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Handling write failure (due to power failures)

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Slideshow:

Intro: solving the write failure caused by power failure

No help from the hard disk itself...
 
 

 
 
 
 
 
Solving the write failure caused by power failure - solution overview

Solving the write failure caused by power failure - write algorithm

Solving the write failure caused by power failure - possible outcomes

Solving the write failure caused by power failure - read algorithm

Solving the write failure caused by power failure - result/summary
 
 
 

 
 
 
 
 
 
 
 
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• Write failure
  • Power failure during a write operation:

    Result: lost of a data stored in a disk block

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  • Fact:

    Hard disks do not handle (support) write failures        Because: Support the write failure will require a high overhead in performance You will need to introduce this write technique yourself if you want to protect against write failure

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• Handling write failure
  • Using the stable storage update policy :

    A sector in the stable storage update policy is a logical unit Each (logical) sector, consists of two physical sectors: Stable storage update policy: Always update the Left sector (xL) first             After successful update, update the Right sector (xR) later

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  • Write operation to a sector x:

    Update xL using this algorithm: successL = false; ntries = 0; while ( not successL ) { ntries++; write data to sector xL; read data back; if ( verified OK ) successL = true; else { if ( ntries > too-many-times ) { replace xL with another sector; ntries = 0; // Repeat !!! } } } Update xR using the same procedure above

    Result: only 3 outcomes are possible

    If there was a power failure during the write xL operation: If there was a power failure during the write xR operation: If no power failure during the write operation:

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  • Read operation to sector x:

    ntries = 0; while ( ntries < too-many-tries ) { /* ============================================ If xL is OK, use xL ============================================ */ read sector xL; if ( verified OK ) return xL; else { /* ====================================== Otherwise, use xL ====================================== */ read sector xR; if ( verified OK ) return xR; } // Comment: one of xL or xR read operation will be successful ! }

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  • Result:

    Stable storage write policy will preserve the old copy or the new copy of the data during power failure