Warm standby done right

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Warm standby done right
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People has been setting up warm standby systems with streaming replication since version 9.0, and even longer with file-based log-shipping. However, there has been a few pitfalls that many people don't know about, while others have simply accepted the risks. PostgreSQL 9.5 brings a bunch of new features and subtle changes that make warm standby setups more robust than ever. In 9.5, the interaction between a WAL archive and failover has been revised. pg rewind makes it possible to resynchronize an old master server after failover - even an unplanned one. Replication slots, already introduced in 9.4, make the behaviour of a standby falling behind nicer. This presentation explains the changes, and why they were needed. Finally, I'm going to walk through setting up a simple, robust, two server hot standby system, using only built-in tools and simple shell scripts, taking advantage of the new features.
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on my namesake limitedness nowadays over people fall on we're working on open source and free 1 even 1 hasn't on but today we today kind of told well this is something I've been hacking 1 4 9 1 5 on this
presentation and we'll talk about some improvements that we do not want violence and changes that are hopefully improvements in 1 5 and this is about this is all about building tools and the server-side behavior that we have imposed this I'm not going to cover anything about manager Wally PC back resources of the tools of of cover of the new monitoring more heartbeats anything like that those over important things you need those of but I'm not gonna cover that important parts today I am going to talk about the pushes internals of understood this presentation into a few parts that are kind of loosely coupled of 100 % some problems that we have had in previous is releases them I'm going to explain how we can all work around them or how they have been fixed in point 5 so that's sort of part 1
continues archiving how many of you use continues archiving how many of you have set up what the a quite a few so there are a few got just that you might not be aware of this simple
procedure where you set of archive is set up while I will talk about things that are promoted on and then you specify archive command and this is where it gets come more complicated would you continue archive command station that that is a good answer and let's say the backrest dust what I'm going to talk about and if it actually does that correctly you come back to on so when you said
that while archived the posters user manual says that is very flexible know the oracle command might store of the day the Michael Doris agent might you might use to some like Pt backrest and all that all that as many users as a circuit somewhere else so and actually know if and when we have a lot of time on the market there all of the of the we read it all that makes a circuit with a lot of Europe refers to that make sense so yes have so you can over there a Briton CV would not but what the manual says
that you have to make sure is that it returns 0 if it succeeds and it must not return 0 if it does not succeed another important point that the manual makes is that it must not through its should refuse to overwrite the existing files it's a pretty broad and safeguards on I think things will work if you don't do that if you will overwrite existing files with it's a sounds dangerous so you know it's a good safety measure not to on post should work if you do but what he adds is it to get its the eyes are easy to get confused if you have multiple classes you point in the same directory and roots of yeah but assuming you do everything correctly this is in no posters doesn't require that for correctness that refused or filed with system very good administration of practice after manual says if you make an administrator error tools for the you
from that so then the manual goes on and use these example which is like the statistical become all and even says that it's a record it's an example of the recommendation of this might not work on all platforms happening and with this just a of have only things words with work or I don't know so that is the 1st got to got
their O P 1 drifting off so if you use this example command just copied to local directory review write your own tool that just copies of the local directory there is no guarantee that when run your program 6 it's really on this so it's possible that spectral possible that right after your commanders returned posters decides OK this has been saved archive I can later and then you crash and now you have to follow the lead from posters side but it's an odd actually Stallman disk so you lose the not series of the sons because all the latest values that way or maybe the latest so it's not no it's not the end of the world but it's still pretty annoying and it does mean that if you then continue running you'll have a missing file in the middle of your archive and then you can you know restore that back up any further than that so it's annoying and I said I won't talk about monitoring tools but that is 1 rated test that you're monitoring tools should be checking do you have all the well for us there are sequentially number so I recommend that you every now and then polio archive and make sure that you have all files and there just who knows you might have problems like this or something else that makes 1 file to go missing and the missing file in the middle all the rest of the velocity have after that are used so the 2nd got that
comes from this arises from the fact that if you follow the good industries in practice that you never override an existing file there is actually no guarantee that posters want try of archived the same file because it's entirely possible that after you've already archived files you crash and the fire was already archived post didn't get the memo it doesn't know that it was safely archives so after you restart of the further requirement again now if you wrote your you're archive command the way the manual says it will turn error because it's already there and now you're archiving is failing hopefully again you have these screw monitoring tools that will detect that and then you go and fix it all but it's something that can happen so that you can all yes I
would recommend exactly that use to compare the old and new files and if they are white but identical return success instead of positive because if you drive the archives and file flies that no there's no point in our carrying the same for all of us so I recommend writing your pocket money if you're writing something like back resource and computer tool of it's worthwhile to do his job and so all the right to probe or stock of command only return 0 such as as the manual says as the manuals assessed refused to override an existing file or less it's identical to the 1 that that exists in that isn't such and if you're writing to locally always of sink to make sure it actually makes that is if you're coping it somewhere else then I guess it depends whether you want I mean isn't enough that you as a a copies over them on the server maybe you trust that it won't crash at the same time I know that I would yeah I just yet still a single 1 of this was but in the 1 yes that's true that's true so if you just write it if you yeah so that's the point is you've have added that there so also make sure that your your input covering the body places of comical you use something like temper filename exiting then renamed it in place and you have that you have that yeah yeah because again why not have my than so better an ontology Denslow so that I think the Madeleine it makes it look deceptively easy to write the robust archive canonical it's not actually that these is that we should do something like Pt command that it gets this thing right or maybe not maybe trust pt backrest and 1 and 1 that is all of these tools that you know flooding into our command and hopefully get this right on while writing this presentation it occurs to me that we should you know maybe at this since the documentation as well as the intuition that so the
part 2 of my presentation i'm going to talk about the new feature in 9 . 5 all its quota archive mode always so in previous releases you can set
Okamoto to on or off and now there is a 3rd option selected trees that lends often on all or always on which means that is also 1 evening a standby server so in previous releases if you said Okamoto on and you have a streaming replication running more just pointing time recover kinds of proprietary system is only the master that's generating the wall that dust archive but he sets are common to all and also the standby server or if you're doing continuum of recovery then also the recovery server will try to our criteria all everything that it has already restored and the and the use case for this is
if you have if you wanna have this set up where you have a master understand by and you want them both to have a separate wall in a monastery all that's the scenario rouge 1 asset Okamoto always make sense if you have no 13 different data centers or what not are definitely something you should be doing but it was not really possible intruders releases the set of archive command that way knowledge so
believe that for a moment locomote always lets you do this separately archive there's more things you can do more tricks you can do with that I'm going to get back to that later so
the path rate is how do you set up the continues archiving with shared archive to the previous
slide was about having separate archives for the math understand why this is probably more common I think of people only 1 1 up while archive with their role in this and what not and they only 1 arc of these files once you want to keep a separate archive we stand by so how do you do this how many of people actually trying to do this today comes up hence few here we go let's see if you got this right so so the
naive approach is to set are common to all 1 and then with the same oracle command on both of those servers and it's not quite good enough and then explain what so let's see what
happens when you promoters that wasn't the case so the media hits your master server hopefully you walk I was not on the center of the master just then you screwed up so you have your archive somewhere else and safe and you have a standby promote that to become the new master at this point is
that was a rule 1st of all it will replay all the while it had already stream from the master in it's going to create a new timeline how many of you know all the time I good hopefully under arrest than before will also all finalized up then what it's going to know the new timeline means that it's gonna bonds tho can number in the wall falls to generate so that they don't clash with the ones generated in the 1st server so you get kind of 2 sets of wall going for and what is going to do next it's going to copy their well last segment those writing through or recovery and renamed it on the new timeline and continue going from there and then this the must restart archiving because you had archive so
you have this this situation you have masses are any of these 3 there yeah the standby server and you have you have have stream replication set up and use and all of these 3 files the standby by and you also have the wall archive the master of cold archive common for all these 3 files and they're all in there so they're all in sync which
up know as time goes on the mass of all this more active in the master and will generate more while so you see a few extra false there and they also get stream to the standby and your was to have is 1 last segment that is in full yet it's currently writing to the file so when you you you insert something it goes to that last while that's still active and if you still replication standbys also keeping that file in saying that might be lacking behind but the point is that it's also right this partial for fully and posters 1 try to archive that file on the becomes for because you don't want to use a hall full in Europe and so it's not going to happen when you crash
and you promote user in 9 . 4 and below what will happen is that the standby 1st of all archive this partial file and it's going to have a name that looks just like any other file which is pretty confusing because you don't know it's a partial and I'm going to do a
little detour on this so you have enough foreign below you have this segment in the wall archive to but the 1 you this segment in the
wall archive of circles that looks just like any other while following the archive but
it's not actually complete yet is only half of it contains garbage on it's not a problem on its own but it it's very confusing human history looks at that hold you tell it's all a complete thought it was all FIL would be people should try Dalton that in note that you know what you can try to open notepad and see if you see the forest all he had all there is to other is the tool of excellent that's it so you can run that but from the looked on both with 0 you back up to look into file OK so is there any back up to the that the day of the week and OK so there's a plan feature that you that have happened felt that's good that's good that's actually a very nice feature it would be nice to run that feature only while for us to make sure that they're all please here yes that was organized in a well while waiting for that he told to appear but there is no way to really easily tell that it's it's a partial segment you could use PTA photons but again really reading that to determine those fooled not trivial I could probably do it but now yeah so there are some problems with this if the master actually continues running it's if if it was in the media that struck at but it's still running it might later try to archive the same file after it's actually complete but now you have a special file with same name archive and you're trying to override it with the complete 1 your command will fail and now you never get the complete segmented archive you might not care because you already fair failed over the standby maybe you just went from the way but then again you usually don't wanna lose like valid transactions if you ever we might wanna go back and look at all so the 2nd problem is that but what also happened is that the master actually had already archived at before you failover bought the standby was lagging behind so now what will happen is that the standby after the failover than the the new master will try to archive the partial statement it's saying name and that will fail and now it's going to keep failing and failing and your new master will never make any progress with its archiving it's going to get stuck non-administrative has to go and fix it and that Mr. there is going to be all fully comes continues because there is a sample of the same name yes the only yeah here I just want to give you the moon of yet and 1 way to avoid this is to actually use like a different directory for the mass the standby so that even though it's from the same archive but you can then have both copies of files of but again then command gets more complicated than it's it's really not ideal so this has been
changed in my . 5 all whenever the standby now archives this partial segment is not complete it will add the dual rename the file to both parts so in that 1 5 if you do a lower so you will start to see don't partial files in the archive don't be alarmed that's perfectly normal usually you don't need to care about this partial files posters doesn't care about them if you could be PCX look closer so just ignore them on you rename and remove a suffix also this is just for for you know emergency situations the partial segment was always like that was never really need for posters itself but it's good to have it in case of an emergency if you if that mystery that has to go and you know do something and then panic along so say that the master that the standby crashed immediately after the promotion and he didn't get around to archive the next segment yet the it's that the new segment on the new timeline then you might want to use this partial segment to recover up to the point just before the faIl were just there is no other copy of that while in that case in Dhaka but as soon as the standby no archives 1st segment on the new timeline you have it there as well so you won't need it but there are and really know where that's going to happen and I don't think anyone know does this happen unnatural but is there a few new on anyway this is much better than few situation where you couldn't tell that surpasses
so that is about a partial segments knowledge you back to what happens at the promotion so after the promotions complete that you will have this situation you have this master the master that crashed you will have a segment with that without the partial suffix that in the PCX looked erectile unit to look at that this defined is right to in standby be renamed to build partial and it also gets archived by the standby acetyl partial and now the standby continues running and generates this wall segments and on the new find 1 uses the little number 2 there that indicates the timeline and no everything's security except if you look carefully at the situation you'll see that
segments 18 and 19 were never archive and that's bad because you know how this gap in your wallet archive and there is no way to get past that point if you try to do point 5 recover anything note this you don't is very often in production but it can happen if for example the master haven't yet of those 18 and 19 us then they won't be there the liver get there are usually hopefully you're coComment keeps up so it doesn't happen but there's always going to be a small window can happen and especially if you're complement slow or something different can happen and you're pretty badly screwed up a point goes
that can happen in this would mean that you can't restore from back up to any point after the the promotion anymore you know although the while that your code archived here on the 2nd
time line is pretty much useless who knew that that 3 people that is for the shock you know I mean I kind of knew this always but when I really started to dig into this it's for the shocking that this happens because this is a pretty standard set up you have the oracle comments about the the same in Boston standby and you would expect this to work so what are we
going to do about it on of water
for while on this for the moment . 4 really cycle imagining an option called our common shares where we would kind of construct vector gaps and then try to archive and understand but also ran out of time for 9 . 5 and I realize that actually there are would always you can use that to work around this it's a little bit complicated but at least there is no it's not least possible to fix this on so if you remember we don't are would always stand but also Cartwright archive everything interested now you can take advantage of that by on having
the same Marco commanding must understand why but you have a set amount so that they're not trying to of compete with 1 archives default 1st but you can within the logic there to check if the follow really exists and then all comically more than place if it doesn't and it's identical then you know we can make it work you are make sure it's identical had actually compressed files yes you do on and it's actually it's not easy to write this because those race conditions you're suddenly going to have to service trying to do they exact same thing at the same time which you don't normally have a proper amount of but it's theoretically possible right that I have not actually tried to write that command by the genome that would be great but so now there is a way of maybe they should get back to having their real are como share it you know the system take care of this in some other fashion but no others work around so summary on the shared
archive problems on you can work around you can use Locke and would always make sure your archive command is concurrence if you do that and it had the stupidest which needs to check that the files are identical so for In nite with life if you start to see this book partial false or just ignore them until if you don't know what to do if you can just ignore them that's basically my advisor but it's good to have and still make sure the oracle command corseting more questions on this past yes on anaphors all I don't know I don't know yeah that's 1 solution don't use anything I don't actually another way to set this thing off it's it's not this performance but you can just set up like 2 directories in same same archive and just always archive everything flies it's waste space but indeed based network network-management flies but you know it might be acceptable and it's a lot easier to set up of yes there we we we we we we can we have the rest of you the yeah so the question was what is the point of having a shared archive in the 1st place to get the right answer yes alignment mission is to use it use to restore command and be able to restore the faster the standby from there the other 1 is just to have a back all yet the backup of thoughts and you you need a bass back up to go with it to have a full Our back up with a with a lot with different when you have a back up and the loss then you can do point-in-time recovery to any point on all kinds of nice things it's in a production system you should always use continues archiving really on the other side of the question is why do you want to use a shared archive and why not always use non-shared archive well of using a shared uses less space was so again 1 way to do this is to set are come always and then exit the fact that everything gets archived flies but then if you care about the share the disk space you might wanna have like a background process work on job or something that that emerges alignment fossils and that the more you use on the internet and training and the like that there there is that this will be so some systems might due duplication for you so you don't care if you still have the network traffic to or colocalize but again it might not the problem yes yes yes desert dust or it might restored it from the archive already which kind of strands consistent and try to free archiv again but of 11 well yes there is a case there is no guarantee that the master had already archived it might be streamed to the standby before it's archived in the past moment so the next part I'm just going to mention that
there is actually a tool called PTVs index along with it you can do the same thing with these is eventual that it's probably actually better are it's different from the archive commanding that you included in the software and you install what comes posters but have exploiting the archive server itself it's a piece of software that runs not on your own database server about and archive server so you just can't can't just use and and NFS monitoring thing like that you have to have a real server running but the way it works is that it connects to your server and if uses streaming replication to get all the data and then it'll start and the files and then result is exactly the same that we get it archive commanding that you get these files and it's going to strike and the directory of your code directory and then result is gonna look the same as with topic among all it's better than it has an of archive command in that it's actually up to date so are command only full segments so you're always gonna lose the last 8 megabytes of 16 megabytes of data or while whenever you crash because we always wait until the segment is full and then archive but of these here is a vector you have that problem is gonna up continuously stream over the changes which is better I MPEG reserve X log that in my .
2 6 9 . 3 it can full-time 1 changes which was pretty important for this very always an hour of all it means that if the master Crassus and PT excellent was connected through the it'll try to reconnect and modal hopefully connector new master and it's gonna this that OK there was a time when change is gonna stream everything over and up is just gonna work it's not point form a this reliable in that we added his political replications lots of the form I put forward was possible in producing the tools that are their masses are already believes the file before it has been streamed over which is used in lot of nothing to stop it from doing so our body not . 4 you can set up a replication slot slot to prevent that and then it's really gonna be real reliable replacement for for setting up the archive command file for always requires a little bit more monitoring I think it is now you have to have this process running all the time in recursive rows of make sure it's running in 1 or more than 1 of the all in different servers I guess OK yes we have a dozen of them right and see what happens if you really care about the world unlike 5 added a margin call synchronous which is so you know the asynchronous replication you can also or you can use synchronous replication knowledge is using excellent which means that it's not an added knowledge to the master that it has the date on the latest absent and the disk which means that you can set up a system so that of whenever you commit the commit will wait until it's safely archived by teaching is that of others I normally recommend using sometimes repetition all but that's a different topic you want you do make sure you have more than 1 and if you think you do you probably don't anyway so if you think again usually there are scenarios I admit that but usually when people say then is singular for players and they don't so what was the court may actually be filed in the form of a powerful that suggest you that carries over and I think that that's a good point there is so there was some president of this book partial files PC resemble lots of excellent has always used the don't partial suffix so whenever it's streaming of file and is not complete yet it to be used to build partial name for that sake so when you store for Morocco I created with these using that lot you will have is 1 . partial file at the end and you want to rename that so that it gets a story or you have your restore command checked for the parts of us some like that but yet no questions on this great use the series Iraq's excellent it's easier to set up problem so the final part
what people were guessing that I'll be talking to 1 today I am but not very much all so another new feature in 9 . 5 is this tool called
PDBbind it's really cool but it's useful failed back salute in previous versions so it's been a problem if you do the failover the 2 years that was there no you stand Soroka gets back up again maybe you upgraded the binaries or something half of you failed back that's a of reverse the roles of the service again and then all the answer was that you have to draw away the masses did directory and restore from this back up and that's pretty slow if you have a life can survive today it's not really feasible and in many cases so there traits c can use are assumed to speed that up our sink is nice but it still has to read all the data up or you have to trust the timestamps which you you feel students are so in practice to make that say if you have to read all the data and again if it's a attend right database you're not my not having all of changes there but if you can read through 10 the rights of data it's in the 1 so in 9 . 5 this talk on things you rewind which solves this
problem of Callisto this
1st of pt reminds tool that replaces are assumed for this particular scenario of doing that but it works pretty much like our sink all except that when are Inc tries to detect what files have changed and what parts of files have changed it calculates these rolling checksums and these rules of the form of all of the data of PT remind users that while for that because the right that log already contains the tree over a record for every modification we make 10 pages so we can just use that so the way it works is status the while on those servers that the 1 that you're that crashed and were failed over from all its gas the well there to do to to determine which blocks have been modified and then it's chemical pure where only the modified blocks and for anything else than their actual tables and indexes anything else than they actually the false it's just a copy everything that includes any configuration files any random readme follows a drop in your data directory anything see a lot more gets ideas of free space maps of all kinds of files that it doesn't handle which means that it just copies the moral with because that's always safe it's always seems to just copy over your data directory this obviously there's that is the way we used to use just taking a bit about but for the data files which hopefully that's where most of your data is most of the Volume comes from except when you have yes I expect that I some of those of you who said that he did some testing with the most exciting box and generated for gigabytes of moved exciting but only have like 10 megabytes of data is 0 yeah there you go so this still is not for you but so
let's begin is a little bit so the problem had failed back is basically that you have your old master and you might have some some commits there was some transactions there that they're not streamed over before the failover happened and those transactions or the problem they're trying to get when you do failed back all your are assumed command or PG everyone will erase those transactions and any modifications they made so that you can then get it in sync with the new master but there's a few points that are important to make here of some people tried to use changes replication with this problem it will not help you even if you synchronous applications there is always going to be song while that has not yet been stream to the standby but has already been fostered the master of options repairs and does not help that that it makes the window smaller but it's still there there's always going to be just another petition only affects comets there's always all the vacuum running there's always checkpoints running they all white well they will modify pages and even your transactions they do stuff before they commit and they're not synchronous you sucrose really correctly at every while recordings of the comments so again to transportation doesn't help here on yet any questions on this it's really nice tool I hope people in protest this and put into action to alpha release goes out hopefully next week your person anyway please try to see if it works for you and we still have the time to fix it before the release of it's been around
for a while now and people or something the use it and has come up with anything major but with the all
that is a good question that's kind of like yeah I think that's equivalent to the halting problem or something it's yeah so you can cooperate yeah you are still at their and PCA rewind which ran because you wanted to avoid renewal you data use then run pt dump on both servers and compare dealt with manually yes exactly so that would be great that's all there are at present tests will be 0 and they are you can run them there useful didn't this 3 much but it kind of tried to capture all the different called class and the usual the usual stuff all but I haven't tried running it on Cantor about databases myself on I hope some of from the yes there is a reason out there on the top I originally wrote this when we're still my country there's a reason and you pop that as far as I know still works for my country and . 4 but there Rosenborg changes in 9 . 5 that I made the wall format to make this more robust and more maintainable really in love if you're interested you can look at the residence on account for 9 . 3 9 . 4 again they should work but I'm not spending any anaphoric myself on that I think Michael Peck here is still maintaining those so if you recall the bottom will probably get this for my continent forceful of but all I'm not I don't really care about that in a moment of any more questions yes just given outcome of the process of writing in meeting we don't have any those people will project this is where you have these these all the during the best that have at all you have to do with the this is that some of the things that all currently right and that's what I want to say yes or no provided that you the complexity of the of the similarly if you want to be in 1 of the most common on what was was there are people these 2 and 1 you PG receiver actual exists of that works and it's produces the set up is a different philosophy than setting up a lot of match and body Texas and works so that perhaps we should start to de-emphasize the soccer common thing and be documented more as the thing if you're writing you know you can plug in into if you're writing symbolic pt backrest all although was even with even if you have a comment like Pt copy that does that correctly you're still gonna be missing like last few megabytes of well so it's not ideal even them all animal to animal solution should be something like PC copy would be nice but again if you do that then you kind of lose the flexibility so it would have to be more like example that you can copy paste from 1 year you have In on the meaning of the way in the the rise of the we all 1 of their that you know the position of the every user the all of which were you don't want to to check out the 1st part of the world yeah know I think the agree on that of those and not yet it's probably should do something like that you come that actually the point if you run if you run pt copy on the target and not on the server itself over as age then you can make it work even over instances of but even then it's kind of limited and if you want if you had to run something on the target server then you might as well run consumers these effects of parts of things all the institutions that's what he's worked out the text of the I want you to find out what the real world and of all what we're going do to focus on the ground then you should In the field of work and need to use and in the the in the form of these were the people who who work all of my answer really the gas this formula as well 1 you might you could run the opposite reserve slope somewhere else then and actually I wonder if you could write on server itself become of beer but you can also get all that all around us could you can't look at that so then you just of actual right at the uraniferous of some kind of fear that 1 of the arrows here but and I guess is running on the same server doesn't help with the display lower scenario we chose the eyes the levels and problems there is no we we so the question was what will be the right way to monitor PG reason log of all the so something like Stevens tool that continues to the polls to archive make sure that there are new follows and arriving there every now and then start with that and it would also be good to actually read through the files and make sure that you don't have any gaps missing segments and that the series is much so that's sort of the this is a on the question that's just get 1 you can do about related to this was ordered held to work on the sphere and actually useful that has held about yeah that makes sense to do end up are coming for us what I get all of the people you know what's going on in the rest of the body of the work we realize that the type of people when they were never so you can store a kind of directly write this story in itself is not about right yes again tries to build be that makes sense but it does not solve the missing segments problem so as to get the any more questions before I think you're all time thank you