Re: pg_upgrade project: high-level design proposal of in-place
От | Bruce Momjian |
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Тема | Re: pg_upgrade project: high-level design proposal of in-place |
Дата | |
Msg-id | 200506042333.j54NXeo21780@candle.pha.pa.us обсуждение исходный текст |
Ответ на | pg_upgrade project: high-level design proposal of in-place upgrade facility ("Serguei A. Mokhov" <mokhov@cs.concordia.ca>) |
Ответы |
Re: pg_upgrade project: high-level design proposal of
("Serguei A. Mokhov" <mokhov@cs.concordia.ca>)
|
Список | pgsql-hackers |
I have added a link to this thread on the TODO list under TODO.detail. --------------------------------------------------------------------------- Serguei A. Mokhov wrote: > Hello dear all, > > [Please CC your replies to me as I am on the digest mode] > > Here's finally a very high-level design proposal of the pg_upgrade feature > I was handwaiving a couple of weeks ago. Since, I am almost done with the > moving, I can allocate some time for this for 8.1/8.2. > > If this topic is of interest to you, please read on until the very end > before flaming or bashing the ideas out. I had designed that thing and > kept updating (the design) more or less regularly, and also reflected some > issues from the nearby threads [1] and [2]. > > This design is very high-level at the moment and is not very detailed. I > will need to figure out more stuff as I go and design some aspects in > finer detail. I started to poke around asking for initdb-forcing code > paths in [3], but got no response so far. But I guess if the general idea > or, rather, ideas accepted I will insist on more information more > aggressively :) if I can't figure something out for myself. > > [1] http://archives.postgresql.org/pgsql-hackers/2004-09/msg00000.php > [2] http://archives.postgresql.org/pgsql-hackers/2004-09/msg00382.php > [3] http://archives.postgresql.org/pgsql-hackers/2004-08/msg01594.php > > Comments are very welcome, especially _*CONSTRUCTIVE*_... > > Thank you, and now sit back and read... > > CONTENTS: > ========= > > 1. The Need > 1. Utilities and User's View of the pg_upgrade Feature > 2. Storage Management > - Storage Managers and the smgr API > 3. Source Code Maintenance Aspects > 2. The Upgrade Sequence > 4. Proposed Implementation Plan > - initdb() API > - upgrade API > > > 1. The Need > ----------- > > It's been a problem for PG for quite awhile now to have a less painful > upgrade procedure with every new revision of PostgreSQL, so the > dump/restore sequence is required. That can take a while for a production > DB, while keeping it offline. The new replication-related solutions, such > as Slony I, pg_pool, and others can remedy the problem somewhat, but > require to roughly double the storage requirements of a given database > while replicating from the older server to a newer one. > > The proposed implementation of an in-server pg_upgrade facility attempts > to address both issues at the same time -- a possibility to keep the > server running and upgrading lazily w/o doubling the storage requirements > (there will be some extra disk space taken, but far from doubling the > size). The in-process upgrade will not take much of down time and won't > require that much memory/disk/network resources as replication solutions > do. > > > Prerequisites > ------------- > > Ideally, the (maybe not so anymore) ambitious goal is to simply be able to > "drop in" the new binaries of the new server and kick off on the older > version of data files. I think is this feasible now a lot more than before > since we have those things available, which should ease up the > implementation: > > - bgwriter > - pg_autovacuum (the one to be integrated into the backend in 8.1) > - smgr API for pluggable storage managers > - initdb in C > - ... > > initdb in C, bgwriter and pg_autovacuum, and pluggable storage manager > have made the possibility of creation of the Upgrade Subsystem for > PostgreSQL to be something more reasonable, complete, feasible, and sane > to a point. > > > Utilities and the User's (DBA) View of the Feature > -------------------------------------------------- > > Two instances exist: > > pg_upgrade (in C) > > A standalone utility to upgrade the binary on-disk format from one > version to another when the database is offline. > We should always have this as an option. > pg_upgrade will accept sub/super set of pg_dump(all)/pg_restore > options that do not require a connection. I haven't > thought through this in detail yet. > > pg_autoupgrade > > a postgres subprocess, modeled after bgwriter and pg_autovacuum > daemons. This will work when the database system is running > on old data directory, and lazily converting relations to the new > format. > > pg_autoupgrade daemon can be triggered by the following events in addition > to the lazy upgrade process: > > "SQL" level: UPGRADE <ALL | relation_name [, relation_name]> [NOW | time] > > While the database won't be offline running over older database files, > SELECT/read-only queries would be allowed using older storage managers*. > Any write operation on old data will act using write-invalidate approach > that will force the upgrade the affected relations to the new format to be > scheduled after the relation-in-progress. > > (* See the "Storage Management" section.) > > Availability of the relations while upgrade is in progress is likely to be > the same as in VACUUM FULL for that relation, i.e. the entire relation is > locked until the upgrade is complete. Maybe we could optimize that by > locking only particular pages of relations, I have to figure that out. > > The upgrade of indices can be done using REINDEX, which seems far less > complicated than trying to convert its on-disk representation. This has > to be done after the relation is converted. Alternatively, the index > upgrade can simply be done by "CREATE INDEX" after the upgrade of > relations. > > The relations to be upgraded are ordered according to some priority, e.g. > system relations being first, then user-owned relations. System relations > upgrade is forced upon the postmaster startup, and then user relations are > processed lazily. > > So, in a sense, pg_autoupgrade will act like a proxy choosing appropriate > storage manager (like a driver) between the new server and the old data > file upgrading them on-demand. For that purpose we might need to add a > pg_upgradeproxy to intercept backend requests and use appropriate storage > manager. There will be one proxy process per backend. > > > Storage Management > ================== > > Somebody has made a possibility to plug a different storage manager in > postgres and we even had two of them at some point . for the magnetic disk > and the main memory. The main memory one is gone, but the smgr API is > still there. Some were dubious why we would ever need another third-party > storage manager, but here I propose to "plug in" storage managers from the > older Postgres versions itself! Here is where the pluggable storage > manager API would be handy once fully resurrected. Instead of trying to > plug some third party storage managers it will primarily be used by the > storage managers of different versions of Postgres. > > We can take the storage manager code from the past maintenance releases, > namely 6.5.3, 7.0.3, 7.1.3, 7.2.5, 7.3.7, 7.4.5, and 8.0, and arrange them > in appropriate fashion and have them implement the API properly. Anyone > can contribute a storage manager as they see fit, there's no need to get > them all at once. As a trial implementation I will try to do the last > three or four maybe. > > > Where to put relations being upgraded? > -------------------------------------- > > At the beginning of the upgrade process if pg detects the old version of > data files, it moves them under $PGDATA/<ver>, and keeps the old relations > there until upgraded. The relations to be upgraded will be kept in the > pg_upgrade_catalog. Once all relations upgraded, the <ver> directory is > removed and the auto and proxy processes are shut down. The contents of > the pg_upgrade_catalog emptied. The only issue remains is how to deal > with tablespaces (or LOCATION in 7.* releases) elsewhere .- this can > probably be addressed in the similar fashion, but having a > /my/tablespace/<ver> directory. > > Source Code Maintenance > ======================= > > Now, after the above some of you may get scared on the amount of similar > code to possibly maintain in all those storage managers, but in reality > they would require as much maintenance as the corresponding releases do > get back-patched in that code area, and some are not being maintained for > quite some time already. Plus, I should be around to maintain it, should > this become realized. > > Release-time Maintenance > ------------------------ > > For maintenance of pg_upgrade itself, one will have to fork out a new > storage manager from the previous stable release and "register" it within > the system. Alternatively, the new storage manager can be forked when the > new release cycle begins. Additionally, a pg_upgrade version has to be > added implementing the API steps outlined in the pg_upgrade API section. > > > Implementation Steps > ==================== > > To materialize the above idea, I'd proceed as follows: > > *) Provide the initdb() API (quick) > > *) Resurrect the pluggable storage manager API to be usable for the > purpose. > > *) Document it > > *) Implement pg_upgrade API for 8.0 and 7.4.5. > > *) Extract 8.0 and 7.4.5 storage managers and have them implement the API > as a proof of concept. Massage the API as needed. > > *) Document the process of adding new storage managers and pg_upgrade > drivers. > > *) Extract other versions storage managers. > > > pg_upgrade sequence > ------------------- > > pg_upgrade API for the steps below to update for the next release. > > What to do with WAL?? Maybe upgrade can simply be done using WAL replay > with old WAL manager? Not, fully, because not everything is in WAL, but > some WAL recovery maybe needed in case the server was not shutdown cleanly > before the upgrade. > > pg_upgrade will proceed as follows: > > - move PGDATA to PGDATA/<major pg version> > - move tablespaces likewise > - optional recovery from WAL in case old server was not shutdown properly > -? Shall I upgrade PITR logs of 8.x??? So one can recover to a > point-in-time in the upgraded database? > - CLUSTER all old data > - ANALYZE all old data > - initdb() new system catalogs > - Merge in modifications from old system catalogs > - upgrade schemas/users > -- variations > - upgrade user relations > > Upgrade API: > ------------ > > First draft, to be refined multiple times, but to convey the ideas behind: > > moveData() > movePGData() > moveTablespaces() 8.0+ > moveDbLocation() < 8.0 > > preliminaryRecovery() > - WAL?? > - PITR 8.0+?? > > preliminaryCleanup() > CLUSTER -- recover some dead space > ANALYZE -- gives us stats > > upgradeSystemInfo() > initdb() > mergeOldCatalogs() > mergeOldTemplates() > > upgradeUsers() > > upgradeSchemas() > - > 7.2, else NULL > > upgradeUserRelations() > upgradeIndices() > DROP/CREATE > > upgradeInit() > { > > } > > The main body in pseudocode: > > upgradeLoop() > { > moveData(); > preliminaryRecovery(); > preliminaryCleanup(); > upgradeSystemInfo(); > upgradeUsers(); > upgradeSchemas(); > upgradeUserRelations(); > } > > Something along these lines the API would be: > > typedef struct t_upgrade > { > bool (*moveData) (void); > bool (*preliminaryRecovery) (void); /* may be NULL */ > bool (*preliminaryCleanup) (void); /* may be NULL */ > bool (*upgradeSystemInfo) (void); /* may be NULL */ > bool (*upgradeUsers) (void); /* may be NULL */ > bool (*upgradeSchemas) (void); /* may be NULL */ > bool (*upgradeUserRelations) (void); /* may be NULL */ > } t_upgrade; > > > The above sequence is executed by either pg_upgrade utility uninterrupted > or by the pg_autoupgrade daemon. In the former the upgrade priority is > simply by OID, in the latter also, but can be overridden by the user using > the UPGRADE command to schedule relations upgrade, write operation can > also change such schedule, with user's selected choice to be first. The > more write requests a relation receives while in the upgrade queue, its > priority increases; thus, the relation with most hits is on top. In case > of tie, OID is the decision mark. > > Some issues to look into: > > - catalog merger > - a crash in the middle of upgrade > - PITR logs for 8.x+ > - ... > > Flames and Handwaiving > ---------------------- > > Okay, flame is on, but before you flame, mind you, this is a very initial > version of the design. Some of the ideas may seem far fetched, the > contents may seem messy, but I believe it's now more doable than ever and > I am willing to put effort in it for the next release or two and then > maintain it afterwards. It's not going to be done in one shot maybe, but > incrementally, using input, feedback, and hints from you, guys. > > Thank you for reading till this far :-) I.d like to hear from you if any > of this made sense to you. > > Truly yours, > > -- > Serguei A. Mokhov | /~\ The ASCII > Computer Science Department | \ / Ribbon Campaign > Concordia University | X Against HTML > Montreal, Quebec, Canada | / \ Email! > > ---------------------------(end of broadcast)--------------------------- > TIP 5: Have you checked our extensive FAQ? > > http://www.postgresql.org/docs/faqs/FAQ.html > -- Bruce Momjian | http://candle.pha.pa.us pgman@candle.pha.pa.us | (610) 359-1001+ If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania19073
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