On Wed, Nov 22, 2023 at 2:48 PM Cedric Blancher <cedric.blancher@xxxxxxxxx> wrote: > > On Sun, 19 Nov 2023 at 19:02, Anna Schumaker <schumaker.anna@xxxxxxxxx> wrote: > > > > On Sun, Nov 19, 2023 at 12:59 PM Cedric Blancher > > <cedric.blancher@xxxxxxxxx> wrote: > > > > > > On Sun, 19 Nov 2023 at 18:48, Anna Schumaker <schumaker.anna@xxxxxxxxx> wrote: > > > > > > > > Hi, > > > > > > > > On Sun, Nov 19, 2023 at 12:38 PM Cedric Blancher > > > > <cedric.blancher@xxxxxxxxx> wrote: > > > > > > > > > > Good evening! > > > > > > > > > > How does READ_PLUS differ from READ? Has anyone made a simpler > > > > > presentation (PowerPoint slides) than the RFCs? > > > > > > > > No slides, but at a high level READ_PLUS can compress out long ranges > > > > of zeroes in a read reply by returning a HOLE segment instead of the > > > > actual zeroes. It's perfectly valid for the server to skip the zero > > > > detection and return everything as a data segment, however. > > > > > > So how do you differ between > > > 1. a hole, aka no filesystem blocks allocated > > > 2. a long sequence of valid data with all zero bytes in them > > > > That's up to the server! It could use something like fiemap or lseek > > with SEEK_HOLE or SEEK_DATA. It could also scan the data to see if > > there are any zeroes that could be compressed out. > > How can the client figure out whether the data in a READ_PLUS reply > are zeros of data, or zeros from a hole? As I understand the RFC, it cannot. Or put another way "a hole is a region that reads as all 0s, which may or may not have allocated blocks on the server file system". Although SEEK_HOLE typically returns the offset of an unallocated region, I don't think either the POSIX draft (was it ever ratified?) nor RFC7862 actually define a "hole" as an unallocated region. On a similar vein, Deallocate can simply write 0s to the region. (It does not actually have to "deallocate data blocks".) At least that is my understanding of POSIX and RFC7862, rick > > Ced > -- > Cedric Blancher <cedric.blancher@xxxxxxxxx> > [https://plus.google.com/u/0/+CedricBlancher/] > Institute Pasteur >
