On Mon, 28 Apr 2014 03:17:48 -0700 Christoph Hellwig <hch@xxxxxxxxxxxxx> wrote: > On Mon, Apr 28, 2014 at 02:58:41PM +0800, Shaohua Li wrote: > > > > The stripe cache has two goals: > > 1. cache data, so next time if data can be found in stripe cache, disk access > > can be avoided. > > I think this is mostly a side effect. We have a much larger and better > tuned page cache to take care of this. > > > 2. stable data. data is copied from bio to stripe cache and calculated parity. > > data written to disk is from stripe cache, so if upper layer changes bio data, > > data written to disk isn't impacted. > > > > In my environment, I can guarantee 2 will not happen. > > Why just in your environment? Now that we got stable pages in the page > cache this should always be the case. Hmm... I hadn't realised that we were guaranteed stabled pages always (if requested). It seems that we are. > > > Of course, this shouldn't be enabled by default, so I added an option to > > control it. > > Unless careful benchmarking in various scenarious shows adverse effects > this should be the default. And if we can find adverse effects we need > to look into them. Certainly some benchmarking is needed. We should set mddev->queue->backing_dev_info.capabilities |= BDI_CAP_STABLE_WRITES if and only iff 'skip_copy' is set. Then test various cases just to confirm that it is generally an improvement. NeilBrown
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