{"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\/ro\/blog\/administrirovanie\/barery-i-zhurnaliruemye-fajlovye-sistemy","title":{"rendered":"Bariere \u0219i sisteme de fi\u0219iere jurnalizate","gt_translate_keys":[{"key":"rendered","format":"text"}]},"content":{"rendered":"
\n

S\u0103 ave\u021bi un weekend excelent!<\/em><\/strong> V\u0103 invit\u0103m la o lec\u021bie demo gratuit\u0103 <\/em>\u201eConfigurarea serverului web (Apache, Nginx, balansarea Nginx)\u201c<\/em><\/a><\/noindex>, care \u00eel va conduce \u0410\u043d\u0434\u0440\u0435\u0439 \u0411\u0443\u0440\u0430\u043d\u043e\u0432 \u2014 specialist \u00een sistemele UNIX de la Mail.Ru Group. De asemenea, public\u0103m articolul lui Jonathan Corbet \u2014 Editor Executiv la LWN.net.<\/em><\/p>\n<\/blockquote>\n

Sistemele de fi\u0219iere jurnalizate promit s\u0103 elibereze administratorii de sistem de problemele cu deteriorarea discului \u00een caz de defectare a sistemului. Chiar \u0219i f\u0103r\u0103 a lansa verificarea integrit\u0103\u021bii sistemului de fi\u0219iere. Totu\u0219i, \u00een realitate, lucrurile sunt pu\u021bin mai complicate. A\u0219a cum sugereaz\u0103 o discu\u021bie recent\u0103, este posibil ca acestea s\u0103 fie chiar mai confuze dec\u00e2t cred mul\u021bi dintre noi, deoarece asigurarea integrit\u0103\u021bii sistemelor de fi\u0219iere jurnalizate afecteaz\u0103 performan\u021ba.<\/p>\n

Sistemul de fi\u0219iere, cum ar fi ext3, folose\u0219te o zon\u0103 separat\u0103 pe disc, numit\u0103 jurnal. Atunci c\u00e2nd se fac modific\u0103ri \u00een metadatele sistemului de fi\u0219iere, aceste modific\u0103ri sunt mai \u00eent\u00e2i scrise \u00een jurnal f\u0103r\u0103 a modifica restul sistemului de fi\u0219iere. Dup\u0103 ce toate modific\u0103rile sunt \u00eenregistrate \u00een jurnal, se adaug\u0103 un «bloc de commit», care indic\u0103 finalizarea tranzac\u021biei. Abia dup\u0103 \u00eenregistrarea blocului de commit, tranzac\u021bia este confirmat\u0103 \u0219i metadatele modificate sunt scrise pe disc. Dac\u0103 sistemul se blocheaz\u0103 \u00eentr-un anumit moment, informa\u021biile din jurnal permit finalizarea \u00een siguran\u021b\u0103 \u0219i evitarea deterior\u0103rii sistemului de fi\u0219iere din cauza faptului c\u0103 doar o parte din metadate a fost actualizat\u0103.<\/p>\n

Cu toate acestea, exist\u0103 o problem\u0103: codul sistemului de fi\u0219iere, \u00eenainte de a scrie blocul de commit, trebuie s\u0103 fie absolut sigur c\u0103 toate informa\u021biile despre tranzac\u021bie sunt deja \u00een jurnal. Simplul fapt de a \u00eenregistra opera\u021biile \u00een ordinea corect\u0103 nu este suficient \u2014 discurile moderne suport\u0103 cache-uri interne mari \u0219i reordoneaz\u0103 opera\u021biile pentru a \u00eembun\u0103t\u0103\u021bi performan\u021ba. De aceea, \u00eenainte de blocul de commit, trebuie s\u0103 se indice explicit transferul tuturor datelor jurnalului pe disc. Dac\u0103 blocul de commit este scris mai devreme, jurnalul poate fi deteriorat. Pentru a rezolva aceast\u0103 problem\u0103, se folosesc bariere. Practic, o barier\u0103 interzice scrierea oric\u0103ror blocuri dup\u0103 barier\u0103, p\u00e2n\u0103 c\u00e2nd toate blocurile scrise \u00eenainte de barier\u0103 au fost transferate pe disc. Folosind bariere, sistemele de fi\u0219iere garanteaz\u0103 coeren\u021ba structurilor de fi\u0219iere.<\/p>\n

Dar exist\u0103 \u00eenc\u0103 o problem\u0103: sistemele de fi\u0219iere ext3 \u0219i ext4 nu folosesc bariere \u00een mod implicit. Op\u021biunea exist\u0103, dar dac\u0103 administratorul nu le activeaz\u0103 explicit, aceste sisteme de fi\u0219iere func\u021bioneaz\u0103 f\u0103r\u0103 bariere, de\u0219i \u00een anumite distribu\u021bii (de exemplu, SUSE) valorile implicite sunt altele. Eric Sandeen a decis recent c\u0103 aceast\u0103 situa\u021bie trebuie schimbat\u0103 \u0219i a f\u0103cut un patch<\/u><\/a><\/noindex>, care modific\u0103 set\u0103rile implicite pentru ext3 \u0219i ext4. \u0218i atunci a \u00eenceput o dezbatere aprins\u0103.<\/p>\n

Andrew Morton a r\u0103spuns<\/u><\/a><\/noindex>, de ce valoarea implicit\u0103 este exact a\u0219a:<\/p>\n

\n

Data trecut\u0103, c\u00e2nd am \u00eencercat s\u0103 facem aceast\u0103 modificare, performan\u021ba a sc\u0103zut cu 30% la multe sarcini, a\u0219a c\u0103 am aruncat toate aceste patch-uri. Cred c\u0103 nu ne putem permite acest lucru \u0219i s\u0103 \u00eencetinim at\u00e2t de grav toate ma\u0219inile...<\/em><\/p>\n

Nu exist\u0103 solu\u021bii perfecte aici, iar eu tind s\u0103 nu deranjez acest c\u00e2ine adormit \u0219i s\u0103 las parametrii implici pe seama dezvoltatorilor distribu\u021biilor.<\/em><\/p>\n<\/blockquote>\n

Astfel, dincolo de barier\u0103 sunt dezactivate \u00een mod implicit, deoarece acestea afecteaz\u0103 grav performan\u021ba. \u00cen plus, sistemele de fi\u0219iere func\u021bioneaz\u0103 destul de bine f\u0103r\u0103 bariere. Raport\u0103rile despre coruperea sistemului de fi\u0219iere ext3 sunt destul de rare.<\/p>\n

Dar nu este doar noroc. Ted Ts'o explic\u0103<\/u><\/a><\/noindex> este faptul c\u0103 jurnalul ext3 \/ ext4 este de obicei plasat continuu. \u00cen primul r\u00e2nd, driverul sistemului de fi\u0219iere \u00eencearc\u0103 s\u0103 creeze un jurnal continuu. \u00cen al doilea r\u00e2nd, jurnalul este, de obicei, creat simultan cu sistemul de fi\u0219iere, c\u00e2nd este u\u0219or s\u0103 g\u0103sim un spa\u021biu continuu. Continuitatea \u0219i ordonarea sunt utile nu doar pentru performan\u021b\u0103, ci \u0219i pentru a preveni reordonarea. De obicei, blocul de commit va fi plasat imediat dup\u0103 celelalte date din jurnal, astfel c\u0103 discul nu are motive s\u0103 reordoneze. Blocul de commit este scris pe disc imediat dup\u0103 celelalte \u00eenregistr\u0103ri din jurnal.<\/p>\n

Cu toate acestea, nimeni nu afirm\u0103 c\u0103 a\u0219a va fi mereu. Discurile pot ac\u021biona diferit. \u00cen plus, jurnalul este un buffer circular. A\u0219adar, c\u00e2nd o tranzac\u021bie este \u00eenregistrat\u0103 la sf\u00e2r\u0219itul jurnalului, blocul de comitere poate fi \u00een parte anterioar\u0103, \u00eenaintea altor \u00eenregistr\u0103ri din jurnal. Deci, riscul de corup\u021bie este \u00eentotdeauna prezent. De fapt, Chris Mason are pentru aceasta teste<\/u><\/a><\/noindex>. Nu exist\u0103 \u00eendoial\u0103 c\u0103 munca f\u0103r\u0103 bariere este mai pu\u021bin sigur\u0103 dec\u00e2t cu ele.<\/p>\n

Dac\u0103 sunte\u021bi preg\u0103ti\u021bi s\u0103 accepta\u021bi o sc\u0103dere a performan\u021bei, atunci pute\u021bi activa barierele. \u00cen acel caz, desigur, c\u00e2nd sistemul dumneavoastr\u0103 de fi\u0219iere nu se bazeaz\u0103 pe LVM (a\u0219a cum se \u00eent\u00e2mpl\u0103 \u00een unele distribu\u021bii implicit). Se pare c\u0103 device mapper nu suport\u0103 bariere. \u00cen celelalte cazuri, ar fi bine s\u0103 reduce\u021bi sc\u0103derea performan\u021bei. \u0218i, se pare, acest lucru se poate face.<\/p>\n

Implementarea curent\u0103 ext3 (c\u00e2nd barierele sunt activate) execut\u0103 urm\u0103toarea secven\u021b\u0103 de opera\u021bii pentru fiecare tranzac\u021bie:<\/p>\n

    \n
  1. \n

    Datele sunt scrise \u00een jurnal<\/p>\n<\/li>\n

  2. \n

    Se execut\u0103 bariera<\/p>\n<\/li>\n

  3. \n

    Se scrie blocul de comitere<\/p>\n<\/li>\n

  4. \n

    Se execut\u0103 urm\u0103toarea barier\u0103<\/p>\n<\/li>\n

  5. \n

    Ulterior, metadatele sunt scrise pe disc<\/p>\n<\/li>\n<\/ol>\n

    \u00cen ext4, prima barier\u0103 (pasul 2) poate fi omis\u0103, deoarece sistemul de fi\u0219iere ext4 suport\u0103 checksum-uri \u00een jurnal.<\/p>\n

    Dac\u0103 datele jurnalului \u0219i blocul de comitere sunt rearanjate, iar opera\u021bia este \u00eentrerupt\u0103 \u00een urma unei erori, atunci checksum-ul jurnalului nu va corespunde cu cel stocat \u00een blocul de comitere, iar tranzac\u021bia va fi anulat\u0103. <\/p>\n

    Chris Mason consider\u0103<\/u><\/a><\/noindex>, ceea ce ar fi \u201e\u00een general sigur\u201d s\u0103 elimin\u0103m aceast\u0103 barier\u0103 \u0219i \u00een ext3, cu o excep\u021bie posibil\u0103 atunci c\u00e2nd jurnalul ajunge la final \u0219i \u00eencepe s\u0103 scrie din nou de la \u00eenceput. <\/p>\n

    O alt\u0103 idee pentru a accelera func\u021bionarea este de a am\u00e2na opera\u021biile cu bariere atunci c\u00e2nd este posibil. Dac\u0103 nu exist\u0103 o necesitate stringent\u0103 de a scrie imediat datele pe disc, atunci se pot crea mai multe tranzac\u021bii \u00een jurnal, \u0219i s\u0103 se scrie pe disc cu o singur\u0103 barier\u0103.<\/p>\n

    De asemenea, exist\u0103 un anumit poten\u021bial de \u00eembun\u0103t\u0103\u021bire prin ordonarea atent\u0103 a opera\u021biunilor, astfel \u00eenc\u00e2t barierele (care sunt de obicei implementate sub form\u0103 de solicit\u0103ri \u201escrie toate opera\u021biunile \u00eent\u00e2rziate pe disc\u201d) s\u0103 nu for\u021beze scrierea blocurilor care nu necesit\u0103 ordonare.<\/p>\n

    Se pare c\u0103 a sosit momentul s\u0103 ne g\u00e2ndim cum s\u0103 facem costul barierelor acceptabil. Ted Tso pare s\u0103 consider\u0103 similar<\/u><\/a><\/noindex>:<\/p>\n

    \n

    Cred c\u0103 ar trebui s\u0103 includem bariere \u00een ext3\/4 \u0219i apoi s\u0103 lucr\u0103m la reducerea cheltuielilor indirecte \u00een ext4\/jbd2. Probabil c\u0103 majoritatea sistemelor nu opereaz\u0103 \u00een condi\u021bii asem\u0103n\u0103toare cu cele folosite de Chris pentru a demonstra problema \u0219i securitatea sistemului de fi\u0219iere ar trebui s\u0103 fie o prioritate.<\/em><\/p>\n<\/blockquote>\n

    Cuv\u00e2ntul bun sim\u021b \u00eemi spune c\u0103 acest c\u00e2ine nu mai doarme \u0219i, probabil, va latra pentru o vreme. Aceasta poate provoca \u00eengrijorare \u00een r\u00e2ndul unor vecini, dar este mai bine dec\u00e2t s\u0103 o la\u0219i s\u0103 mu\u0219te.<\/p>\n

    Interesat s\u0103 dezvolta\u021bi \u00een aceast\u0103 direc\u021bie?<\/em><\/strong> \u00censcrie\u021bi-v\u0103 pentru un demo gratuit <\/em>\u201eConfigurarea serverului web (Apache, Nginx, balansarea Nginx)\u201c<\/em><\/a><\/noindex> \u0219i participa\u021bi la transmisiune <\/em>\u00abLucrul cu jurnale \u00een Linux\u00bb<\/em><\/a><\/noindex>, care va fi condus\u0103 de Pavel Vikiryuk \u2014 operator de re\u021bea MVNO, inginer DevOps. <\/em><\/p>\n

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

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