| OCR Text |
Show 10 coJJtai u inform at ion about physical locations and these locations might be reused, it Jlc.t 'C: <J :):2 !Ji t ti!Ji<jl l< ' Jlli!Iliwr ill sicl<' tlw OlD to dd <'ct d<tiigling ur co rlli pLcd O!Ds. 01J.i <'c1 In ovcnH'nL i11 1 ~ :-JJ\11 is a problem since OIDs contain inform at ion about the phys icctl location or object. Objects in ESM are allocated 111 an esm-file, whicb is a col lect ion of related storage objects . Objects in an esm-file can be scanned in logical or physical order. \Niwn creating an object, the t1ser can ask ESM to cluster it near some other object or ne<tr Lbe first or last , logical or physical object. Objects which are larger than a di sk pag<' ar<-' repres<~ n L<~ cl using a B+ t ree-like index structure. This ensures that operations can be executed efficien t ly on large obj ects. ESM, however, provides a uniform call interface for both large and small objects. ESM provides t he concept of buffer groups. A buffer group identifies the bufFer In<-~. I J (lgcnwnt policy for a pa.rtic11lar gro 11p of o hj t~ cts . With t his fac ility, t he size i1l1d ll'pJact•!llC'iiL policy of' <t httff't~ l' grOilj) C(l.IJ h<~ spec ifi<c~ d for a part icul ar a.ctivity, <'g .. fur efFic ient scannin g, ESM uses a buffer group which bas most recent! used (MRU) as its replacement poli cy. For concurrency, ESM employs a two-phase locking protocol. It provides locks at two granul arities: fil e-level and page-level. Files can be locked in six lock modes: ;'\L (no lock), S (sha.nxl), X ( <~xcl u s i ve), IS (in tent to sha re) , IX (in tent to be ex clu sive), and SIX (sha red with intent to be exclusive) while pages can only be locked i 11 S and X mode. For recovery, ESl\ll uses a "log" based mechanisrn for objects and "shadowing" for in dices. In shadowing, a backup copy of the page Lahle (which C011Laius Pntri es co rresponding to database pag<~s ) is nlctintainecl and t lJ is copy is never modified . During updates , pages are modified as usual. lf recovery i;, JJ('(('SSl:LI.JI, tlw lll odili<'d pet.ge is rr<'cd (\lid Lhc s!Id.dow page is n~ im; LdL(~d. I ~ Sl\ 1 di!ows It sc~rs to huild indic<-~s on objects based on user speci fied keys. Tbe iu cl <~ x associates OlDs with the keys. ESM provides both a B+ tree index and a |
