Re: [libvirt] [PATCH 3/4] vol-info: Check for NFS FS type

> Testing seems to indicate that posix_fallocate() either doesn't work as > expected on the target or using the target.path is incorrect... > > Before posix_fallocate > stat st_blocks=0 st_blksize=1048576 st_size=10485760 > lseek end=10485760 > > posix_fallocate of 10485760 bytes on /home/nfs_pool/target/test-vol1 > > After posix_fallocate > stat st_blocks=88 st_blksize=1048576 st_size=10485760 > lseek end=10485760 > > > Hmm... would going at the target be correct in this instance? Same test > but use the source path:

> > Before posix_fallocate > stat st_blocks=0 st_blksize=4096 st_size=10485760 > lseek end=10485760 > > posix_fallocate of 10485760 bytes on /home/nfs_pool/nfs-export/test-vol1 > > After posix_fallocate > stat st_blocks=20480 st_blksize=4096 st_size=10485760 > lseek end=10485760 > > ... > > hmm.... 20480 * 512 = 10485760 > > # df > ... > localhost:/home/nfs_pool/nfs-export 140979200 35521536 98273280 27% > /home/nfs_pool/target > # > Well it's a tangled web that's being weaved... The blksize of the target volume comes from the 'wsize' value in the mount: localhost:/home/nfs_pool/nfs-export on /home/nfs_pool/target type nfs4 (rw,relatime,vers=4.0,rsize=1048576,wsize=1048576,...) Further testing shows if I change the wsize to 4096, then I get what I expect; however, starting at 8192 I'll get decreasingly smaller allocations. So this is a "math problem". So going back to writing via posix_fallocate() to the "/home/nfs_pool/target/test-vol" the "issue" is the blksize of the "source" (nfs-export) is 4096 while the blksize of the "target" (target) is 1048576 (as a result of the nfs mount settings). What "seems" to happen is the posix_fallocate() makes 11 "writes" - I assume because blksize*10 < desired_size (10485760) (instead of <=...). Thus 11 * 4096 = 45056 (bytes) / 1024 = 44 KiB which was displayed. Why all this happens I'm not sure. Bug in posix_fallocate()? Bug in configuration? I have to assume that when this code was first added NFS probably was still using smaller block sizes.

Whether anyone has noticed or not beyond the virt-test which discovered the issue - I'm not sure. In any case, does anyone have feedback/thoughts for next steps? I can put together something that avoids posix_fallocate() for the create-as and resize paths.