{"id":30356,"date":"2019-10-31T21:35:06","date_gmt":"2019-10-31T18:35:06","guid":{"rendered":"https:\/\/prohoster.info\/blog\/kak-my-ispolzovali-otlozhennuyu-replikatsiyu-dlya-avarijnogo-vosstanovleniya-s-postgresql\/"},"modified":"2019-10-31T21:35:06","modified_gmt":"2019-10-31T18:35:06","slug":"kak-my-ispolzovali-otlozhennuyu-replikatsiyu-dlya-avarijnogo-vosstanovleniya-s-postgresql","status":"publish","type":"post","link":"https:\/\/prohoster.info\/hr\/blog\/administrirovanie\/kak-my-ispolzovali-otlozhennuyu-replikatsiyu-dlya-avarijnogo-vosstanovleniya-s-postgresql","title":{"rendered":"Kako smo koristili odlo\u017eenu replicaciju za oporavak od katastrofe s PostgreSQL","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"<p><img decoding=\"async\" alt=\"Kako smo koristili odlo\u017eenu replicaciju za oporavak od katastrofe s PostgreSQL\" src=\"\/wp-content\/uploads\/2019\/03\/773f3b6c8d173be91c49066d7067bad3.png\" style=\"display:block;margin: 0 auto;\" \/><br \/>\nReplikacija nije backup. Ili mo\u017eda jeste? Evo kako smo koristili odlo\u017eenu replikaciju za oporavak nakon \u0161to smo slu\u010dajno obrisali pre\u010dace.<\/p>\n<p><\/p>\n<p><noindex><a rel=\"nofollow\" href=\"https:\/\/about.gitlab.com\/handbook\/engineering\/infrastructure\/\">Stru\u010dnjaci za infrastrukturu<\/a><\/noindex> na GitLabu odgovaraju za rad <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/\">GitLab.com<\/a><\/noindex> \u2013 najve\u0107eg GitLab instance ikada. Ovdje se nalazi 3 miliona korisnika i gotovo 7 miliona projekata, i to je jedan od najve\u0107ih open-source SaaS sa specijaliziranom arhitekturom. Bez sustava baza podataka PostgreSQL, infrastruktura GitLab.com ne bi daleko dogurala, a sve \u0161to radimo za otpornost na kvarove kada mo\u017eemo izgubiti podatke. Vjerojatno se takva katastrofa ne\u0107e dogoditi, ali smo se dobro pripremili i opskrbili raznim mehanizmima backup-a i replikacije.<\/p>\n<p><\/p>\n<p>Replikacija nije na\u010din za backup baza podataka (<noindex><a rel=\"nofollow\" href=\"https:\/\/about.gitlab.com\/2019\/02\/13\/delayed-replication-for-disaster-recovery-with-postgresql\/#summing-up\">vidi dolje<\/a><\/noindex>). No, sada \u0107emo vidjeti kako brzo obnoviti slu\u010dajno obrisane podatke pomo\u0107u odlo\u017eene replikacije: na <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/\">GitLab.com<\/a><\/noindex> korisnik <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/gitlab-com\/gl-infra\/production\/issues\/509\">izbrisao pre\u010dac<\/a><\/noindex> za projekt <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/gitlab-org\/gitlab-ce\/\"><code>gitlab-ce<\/code><\/a><\/noindex> i izgubio vezu s merge requestovima i zadacima.<\/p>\n<p><\/p>\n<p>Sa odlo\u017eenom replikom obnovili smo podatke u svega 1,5 sata. Pogledajte kako je to bilo.<\/p>\n<p><noindex><a rel=\"nofollow\" name=\"habracut\"><\/a><\/noindex><\/p>\n<h3 id=\"vosstanovlenie-na-moment-vremeni-s-postgresql\">Obnova na odre\u0111eno vrijeme s PostgreSQL<\/h3>\n<p><\/p>\n<p>PostgreSQL ima ugra\u0111enu funkciju koja obnavlja stanje baze podataka na odre\u0111eni trenutak. Zove se <noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/current\/continuous-archiving.html\">Point-in-Time Recovery<\/a><\/noindex> (PITR) i koristi iste mehanizme koji podr\u017eavaju a\u017eurnost replike: zapo\u010dinjemo s pouzdanim snimkom cijelog klastera baza podataka (osnovni backup), a zatim primjenjujemo niz promjena stanja do odre\u0111enog trenutka.<\/p>\n<p><\/p>\n<p>Kako bismo koristili ovu funkciju za hladni backup, redovito radimo osnovni backup baze podataka i pohranjujemo ga u arhivu (arhive GitLab-a \u017eive u <noindex><a rel=\"nofollow\" href=\"https:\/\/cloud.google.com\/storage\/\">Google Cloud Storage<\/a><\/noindex>). Osim toga, pratimo promjene stanja baze podataka, arhiviraju\u0107i zapisni dnevnik (<noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/current\/wal-intro.html\">write-ahead log<\/a><\/noindex>, WAL). I sa svime ovim mo\u017eemo izvesti PITR za hitnu obnovu: po\u010dinjemo od snimka napravljenog prije gre\u0161ke i primjenjujemo promjene iz WAL arhive do kvara.<\/p>\n<p><\/p>\n<h3 id=\"chto-takoe-otlozhennaya-replikaciya\">\u0160to je odlo\u017eena replikacija?<\/h3>\n<p><\/p>\n<p>Odlo\u017eena replikacija je primjena promjena iz WAL-a s odgodom. Drugim rije\u010dima, transakcija se dogodila u satu <code>X<\/code>, ali u replici \u0107e se pojaviti s odgodom <code>d<\/code> za sat <code>X + d<\/code>.<\/p>\n<p><\/p>\n<p>U PostgreSQL-u postoje 2 na\u010dina za postavljanje fizi\u010dke replike baze podataka: obnova iz arhive i streaming replikacija. <noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/current\/archive-recovery-settings.html\">Obnova iz arhive<\/a><\/noindex>, u biti, radi kao PITR, ali neprekidno: stalno izvla\u010dimo promjene iz WAL arhive i primjenjujemo ih na repliku. A <noindex><a rel=\"nofollow\" href=\"https:\/\/wiki.postgresql.org\/wiki\/Streaming_Replication\">streaming replikacija<\/a><\/noindex> direktno izvla\u010di WAL tok s vi\u0161eg hosta baze podataka. Preferiramo obnavljanje iz arhive - njima je lak\u0161e upravljati i imaju normalne performanse, koje ne zaostaju za radnim klastra.<\/p>\n<p><\/p>\n<h3 id=\"kak-nastroit-otlozhennoe-vosstanovlenie-iz-arhiva\">Kako postaviti odgo\u0111eno obnavljanje iz arhive<\/h3>\n<p><\/p>\n<p><noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/current\/recovery-config.html\">Opcije obnove<\/a><\/noindex> opisane su u datoteci <code>recovery.conf<\/code>. Primjer:<\/p>\n<p><\/p>\n<pre><code class=\"plaintext\">standby_mode = 'on'\nrestore_command = ' \/usr\/bin\/envdir \/etc\/wal-e.d\/env \/opt\/wal-e\/bin\/wal-e wal-fetch -p 4 \"%f\" \"%p\"'\nrecovery_min_apply_delay = '8h'\nrecovery_target_timeline = 'latest'<\/code><\/pre>\n<p><\/p>\n<p>S ovim parametrima postavili smo odgo\u0111enu repliku s obnavljanjem iz arhive. Ovdje se koristi <noindex><a rel=\"nofollow\" href=\"https:\/\/github.com\/wal-e\/wal-e\">wal-e<\/a><\/noindex> za va\u0111enje WAL segmenata (<code>restore_command<\/code>) iz arhive, a promjene \u0107e se primjenjivati nakon osam sati (<code>recovery_min_apply_delay<\/code>). Replika \u0107e pratiti promjene vremenskog okvira u arhivi, na primjer, zbog obrade otkaza u klasteru (<code>recovery_target_timeline<\/code>).<\/p>\n<p><\/p>\n<p>S <code>recovery_min_apply_delay<\/code> mo\u017ee se postaviti streaming replikacija sa zaka\u0161njenjem, ali ovdje postoje neki trikovi koji se odnose na replika\u017ene slotove, povratne informacije s vru\u0107ih rezervi itd. WAL arhiva omogu\u0107uje izbjegavanje tih problema.<\/p>\n<p><\/p>\n<p>Parametar <code>recovery_min_apply_delay<\/code> pojavio se samo u PostgreSQL 9.3. U prethodnim verzijama za odgo\u0111enu replikaciju trebate postaviti kombinaciju <noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/9.3\/functions-admin.html\">funkcija upravljanja oporavkom<\/a><\/noindex> (<code>pg_xlog_replay_pause(), pg_xlog_replay_resume()<\/code>) ili zadr\u017eati WAL segmente u arhivi tijekom zadr\u0161ke.<\/p>\n<p><\/p>\n<h3 id=\"kak-postgresql-eto-delaet\">Kako to PostgreSQL radi?<\/h3>\n<p><\/p>\n<p>Zanimljivo je vidjeti kako PostgreSQL implementira odgo\u0111eno obnavljanje. Pogledajmo na <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/postgres\/postgres\/blob\/c24dcd0cfd949bdf245814c4c2b3df828ee7db36\/src\/backend\/access\/transam\/xlog.c#L6124\"><code>recoveryApplyDelay(XlogReaderState)<\/code><\/a><\/noindex>. Poziva se iz <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/postgres\/postgres\/blob\/c24dcd0cfd949bdf245814c4c2b3df828ee7db36\/src\/backend\/access\/transam\/xlog.c#L7196\">glavnog kruga ponavljanja<\/a><\/noindex> za svaki zapis iz WAL.<\/p>\n<p><\/p>\n<pre><code class=\"plaintext\">static bool\nrecoveryApplyDelay(XLogReaderState *record)\n{\n    uint8       xact_info;\n    TimestampTz xtime;\n    long        secs;\n    int         microsecs;\n\n    \/* nothing to do if no delay configured *\/\n    if (recovery_min_apply_delay &lt;= 0)\n        return false;\n\n    \/* no delay is applied on a database not yet consistent *\/\n    if (!reachedConsistency)\n        return false;\n\n    \/*\n     * Is it a COMMIT record?\n     *\n     * We deliberately choose not to delay aborts since they have no effect on\n     * MVCC. We already allow replay of records that don&#039;t have a timestamp,\n     * so there is already opportunity for issues caused by early conflicts on\n     * standbys.\n     *\/\n    if (XLogRecGetRmid(record) != RM_XACT_ID)\n        return false;\n\n    xact_info = XLogRecGetInfo(record) &amp; XLOG_XACT_OPMASK;\n\n    if (xact_info != XLOG_XACT_COMMIT &amp;&amp;\n        xact_info != XLOG_XACT_COMMIT_PREPARED)\n        return false;\n\n    if (!getRecordTimestamp(record, &amp;xtime))\n        return false;\n\n    recoveryDelayUntilTime =\n        TimestampTzPlusMilliseconds(xtime, recovery_min_apply_delay);\n\n    \/*\n     * Exit without arming the latch if it&#039;s already past time to apply this\n     * record\n     *\/\n    TimestampDifference(GetCurrentTimestamp(), recoveryDelayUntilTime,\n                        &amp;secs, &amp;microsecs);\n    if (secs &lt;= 0 &amp;&amp; microsecs &lt;= 0)\n        return false;\n\n    while (true)\n    {\n        \/\/ Shortened:\n        \/\/ Use WaitLatch until we reached recoveryDelayUntilTime\n        \/\/ and then\n        break;\n    }\n    return true;\n}<\/code><\/pre>\n<p><\/p>\n<p>Bitno je da je odgoda temeljena na fizi\u010dkom vremenu zabilje\u017eenom u vremenskoj oznaci komita transakcije (<code>xtime<\/code>). Kao \u0161to je vidljivo, odgoda se primjenjuje samo na komite i ne doti\u010de druge zapise \u2014 sve promjene se primjenjuju izravno, a komit se odga\u0111a, tako da \u0107emo vidjeti promjene tek nakon postavljene odgode.<\/p>\n<p><\/p>\n<h3 id=\"kak-ispolzovat-otlozhennuyu-repliku-dlya-vosstanovleniya-dannyh\">Kako koristiti odgo\u0111enu replicu za oporavak podataka<\/h3>\n<p><\/p>\n<p>Pretpostavimo da imamo klaster baze podataka u produkciji i replicu s osmosatnom odgodom. Pogledajmo kako oporaviti podatke na primjeru <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/gitlab-com\/gl-infra\/production\/issues\/509\">nasumi\u010dnog brisanja oznaka<\/a><\/noindex>.<\/p>\n<p><\/p>\n<p>Kada smo saznali za problem, <noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/9.3\/functions-admin.html\">zaustavili smo obnovu iz arhiva<\/a><\/noindex> za odgo\u0111enu replicu:<\/p>\n<p><\/p>\n<pre><code class=\"plaintext\">SELECT pg_xlog_replay_pause();<\/code><\/pre>\n<p><\/p>\n<p>S pauzom nismo imali rizika da \u0107e replika ponoviti zahtjev <code>DELETE<\/code>. Korisna stvar, ako treba vremena da se sve razjasni.<\/p>\n<p><\/p>\n<p>Bitno je da odgo\u0111ena replika do\u0111e do trenutka prije zahtjeva. <code>DELETE<\/code>Otprilike smo znali fizi\u010dko vrijeme brisanja. Izbrisali smo <code>recovery_min_apply_delay<\/code> i dodali <code>recovery_target_time<\/code> u <code>recovery.conf<\/code>. Tako replika dolazi do potrebnog trenutka bez odgoda:<\/p>\n<p><\/p>\n<pre><code class=\"plaintext\">recovery_target_time = '2018-10-12 09:25:00+00'<\/code><\/pre>\n<p><\/p>\n<p>S vremenskim oznakama bolje je smanjiti suvi\u0161no kako bismo izbjegli proma\u0161aj. Istina, \u0161to vi\u0161e smanjujemo, to vi\u0161e podataka gubimo. Opet, ako prokockamo zahtjev, <code>DELETE<\/code>sve \u0107e ponovno biti obrisano i morat \u0107emo po\u010deti iznova (ili uop\u0107e uzeti hladnu sigurnosnu kopiju za PITR).<\/p>\n<p><\/p>\n<p>Ponovo smo pokrenuli odlo\u017eeni Postgres instancu, a WAL segmenti su se ponavljali do navedenog vremena. Napredak u ovoj fazi mo\u017ee se pratiti upitom:<\/p>\n<p><\/p>\n<pre><code class=\"plaintext\">SELECT\n  -- trenutna lokacija u WAL\n  pg_last_xlog_replay_location(),\n  -- trenutni vremenski pe\u010dat transakcije (stanje replike)\n  pg_last_xact_replay_timestamp(),\n  -- trenutno fizi\u010dko vrijeme\n  now(),\n  -- koli\u010dina vremena koja jo\u0161 treba biti primenjena do postizanja recovery_target_time\n  '2018-10-12 09:25:00+00'::timestamptz - pg_last_xact_replay_timestamp() as delay;<\/code><\/pre>\n<p><\/p>\n<p>Ako se vremenska oznaka vi\u0161e ne menja, obnova je zavr\u0161ena. Mo\u017eete postaviti akciju <noindex><a rel=\"nofollow\" href=\"https:\/\/www.postgresql.org\/docs\/current\/recovery-target-settings.html\"><code>recovery_target_action<\/code><\/a><\/noindex>, da se zatvori, unapredi ili pauzira instanca nakon ponavljanja (po defaultu se pauzira).<\/p>\n<p><\/p>\n<p>Baza podataka se vratila u stanje pre tog nesretnog upita. Sada je mogu\u0107e, na primer, izvesti podatke. Izvezli smo uklonjene podatke o oznakama i sve veze s zadacima i merge zahtevima i preneli ih u radnu bazu podataka. Ako su gubici veliki, mo\u017eemo jednostavno unaprediti replikaciju i koristiti je kao glavnu. Ali tada \u0107e svi podaci nakon trenutka do kojeg smo se vratili biti izgubljeni.<\/p>\n<p><\/p>\n<p>Umesto vremenskih oznaka, bolje je koristiti ID transakcija. Korisno je zabele\u017eiti ove ID, na primer, za DDL operatore (tipa <code>DROP TABLE<\/code>), pomo\u0107u <code>log_statements = 'ddl'<\/code>. Da smo imali ID transakcije, uzeli bismo <code>recovery_target_xid<\/code> i izvr\u0161ili sve do transakcije pre upita <code>DELETE<\/code>.<\/p>\n<p><\/p>\n<p>Povratak na rad je vrlo jednostavan: uklonite sve promene iz <code>recovery.conf<\/code> i ponovo pokrenite Postgres. Ubrzo \u0107e se na replici ponovo pojaviti osmo\u010dasovno ka\u0161njenje, i spremni smo za budu\u0107e nevolje.<\/p>\n<p><\/p>\n<h3 id=\"preimuschestva-dlya-vosstanovleniya\">Prednosti za oporavak<\/h3>\n<p><\/p>\n<p>Sa odlo\u017eenom replikom umesto hladnog backupa ne treba nam sate za obnavljanje celog snimka iz arhive. Na primer, treba nam pet sati da izvu\u010demo celi osnovni backup od 2 TB. A zatim \u0107emo jo\u0161 morati primeniti celu dnevnu WAL, da bismo se oporavili do potrebnog stanja (u najgorem slu\u010daju).<\/p>\n<p><\/p>\n<p>Odlo\u017eena replika je bolja od hladnog backupa iz dva razloga:<\/p>\n<p><\/p>\n<ol>\n<li>Nije potrebno izvla\u010diti ceo osnovni backup iz arhive.<\/li>\n<li>Postoji fiksno osmo\u010dasovno okno WAL segmenata koje treba ponoviti.<\/li>\n<\/ol>\n<p><\/p>\n<p>Osim toga, stalno proveravamo mo\u017ee li se PITR obaviti iz WAL, i brzo bismo primetili o\u0161te\u0107enja ili druge probleme s arhivom WAL, prate\u0107i ka\u0161njenje odlo\u017eene replike.<\/p>\n<p><\/p>\n<p>U ovom primeru je trebalo 50 minuta za oporavak, \u0161to zna\u010di da je brzina bila 110 GB podataka WAL po satu (arhiv je tada i dalje bio na <noindex><a rel=\"nofollow\" href=\"https:\/\/aws.amazon.com\/s3\/\">AWS S3<\/a><\/noindex>). Sveukupno smo rije\u0161ili problem i povratili podatke u 1,5 sati.<\/p>\n<p><\/p>\n<h3 id=\"itogi-gde-prigoditsya-otlozhennaya-replika-a-gde-net\">Zaklju\u010dci: gdje \u0107e se koristiti odgo\u0111ena replikacija (a gdje ne)<\/h3>\n<p><\/p>\n<p>Koristite odgo\u0111enu replikaciju kao sredstvo prve pomo\u0107i ako slu\u010dajno izgubite podatke i primijetite ovu nesre\u0107u unutar postavljene ka\u0161njenja.<\/p>\n<p><\/p>\n<blockquote><p>Ali imajte na umu: replikacija nije backup.<\/p><\/blockquote>\n<p>Backup i replikacija imaju razli\u010dite ciljeve. Hladni backup \u0107e biti koristan ako ste slu\u010dajno napravili <code>DELETE<\/code> ili <code>DROP TABLE<\/code>. Radimo backup iz hladnog skladi\u0161ta i obnavljamo prethodno stanje tablice ili cijele baze podataka. Me\u0111utim, upit <code>DROP TABLE<\/code> se gotovo trenutno reproducira na svim replikama u radnom klasteru, tako da \u0107e uobi\u010dajena replikacija ovdje pomo\u0107i. Sama po sebi replikacija odr\u017eava bazu podataka dostupnom kada pojedini serveri otkazuju i raspore\u0111uje optere\u0107enje.<\/p>\n<p><\/p>\n<p>\u010cak i s odgo\u0111enom replikom, ponekad jako trebamo hladni backup na sigurnom mjestu ako se dogodi kvar datacentra, skriveno o\u0161te\u0107enje ili drugi doga\u0111aji koje odmah ne primijetite. Ovdje od same replikacije nema koristi.<\/p>\n<p><\/p>\n<p><strong>Napomena<\/strong>. Na <noindex><a rel=\"nofollow\" href=\"https:\/\/gitlab.com\/\">GitLab.com<\/a><\/noindex> sada \u0161titimo od gubitka podataka samo na razini sustava i ne obnavljamo podatke na razini korisnika.<\/p>\n<p>Izvor: <a content=\"nofollow\" rel=\"nofollow\" href=\"https:\/\/habr.com\/ru\/company\/southbridge\/blog\/445446\/\">habr.com<\/a><\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"<p>\u0420\u0435\u043f\u043b\u0438\u043a\u0430\u0446\u0438\u044f \u2014 \u043d\u0435 \u0431\u044d\u043a\u0430\u043f. \u0418\u043b\u0438 \u043d\u0435\u0442? \u0412\u043e\u0442 \u043a\u0430\u043a \u043c\u044b \u0438\u0441\u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u043b\u0438 \u043e\u0442\u043b\u043e\u0436\u0435\u043d\u043d\u0443\u044e \u0440\u0435\u043f\u043b\u0438\u043a\u0430\u0446\u0438\u044e \u0434\u043b\u044f \u0432\u043e\u0441\u0441\u0442\u0430\u043d\u043e\u0432\u043b\u0435\u043d\u0438\u044f, \u0441\u043b\u0443\u0447\u0430\u0439\u043d\u043e \u0443\u0434\u0430\u043b\u0438\u0432 \u044f\u0440\u043b\u044b\u043a\u0438. \u0421\u043f\u0435\u0446\u0438\u0430\u043b\u0438\u0441\u0442\u044b \u043f\u043e \u0438\u043d\u0444\u0440\u0430\u0441\u0442\u0440\u0443\u043a\u0442\u0443\u0440\u0435 \u043d\u0430 GitLab \u043e\u0442\u0432\u0435\u0447\u0430\u044e\u0442 \u0437\u0430 \u0440\u0430\u0431\u043e\u0442\u0443 GitLab.com \u2014 \u0441\u0430\u043c\u043e\u0433\u043e \u0431\u043e\u043b\u044c\u0448\u043e\u0433\u043e \u044d\u043a\u0437\u0435\u043c\u043f\u043b\u044f\u0440\u0430 GitLab \u0432 \u043f\u0440\u0438\u0440\u043e\u0434\u0435. \u0417\u0434\u0435\u0441\u044c 3 \u043c\u0438\u043b\u043b\u0438\u043e\u043d\u0430 \u043f\u043e\u043b\u044c\u0437\u043e\u0432\u0430\u0442\u0435\u043b\u0435\u0439 \u0438 \u043f\u043e\u0447\u0442\u0438 7 \u043c\u0438\u043b\u043b\u0438\u043e\u043d\u043e\u0432 \u043f\u0440\u043e\u0435\u043a\u0442\u043e\u0432, \u0438 \u044d\u0442\u043e \u043e\u0434\u0438\u043d \u0438\u0437 \u0441\u0430\u043c\u044b\u0445 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