoint
record and the previous checkpoint record are stored
separately in different segment file, so I had to
search from the oldest one.)

3. Search the records from the oldest xlog file of

latest time line to the latest xlog file of latest
time line, if the checkpoint record has been found,
update the latest checkpoint and previous
checkpoint.

Some of the code is borrowed from Tom Lane

xlogdump.c file.

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