On Saturday 10 September 2011, Kent Overstreet wrote: > Short overview: > Bcache does both writethrough and writeback caching. It presents itself > as a new block device, a bit like say md. You can cache an arbitrary > number of block devices with a single cache device, and attach and > detach things at runtime - it's quite flexible. > > It's very fast. It uses a b+ tree for the index, along with a journal to > coalesce index updates, and a bunch of other cool tricks like auxiliary > binary search trees with software floating point keys to avoid a bunch > of random memory accesses when doing binary searches in the btree. It > does over 50k iops doing 4k random writes without breaking a sweat, > and would do many times that if I had faster hardware. > > It (configurably) tracks and skips sequential IO, so as to efficiently > cache random IO. It's got more cool features than I can remember at this > point. It's resilient, handling IO errors from the SSD when possible up > to a configurable threshhold, then detaches the cache from the backing > device even while you're still using it. Hi Kent, What kind of SSD hardware do you target here? I roughly categorize them into two classes, the low-end (USB, SDHC, CF, cheap ATA SSD) and the high-end (SAS, PCIe, NAS, expensive ATA SSD), which have extremely different characteristics. I'm mainly interested in the first category, and a brief look at your code suggests that this is what you are indeed targetting. If that is true, can you name the specific hardware characteristics you require as a minimum? I.e. what erase block (bucket) sizes do you support (maximum size, non-power-of-two), how many buckets do you have open at the same time, and do you guarantee that each bucket is written in consecutive order? On a different note, we had discussed at the last storage/fs summit about using an SSD cache either without a backing store or having the backing store on the same drive as the cache in order to optimize traditional file system on low-end flash media. Have you considered these scenarios? How hard would it be to support this in a meaningful way? My hope is that by sacrificing some 10% of the drive size, you would get significantly improved performance because you can avoid many internal GC cycles within the drive. Arnd -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
