Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote: > No, that is very much not the same thing. Look at what NFS does, like > Linus said. Consider this test program: > > fd = open(); > lseek(fd, 5, SEEK_SET); > write(fd, buf, 3); > write(fd, buf2, 10); > write(fd, buf3, 2); > close(fd); Yes, I get that. I can do that when there isn't a local cache or content encryption. Note that, currently, if the pages (or cache blocks) being read/modified are beyond the EOF at the point when the file is opened, truncated down or last subject to 3rd-party invalidation, I don't go to the server at all. > > But that kind of screws with local caching. The local cache might need to > > track the missing bits, and we are likely to be using blocks larger than a > > page. > > There's nothing to cache. Pages which are !Uptodate aren't going to get > locally cached. Eh? Of course there is. You've just written some data. That need to get copied to the cache as well as the server if that file is supposed to be being cached (for filesystems that support local caching of files open for writing, which AFS does). > > Basically, there are a lot of scenarios where not having fully populated > > pages sucks. And for streaming writes, wouldn't it be better if you used > > DIO writes? > > DIO can't do sub-512-byte writes. Yes it can - and it works for my AFS client at least with the patches in my fscache-iter-2 branch. This is mainly a restriction for block storage devices we're doing DMA to - but we're not doing direct DMA to block storage devices typically when talking to a network filesystem. For AFS, at least, I can just make one big FetchData/StoreData RPC that reads/writes the entire DIO request in a single op; for other filesystems (NFS, ceph for example), it needs breaking up into a sequence of RPCs, but there's no particular reason that I know of that requires it to be 512-byte aligned on any of these. Things get more interesting if you're doing DIO to a content-encrypted file because the block size may be 4096 or even a lot larger - in which case we would have to do local RMW to handle misaligned writes, but it presents no particular difficulty. > You might not be trying to do anything for block filesystems, but we > should think about what makes sense for block filesystems as well as > network filesystems. Whilst that's a good principle, they have very different characteristics that might make that difficult. David
