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Why Write-Through is still the default Flash Cache Mode on #Exadata X-4

The Flash Cache Mode still defaults to Write-Through on Exadata X-4 because most customers are better suited that way – not because Write-Back is buggy or unreliable. Chances are that Write-Back is not required, so we just save Flash capacity that way. So when you see this

CellCLI> list cell attributes flashcachemode
         WriteThrough

it is likely to your best :-)
Let me explain: Write-Through means that writing I/O coming from the database layer will first go to the spinning drives where it is mirrored according to the redundancy of the diskgroup where the file is placed that is written to. Afterwards, the cells may populate the Flash Cache if they think it will benefit subsequent reads, but there is no mirroring required. In case of hardware failure, the mirroring is already sufficiently done on the spinning drives, as the pictures shows:

Flash Cache Mode Write-Through

Flash Cache Mode WRITE-THROUGH

That changes with the Flash Cache Mode being Write-Back: Now writes go primarily to the Flashcards and popular objects may even never get aged out onto the spinning drives. At least that age out may happen significantly later, so the writes on flash must be mirrored now. The redundancy of the diskgroup where the object in question was placed on determines again the number of mirrored writes. The two pictures assume normal redundancy. In other words: Write-Back reduces the usable capacity of the Flashcache at least by half.

Flash Cache Mode Write-Back

Flash Cache Mode WRITE-BACK

Only databases with performance issues on behalf of writing I/O will benefit from Write-Back, the most likely symptom of which would be high numbers of the Free Buffer Waits wait-event. And Flash Logging is done with both Write-Through and Write-Back. So there is a good reason behind turning on the Write-Back Flash Cache Mode only on demand. I have explained this just very similar during my present Oracle University Exadata class in Frankfurt, by the way :-)

Tagged: exadata