Hi, We've been discussing how i_blocks is set because of integrity problems on 32bit smp and noticed some problems with v9fs as described here[1]; I could use some help to sort this out. [1] https://marc.info/?l=linux-fsdevel&m=154834116110451&w=2 Long story short, with 9p and caching activated (e.g. -o cache=fscache), creating a new file and writing a few bytes will start at 0 and increment blocks count with inode_add_bytes; so a file with e.g. 200 bytes of data will have i_blocks = 0 and i_bytes = 200. (added linux-nfs@ in Cc because there is a small, few seconds window where I can reproduce a similar issue on nfs: $ echo foo > bar; stat -c %b bar; sleep 3; stat -c %b bar 0 8 with a 4.14.87 knfsd and 4.19.15-300.fc29 client, nfs 4.2, both x86_64, no export option or explicit client option. I'm honestly not sure we care about this single-client problem for a few seconds but figured it's worth reporting) I believe that from the first byte onwards there should be at least one block in i_blocks: - that's how all the local filesystems I know work, when you write a single byte you're actualy reserving a few blocks and that is the number reported with st_blocks ; - tools like du show the file as empty ; - and most importantly there still are some tools looking at i_blocks and not doing any read at all if i_blocks is zero (e.g. gnu tar with some options, see code[2]); I know there's been some discussions around this for btrfs but I'm not sure how these ended. [2] http://git.savannah.gnu.org/cgit/tar.git/tree/src/sparse.c#n273 There also is a weird related behaviour since we use inode_add_bytes is that if you start with an existing file, it'll initially report the right number of blocks but then the evolution is 100% client-write-driven e.g. you start with a file with 200 bytes and 8 blocks; then write another 1k and stat will report 10 blocks when the fs really still only has 8 blocks. I believe this does not matter as much as this probably doesn't cause much problem except du confusion. For what it's worth, cache=none doesn't have the problem because every stat will send a getattr to the server, so it'd always report the number of blocks as seen on the server. Anyway, how is one supposed to use inode_set_bytes/add_bytes for that? Should we remove our uses of inode_add_bytes ? Given 9p does not do quota on the client (if required the server can enforce it), do we care about i_bytes at all? For cached mounts I'd be open to blatanlty lie and always print the rounded up value of block based on i_size (e.g. i_size + 511 >> 9 and i_bytes to 0) thus ignoring what the server report; I don't see much way around that to have something consistent... Thanks, -- Dominique Martinet | Asmadeus
