On Thu, Feb 25, 2021 at 06:48:28PM -0800, Darrick J. Wong wrote: > On Thu, Feb 25, 2021 at 09:34:47AM +0100, Christoph Hellwig wrote: > > On Thu, Feb 25, 2021 at 08:44:17AM +1100, Dave Chinner wrote: > > > > Also, do you have any idea what was Christoph talking about wrt devices > > > > with no-op flushes the last time this patch was posted? This change > > > > seems straightforward to me (assuming the answers to my two question are > > > > 'yes') but I didn't grok what subtlety he was alluding to...? > > > > > > He was wondering what devices benefited from this. It has no impact > > > on highspeed devices that do not require flushes/FUA (e.g. high end > > > intel optane SSDs) but those are not the devices this change is > > > aimed at. There are no regressions on these high end devices, > > > either, so they are largely irrelevant to the patch and what it > > > targets... > > > > I don't think it is that simple. Pretty much every device aimed at > > enterprise use does not enable a volatile write cache by default. That > > also includes hard drives, arrays and NAND based SSDs. > > > > Especially for hard drives (or slower arrays) the actual I/O wait might > > matter. What is the argument against making this conditional? > > I still don't understand what you're asking about here -- > > AFAICT the net effect of this patchset is that it reduces the number of > preflushes and FUA log writes. To my knowledge, on a high end device > with no volatile write cache, flushes are a no-op (because all writes > are persisted somewhere immediately) and a FUA write should be the exact > same thing as a non-FUA write. Because XFS will now issue fewer no-op > persistence commands to the device, there should be no effect at all. > Except the cost of the new iowaits used to implement iclog ordering... which I think is what Christoph has been asking about..? IOW, considering the storage configuration noted above where the impact of the flush/fua optimizations is neutral, the net effect of this change is whatever impact is introduced by intra-checkpoint iowaits and iclog ordering. What is that impact? Note that it's not clear enough to me to suggest whether that impact might be significant or not. Hopefully it's neutral (?), but that seems like best case scenario so I do think it's a reasonable question. Brian > In contrast, a dumb stone tablet with a write cache hooked up to SATA > will have agonizingly slow cache flushes. XFS will issue fewer > persistence commands to the rock, which in turn makes things faster > because we're calling the engravers less often. > > What am I missing here? Are you saying that the cost of a cache flush > goes up much faster than the amount of data that has to be flushed? > > --D >
