On Mon, Apr 28, 2014 at 05:06:28PM +1000, NeilBrown wrote: > On Mon, 28 Apr 2014 14:58:41 +0800 Shaohua Li <shli@xxxxxxxxxx> 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. > > 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. For 1, it's not common > > too. block plug mechanism will dispatch a bunch of sequentail small requests > > together. And since I'm using SSD, I'm using small chunk size. It's rare case > > stripe cache is really useful. > > > > So I'd like to avoid the copy from bio to stripe cache and it's very helpful > > for performance. In my 1M randwrite tests, avoid the copy can increase the > > performance more than 30%. > > > > Of course, this shouldn't be enabled by default, so I added an option to > > control it. > > I'm happy to avoid copying when we know that we can. > > I'm not really happy about using a sysfs attribute to control it. > > How do you guarantee that '2' won't happen? > > BTW I don't see '1' as important. The stripe cache is really for gathering > writes together to increase the chance of full-stripe writes, and for > handling synchronisation between IO and resync/reshape/etc. The copying is > primarily for stability. We are using raid5 in a SCSI target appliance. BIO is dispatched from a SCSI target layer (like LIO) and no filesytem is involved, so I can guarantee the BIO data is stable. What's your favorite way to control it? Thanks, Shaohua -- 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
