On Thu 13 Apr 2017 03:09:53 PM CEST, Denis V. Lunev wrote: >>> For nowadays SSD we are facing problems somewhere else. Right now I >>> can achieve only 100k IOPSes on SSD capable of 350-550k. 1 Mb block >>> with preallocation and fragmented L2 cache gives same 100k. Tests >>> for initially empty image gives around 80k for us. >> Preallocated images aren't particularly interesting to me. qcow2 is >> used mainly for two reasons. One of them is sparseness (initially >> small file size) mostly for desktop use cases with no serious I/O, so >> not that interesting either. The other one is snapshots, i.e. backing >> files, which doesn't work with preallocation (yet). >> >> Actually, preallocation with backing files is something that >> subclusters would automatically enable: You could already reserve the >> space for a cluster, but still leave all subclusters marked as >> unallocated. > > I am spoken about fallocate() for the entire cluster before actual > write() for originally empty image. This increases the performance of > 4k random writes 10+ times. In this case we can just write those 4k > and do nothing else.
You're talking about using fallocate() for filling a cluster with zeroes before writing data to it. As noted earlier in this thread, this works if the image is empty or if it doesn't have a backing file. And if the image is not empty you cannot guarantee that the cluster contains zeroes (you can use FALLOC_FL_ZERO_RANGE, but that won't work in all cases). Berto
