RAID sync

[V13]

On Saturday 12 July 2008, Apollon Koutlidis wrote:
> Den eixa data corruption sta dedomena tou RAID - th deuterh fora
> toulaxiston. To problhma htan sta metadata tou array sumfwna me ta opoia
> to array twn 3 diskwn apoteloutan apo 4 diskous apo tous opoious oi 2
> htan mh-dia8esimoi kai to array twn 4 diskwn apo 5 apo tous opoious pali
> oi 2 htan mh-dia8esimoi. Ekana re-assemble to array me ton pragmatiko
> ari8mo diskwn kai enan missing kai ... afou ekane re-sync, tou perase
> (auta ap' oso mporw na 8umh8w, profanws). Kapoia stigmh elpizw na brw
> xrono (kai dia8esh) gia na peiramatistw peraiterw.

To na einai out-of-sync *einai* en meri curruption sta dedomena (h 
synepagetai). Kapoia metadata den graftikan eno to leitoyrgiko nomize oti 
graftikan (efoson den prokeitai gia apotomi diakopi reymatos).

Dose mdadm --examine /dev/sdX kai tha deis kapoy na leei kati toy styl:

Events : 0.141868
h
Events : 4368

ean ayto den einai idio se oloys toys diskoys tote kati exei graftei ston enan 
kai oxi ston allon kai oi diskoi theoroyntai out-of-sync. Katalabaineis oti 
ayto einai mia pliroforia h opoia brisketai pano sto disko. Ean ayth h 
pliroforia einai diaforetiki se 2+ diskoys (ara theleis RAID5 me >3 diskoys) 
tote to array den mporei na ksanaginei assembly. Mazi me ayth th pliroforia, 
akomi kai an h write cache toy diskoy den kanei reordering sta dedomena poy 
grafontai, yparxoyn pragmata ta opoia den graftikan sto disko.

To oti to 'anagkases' na to kanei assembly den shmainei oti ola einai OK. Kane 
ena fsck se otidipote brisketai mesa sto array ean den theleis na exeis  
dysarestes ekplikseis sto mellon .

<<V13>>

p.s.1 To oti ayto poy prosekses (anagkastika) einai to oti den ginetai 
assembly to array, den synepagetai pos einai to mono problima poy 
dimioyrgithike.
p.s.2 To problima mporei na emfanistei kai me 3 mono diskoys ean oloi exoyn 
diaforetikh timi ston event counter. Sth praksh mporei na symbei alla 
kalitera na min to analysoyme.