Hello, On Tue, Sep 08, 2015 at 10:07:36AM -0700, Shaohua Li wrote: > I need double confirm. But for write + flush, we aggregrate several > writes and do a flush; for FUA, we do every meta write with FUA. So this > is not apple to apple comparison. Does that mean that upper layers are taking different actions depending on whether the underlying device supports FUA? That at least wasn't the original model that I had on mind when implementing the current incarnation of REQ_FLUSH and FUA. The only difference FUA was expected to make was optimizing out the flush after REQ_FUA and the upper layers were expected to issue REQ_FLUSH/FUA the same whether the device supports FUA or not. Hmmm... grep tells me that dm and md actually are branching on whether the underlying device supports FUA. This is tricky. I didn't even mean flush_flags to be used directly by upper layers. For rotational devices, doing multiple FUAs compared multiple writes followed by REQ_FLUSH is probably a lot worse - the head gets moved multiple times likely skipping over data which can be written out while traversing and it's not like stalling write pipeline and draining write queue has much impact on hard drives. Maybe it's different on SSDs. I'm not sure about how expensive flush itself would be given that a lot of write cost is paid asynchronously anyway during gc but flush stalls IO pipeline and that could be very noticeable on high iops devices. Also, unless the implementation is braindead FUA IOs are unlikely to be expensive on SSDs, so maybe what we should do is making block layer hint upper layers regarding what's likely to perform better. But, ultimately, I don't think it'd be too difficult for high-depth SSD devices to report write-through w/o losing any performance one way or the other and it'd be great if we eventually can get there. Thanks. -- tejun -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
