Обсуждение: pg_standby: Question about truncation of trigger file in fast failover
<div dir="ltr"><p>I was trying to understand (and then perhaps mimic) how pg_standby does a fast failover. <br /><p>My currentunderstanding is that when a secondary db is in standby mode, it will exhaust all the archive log to be replayed fromthe primary and then start streaming. It is at this point that xlog.c checks for the existence of a trigger file to promotethe secondary. This was been a cause of some irritation for some of our customers who do not really care about catchingup all the way. I want to achieve the exact semantics of pg_standby's fast failover option.<p><p>I manipulated therestore command to return 'failure' when the word "fast" is present in the trigger file (see below), hoping that whenI want a secondary database to come out fast, I can just echo the word "fast" into the trigger file thereby simulatingpg_standby's fast failover behavior. However, that did not work. Techically, I did not truncate the trigger filelike how pg_standby. <br /><p><span style="font-family:courier new,monospace"><New restore_command> = ! fgrep -qsifast <trigger_file> && <Old restore_command></span><p><br />And that is where I have a question.I noticed that in pg_standby.c when we detect the word "fast" in the trigger file we truncate the file. <p><a href="https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L456" target="_blank">https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L456</a><p>Thereis alsoa comment above it about not "upsetting" the server.<p><a href="https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L454" target="_blank">https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L454</a><p>Whatis thepurpose of truncating the file? To do a smart failover once you come out of standby? But, when I look at xlog.c, whenwe come out of standby due to a failure returned by restore_command, we call CheckForStandbyTrigger() here: <p><a href="https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L10441" target="_blank">https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L10441</a><p>Now, CheckForStandbyTrigger()unlinks the trigger file. I noticed through the debugger that the unlinking happens before xlog.cmakes a call to the next restore_command. So, what is the reason for truncating the "fast" word from the trigger fileif the file is going to be deleted soon after it is discovered? How will we "upset" the server if we don't?<br /><p><br/><p>Assuming this question is answered and I get a better understanding, I have a follow up question. If truncationis indeed necessary, can I simulate the truncation by manipulating restore_command and achieve the same effectas a fast failover in pg_standby?<p><br /><p><br /><p>Thanks in advance for the help.<p>Neil<br /></div>
Re: pg_standby: Question about truncation of trigger file in fast failover
От
Heikki Linnakangas
Дата:
On 02/19/2014 11:15 PM, Neil Thombre wrote: > And that is where I have a question. I noticed that in pg_standby.c when we > detect the word "fast" in the trigger file we truncate the file. > > https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L456 > > There is also a comment above it about not "upsetting" the server. > > https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L454 > > What is the purpose of truncating the file? To do a smart failover once you > come out of standby? But, when I look at xlog.c, when we come out of > standby due to a failure returned by restore_command, we call > CheckForStandbyTrigger() here: > > https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L10441 > > Now, CheckForStandbyTrigger() unlinks the trigger file. I noticed through > the debugger that the unlinking happens before xlog.c makes a call to the > next restore_command. So, what is the reason for truncating the "fast" > word from the trigger file if the file is going to be deleted soon after it > is discovered? How will we "upset" the server if we don't? At end-of-recovery, the server will fetch again the last WAL file that was replayed. If it can no longer find it, because restore_command now returns an error even though it succeeded for the same file few seconds earlier, it will throw an error and refuse to start up. That's the way it used to be until 9.2, anyway. In 9.2, the behavior was changed, so that the server keeps all the files restored from archive, in pg_xlog, so that it can access them again. I haven't tried, but it's possible that the truncation is no longer necessary. Try it, with 9.1 and 9.3, and see what happens. - Heikki
On Wed, Feb 19, 2014 at 1:42 PM, Heikki Linnakangas <hlinnakangas@vmware.com> wrote:
On 02/19/2014 11:15 PM, Neil Thombre wrote:At end-of-recovery, the server will fetch again the last WAL file that was replayed. If it can no longer find it, because restore_command now returns an error even though it succeeded for the same file few seconds earlier, it will throw an error and refuse to start up.And that is where I have a question. I noticed that in pg_standby.c when we
detect the word "fast" in the trigger file we truncate the file.
https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L456
There is also a comment above it about not "upsetting" the server.
https://github.com/postgres/postgres/blob/REL9_1_11/contrib/pg_standby/pg_standby.c#L454
What is the purpose of truncating the file? To do a smart failover once you
come out of standby? But, when I look at xlog.c, when we come out of
standby due to a failure returned by restore_command, we call
CheckForStandbyTrigger() here:
https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L10441
Now, CheckForStandbyTrigger() unlinks the trigger file. I noticed through
the debugger that the unlinking happens before xlog.c makes a call to the
next restore_command. So, what is the reason for truncating the "fast"
word from the trigger file if the file is going to be deleted soon after it
is discovered? How will we "upset" the server if we don't?
The restore_command returns error exactly once in my setup. So the next time around, it does go back and is able to fetch the last segment successfully applied. Let me go through the steps:
<New restore_command> = ! fgrep -qsi fast <trigger_file> && <Old restore_command>
1. Until it finds "fast" in the trigger file it will continue running the (old) restore_command. It applies 0000000300000C3100000099 successfully from the archive.
2. I echo "fast" >> trigger_file
3. The next restore_command returns failure (because of the first part of my &&) so it never ends up applying the next segment it was supposed to apply,i.e., 0000000300000C310000009A
4. Db comes out of standby and checks for trigger file in CheckForStandbyTrigger() that unlinks it. Now the trigger file is gone!
5. Next, it tries to read the last applied segment 0000000300000C3100000099 again - the new restore_command WILL NOT return failure because the trigger file is gone and the first part (before the &&) is true and it will run the old restore_command and try get the same file 0000000300000C3100000099
6. It is during reapplying this file that I get the following error:
Feb 7 00:37:45 XXXXXXXXXXXXXXXXXXXXXXXX LOG: restored log file "0000000300000C3100000099" from archive
Feb 7 00:37:45 XXXXXXXXXXXXXXXXXXXXXXXX FATAL: WAL ends before consistent recovery point
This error comes from:
https://github.com/postgres/postgres/blob/REL9_1_11/src/backend/access/transam/xlog.c#L6782-L6783
Therefore, I feel that something was amiss in my setup. And I wanted to understand the motive/tribal-knowledge behind the truncation part of pg_standby's fast failover so as not to upset the server. In other words, I have a feeling that by not truncating the trigger file I am inadvertently upsetting the server which is the cause of my FATAL error.
That's the way it used to be until 9.2, anyway. In 9.2, the behavior was changed, so that the server keeps all the files restored from archive, in pg_xlog, so that it can access them again. I haven't tried, but it's possible that the truncation is no longer necessary. Try it, with 9.1 and 9.3, and see what happens.
It may very well be that 9.3 will not have this problem. I will definitely try this out when I have a chance - this problem is pretty obscure and rarely happens on our customer databases, therefore we just work with whatever database version we have at the moment to get as much forensics and try out any solutions.
- Heikki
Thanks a lot for your help.