Comment by Qantourisc
10 years ago
IIRC I have seen this discussion before. And the answer was do a fsync. But for sakes of performance we want to be able to issue write-barriers to a group of file handles. So we know the commands will be ran in order, and no other order.
Correct, we want to be able to write in deterministic order. SCSI has supported this natively for decades. Unfortunately SATA doesn't, and the Linux kernel pretty much doesn't, because it can't rely on the storage devices to support it.
Which is just lame, its mostly because the block layer doesn't support file based fencing. A mistake made a decade an a half ago, and no one has the will/political power to fix it.
If the block layer supported it, solving the problem of fencing an ATA device would be as simple as issuing a whole device flush instead of a SYNC_CACHE with range. Which for soft raid devices would make a huge impact because only the devices with dirty data need to be flushed.
Of course the excuse today, is that most scsi devices can't do range based sync either, and just fall back to whole device because no one uses the command. Chicken/egg.