Slideshow:
Solving intermittent failure:
checksumming
Implementation: add
checksum to data
Implementation:
read algorithm
Implementation: add
checksum to data
- Intermittent
read failure:
Solution:
- Verify the the data read for correctness (using a embedded code)
- Retry if verification fails
- Implementation:
- Every sector
of the disk contains
a check sum
on the stored data in the
sector:
Data of 1 sector: +---------------------------------------------+--------+ | data stored in 1 sector | chksum | +---------------------------------------------+--------+
- Read:
success = false; NTries = 0; /* =============================================== Try reading a sector for upto N times =============================================== */ while ( not success && NTries < N ) // Try up to N times !!! { read sector; compute checksum of the data; if ( computed checksum == recorded checksum ) success = true; else NTries++; }
- Write:
1. compute checksum on data; 2. Write data + check sum Optional: Read the data back and verify....
(The optional part is usually not done because to heavy price on performance !!!)
- Every sector
of the disk contains
a check sum
on the stored data in the
sector: