On Tue, Jun 26, 2012 at 07:58:12PM -0300, Pedro Eugênio Rocha wrote: > > I know that ext4 postpones checkpoints in data journaling > mode, i.e., most of data (and meta-data) are only written > to the journal and checkpointed to their final location only > when certain thresholds are crossed. My question is: how > do ext4 handle read operations to data blocks that are only > written to the journal (supposing that its cache has been > evicted)? Does it execute the read operation in the journaled > data? Does it trigger a checkpoint? It's not quite true that we postpone writing blocks to their final location on disk until a checkpoint operation; what happens instead is that we _forbid_ blocks from getting written to disk until the transaction has committed. At that point, the buffer cache writeback threads are free to write them to their final location on disk whenever they want. We only allow a buffer to be evicted from the buffer cache once it is written to the final location on disk, so this isn't a problem. When we do a checkpoint, we *force* all dirty blocks to be written to disk, before we reuse that portion of the journal. However, some or many of these blocks may have already been written out to disk, and they certainly will be written to disk before their in-memory copy is allowed to be evicted from memory. Finally, note that normally data blocks do not get written into the journal at all; this only happens in the data journalling mode, which is not the default. Regards, - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
