{"id":97454,"date":"2020-10-18T14:42:35","date_gmt":"2020-10-18T12:42:35","guid":{"rendered":"https:\/\/prohoster.info\/blog\/administrirovanie\/barery-i-zhurnaliruemye-fajlovye-sistemy"},"modified":"2020-10-18T14:42:35","modified_gmt":"2020-10-18T12:42:35","slug":"barery-i-zhurnaliruemye-fajlovye-sistemy","status":"publish","type":"post","link":"https:\/\/prohoster.info\/hr\/blog\/administrirovanie\/barery-i-zhurnaliruemye-fajlovye-sistemy","title":{"rendered":"Barijere i journalirane datote\u010dne sustave","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"
\n

Svim \u017eelim odli\u010dan vikend!<\/em><\/strong> Pozivamo vas na besplatni Demo-\u010das <\/em>\u201eKonfiguracija web poslu\u017eitelja (Apache, Nginx, uravnote\u017eenje Nginx)\u201c<\/em><\/a><\/noindex>, koji \u0107e voditi Andrey Buranov \u2014 stru\u010dnjak za UNIX sustave u Mail.Ru Group. Tako\u0111er objavljujemo \u010dlanak Jonathana Corbeta \u2014 izvr\u0161nog urednika na LWN.net.<\/em><\/p>\n<\/blockquote>\n

Jurnalizirane datote\u010dne sustave obe\u0107avaju osloboditi sistemske administratore problema s o\u0161te\u0107enjem diska prilikom kvara sustava. \u010cak i bez pokretanja provjere cjelovitosti datote\u010dnog sustava. Iako, u stvarnosti, sve je to malo slo\u017eenije. Kako pokazuje nedavna rasprava, mo\u017ee biti i zamr\u0161enije nego \u0161to mnogi od nas misle, jer osiguranje cjelovitosti jurnaliziranih datote\u010dnih sustava utje\u010de na performanse.<\/p>\n

Datote\u010dni sustav, poput ext3, koristi odvojeno podru\u010dje na disku, nazvano dnevnik. Kada se izvr\u0161e promjene u metapodacima datote\u010dnog sustava, te promjene se prvo zapisuju u dnevnik bez modificiranja ostatka datote\u010dnog sustava. Nakon \u0161to su sve promjene zabilje\u017eene u dnevniku, dodaje se «commit blok», ozna\u010davaju\u0107i zavr\u0161etak transakcije. Tek nakon zapisivanja commit bloka, transakcija se fiksira, a izmijenjeni metapodaci se zapisuju na disk. Ako sustav u nekom trenutku otka\u017ee, informacije u dnevniku mogu se koristiti za sigurno zavr\u0161avanje rada i izbjegavanje o\u0161te\u0107enja datote\u010dnog sustava zbog toga \u0161to su a\u017eurirani samo neki metapodaci.<\/p>\n

Me\u0111utim, postoji jedan zamka: kod datote\u010dnog sustava prije zapisivanja commit bloka mora biti apsolutno siguran da su sve informacije o transakciji ve\u0107 dospjele u dnevnik. Samo zapisivanje operacija u ispravnom redoslijedu nije dovoljno \u2014 moderni diskovi podr\u017eavaju velike unutarnje predmemorije i preure\u0111uju operacije za pobolj\u0161anje performansi. Stoga, prije commit bloka, potrebno je izri\u010dito nazna\u010diti premje\u0161tanje svih podataka iz dnevnika na disk. Ako se commit blok zabilje\u017ei ranije, dnevnik mo\u017ee biti o\u0161te\u0107en. Za rje\u0161avanje ovog problema koriste se barijere. U su\u0161tini, barijera onemogu\u0107ava zapisivanje bilo kojih blokova nakon barijere, dok svi blokovi zapisani prije barijere ne budu premje\u0161teni na disk. Kori\u0161tenjem barijera, datote\u010dni sustavi jam\u010de dosljednost struktura datoteka.<\/p>\n

No, postoji jo\u0161 jedan problem: datote\u010dni sustavi ext3 i ext4 prema zadanim postavkama ne koriste barijere. Opcija postoji, ali ako administrator nije izri\u010dito uklju\u010dio, ti datote\u010dni sustavi rade bez barijera, iako u nekim distribucijama (poput SUSE) zadane vrijednosti su druga\u010dije. Erik Sandeen nedavno je odlu\u010dio da je potrebno promijeniti ovu situaciju i napravio je zakrpu<\/u><\/a><\/noindex>, koja modificira zadane postavke za ext3 i ext4. Tada je zapo\u010dela \u017eestoka rasprava.<\/p>\n

Andrew Morton vrlo detaljno odgovorio<\/u><\/a><\/noindex>, obja\u0161njava\u0107i za\u0161to je zadana vrijednost takva:<\/p>\n

\n

Posljednji put kada smo poku\u0161ali to promijeniti, performanse su se na mnogim optere\u0107enjima pogor\u0161ale za 30%, pa sam u strahu odbacio sve te zakrpe. Mislim da ne mo\u017eemo i\u0107i na to i zna\u010dajno usporiti rad svih strojeva...<\/em><\/p>\n

Nemamo idealna rje\u0161enja i sklon sam tome da ne budim tu uspavanu psa i ostavim zadane postavke na uvid razvojnim timovima distribucija.<\/em><\/p>\n<\/blockquote>\n

Dakle, prema zadanim postavkama, barijere su isklju\u010dene, jer ozbiljno utje\u010du na performanse. Osim toga, datote\u010dni sustavi se prili\u010dno uspje\u0161no koriste bez barijera. Izvje\u0161taji o o\u0161te\u0107enju datote\u010dnog sustava ext3 su malo brojne i rijetke.<\/p>\n

Ali to nije samo sre\u0107a. Ted Tso obja\u0161njava<\/u><\/a><\/noindex> to je ono \u0161to je dnevnik ext3 \/ ext4 obi\u010dno kontinuiran. Prvo, upravlja\u010dki program datote\u010dnog sustava poku\u0161ava ga stvoriti kontinuiranim. Drugo, dnevnik se obi\u010dno stvara istovremeno s datote\u010dnim sustavom, kada je lako prona\u0107i kontinuirani prostor. Kontinuitet i urednost korisni su ne samo za performanse, ve\u0107 i za spre\u010davanje preure\u0111enja. Obi\u010dno \u0107e commit blok biti postavljen odmah nakon ostalih podataka u dnevniku, tako da disk nema razloga za preure\u0111enje. Commit blok se prirodno zapisuje na disk odmah nakon drugih zapisa u dnevniku.<\/p>\n

Ipak, nitko ne tvrdi da \u0107e uvijek biti tako. Diskovni ure\u0111aji mogu se pona\u0161ati druga\u010dije. Osim toga, dnevnik predstavlja kru\u017eni me\u0111uspremnik. Kada se transakcija zapi\u0161e na kraj dnevnika, commit blok mo\u017ee se na\u0107i u ranijem bloku, ispred drugih zapisa u dnevniku. Dakle, uvijek postoji mogu\u0107nost o\u0161te\u0107enja. U stvarnosti, Chris Mason ima za to testove<\/u><\/a><\/noindex>Nema sumnje da je rad bez barijera manje siguran nego s njima.<\/p>\n

Ako ste spremni na kompromis u performansama, mo\u017eete uklju\u010diti barijere. U tom slu\u010daju, naravno, kada va\u0161 datote\u010dni sustav nije zasnovan na LVM (kao u nekim distribucijama po defaultu). Ispostavilo se da device mapper ne podr\u017eava barijere. U ostalim slu\u010dajevima, bilo bi dobro smanjiti smanjenje performansi. \u010cini se da je to i mogu\u0107e.<\/p>\n

Trenutna implementacija ext3 (kada su barijere uklju\u010dene) za svaku transakciju izvodi sljede\u0107i niz operacija:<\/p>\n

    \n
  1. \n

    Podaci se zapisuj\u0105 u dnevnik<\/p>\n<\/li>\n

  2. \n

    Izvodi se barijera<\/p>\n<\/li>\n

  3. \n

    Zapisuje se commit blok<\/p>\n<\/li>\n

  4. \n

    Izvodi se sljede\u0107a barijera<\/p>\n<\/li>\n

  5. \n

    Kasnije se metapodaci ispisuju na disk<\/p>\n<\/li>\n<\/ol>\n

    U ext4 prvi barijer (korak 2) mo\u017ee se izostaviti, jer datote\u010dni sustav ext4 podr\u017eava kontrolne zbrojeve u dnevniku.<\/p>\n

    Ako su podaci dnevnika i commit blok preure\u0111eni, a operacija prekinuta uslijed kvara, kontrolni zbroj dnevnika ne\u0107e odgovarati onome koji se \u010duva u commit bloku, i transakcija \u0107e biti odbijena. <\/p>\n

    Kris Mason smatra<\/u><\/a><\/noindex>, da bi bilo «op\u0107enito sigurno» ukloniti ovu barijeru i u ext3, s mogu\u0107im iznimkama kada dnevnik dosegne kraj i po\u010dne se pisati ispo\u010detka. <\/p>\n

    Jo\u0161 jedna ideja za pobolj\u0161anje performansi je odlaganje operacija s barijerama kada je to mogu\u0107e. Ako nema hitne potrebe odmah ispisivati podatke na disk, mogu\u0107e je stvoriti nekoliko transakcija u dnevniku, te ispisivati na disk jednim barijerom.<\/p>\n

    Tako\u0111er postoji potencijal za pobolj\u0161anje pa\u017eljivim organiziranjem operacija kako bi se osiguralo da prepreke (koje se obi\u010dno implementiraju kao zahtjevi \u201eresetiraj sve odgo\u0111ene operacije na disk\u201d) ne prisiljavaju zapisivanje blokova koji ne zahtijevaju redoslijed.<\/p>\n

    Izgleda da je do\u0161lo vrijeme da razmislimo kako u\u010diniti tro\u0161kove barijera prihvatljivima. Ted Tso, \u010dini se, mislite na sli\u010dan na\u010din<\/u><\/a><\/noindex>:<\/p>\n

    \n

    Mislim da bismo trebali uklju\u010diti barijere u ext3\/4, a zatim raditi na smanjenju tro\u0161kova u ext4 \/ jbd2. Vjerojatno velika ve\u0107ina sustava ne radi u uvjetima sli\u010dnim onima koje je Kris koristio za demonstraciju problema i sigurnost datote\u010dnog sustava po defaultu trebala bi biti prioritet.<\/em><\/p>\n<\/blockquote>\n

    Zdrav razum mi govori da ta pas ve\u0107 ne spava i vjerojatno \u0107e neko vrijeme lajati. To mo\u017ee izazvati zabrinutost nekih susjeda, ali bolje je nego dopustiti da ugrize.<\/p>\n

    Zanima vas razvoj u ovom smjeru?<\/em><\/strong> Prijavite se za besplatnu Demo lekciju <\/em>\u201eKonfiguracija web poslu\u017eitelja (Apache, Nginx, uravnote\u017eenje Nginx)\u201c<\/em><\/a><\/noindex> i sudjelujte u prijenosu <\/em>\u201eRad s logovima u Linuxu\u201c<\/em><\/a><\/noindex>, koju \u0107e provesti Pavel Vikiryuk \u2014 operator MVNO, DevOps in\u017eenjer. <\/em><\/p>\n

    Izvor: habr.com<\/a> <\/p>","protected":false,"gt_translate_keys":[{"key":"rendered","format":"html"}]},"excerpt":{"rendered":"

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