On Wed, Oct 12, 2016 at 01:19:37PM -0400, Zygo Blaxell wrote: > On Wed, Oct 12, 2016 at 01:48:58PM +0800, Qu Wenruo wrote: > > In fact, the _concept_ to solve such RMW behavior is quite simple: > > > > Make sector size equal to stripe length. (Or vice versa if you like) > > > > Although the implementation will be more complex, people like Chandan are > > already working on sub page size sector size support. > > So...metadata blocks would be 256K on the 5-disk RAID5 example above, > and any file smaller than 256K would be stored inline? Ouch. That would > also imply the compressed extent size limit (currently 128K) has to become > much larger. > > I had been thinking that we could inject "plug" extents to fill up > RAID5 stripes. This lets us keep the 4K block size for allocations, > but at commit (or delalloc) time we would fill up any gaps in new RAID > stripes to prevent them from being modified. As the real data is deleted > from the RAID stripes, it would be replaced by "plug" extents to keep any > new data from being allocated in the stripe. When the stripe consists > entirely of "plug" extents, the plug extent would be deleted, allowing > the stripe to be allocated again. The "plug" data would be zero for > the purposes of parity reconstruction, regardless of what's on the disk. > Balance would just throw the plug extents away (no need to relocate them).
Your idea sounds good, but there's one problem: most real users don't balance. Ever. Contrary to the tribal wisdom here, this actually works fine, unless you had a pathologic load skewed to either data or metadata on the first write then fill the disk to near-capacity with a load skewed the other way. Most usage patterns produce a mix of transient and persistent data (and at write time you don't know which file is which), meaning that with time every stripe will contain a smidge of cold data plus a fill of plug extents. Thus, while the plug extents idea doesn't suffer from problems of big sectors you just mentioned, we'd need some kind of auto-balance. -- A MAP07 (Dead Simple) raspberry tincture recipe: 0.5l 95% alcohol, 1kg raspberries, 0.4kg sugar; put into a big jar for 1 month. Filter out and throw away the fruits (can dump them into a cake, etc), let the drink age at least 3-6 months. -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
