On Sat, Mar 21, 2020 at 2:40 PM Nikolaus Rath <Nikolaus@xxxxxxxx> wrote: > > Hello, > > When issuing a 16 kB read request from userspace and the default FUSE > readahead settings, data is read in batches of 32k: > > $ example/passthrough_ll -d mnt > FUSE library version: 3.9.1 > unique: 1, opcode: INIT (26), nodeid: 0, insize: 56, pid: 0 > INIT: 7.27 > flags=0x003ffffb > max_readahead=0x00020000 > INIT: 7.31 > flags=0x0000f439 > max_readahead=0x00020000 > max_write=0x00020000 > max_background=0 > congestion_threshold=0 > time_gran=1 > unique: 1, success, outsize: 80 > unique: 2, opcode: LOOKUP (1), nodeid: 1, insize: 44, pid: 20822 > lo_lookup(parent=1, name=bin) > 1/bin -> 140290677541808 > unique: 2, success, outsize: 144 > unique: 3, opcode: LOOKUP (1), nodeid: 140290677541808, insize: 45, pid: 20822 > lo_lookup(parent=140290677541808, name=bash) > 140290677541808/bash -> 140290677542048 > unique: 3, success, outsize: 144 > unique: 4, opcode: OPEN (14), nodeid: 140290677542048, insize: 48, pid: 20822 > lo_open(ino=140290677542048, flags=32768) > unique: 4, success, outsize: 32 > unique: 5, opcode: FLUSH (25), nodeid: 140290677542048, insize: 64, pid: 20822 > unique: 5, success, outsize: 16 > unique: 6, opcode: READ (15), nodeid: 140290677542048, insize: 80, pid: 20822 > lo_read(ino=140290677542048, size=32768, off=0) > unique: 6, success, outsize: 32784 > unique: 7, opcode: FLUSH (25), nodeid: 140290677542048, insize: 64, pid: 20822 > unique: 7, success, outsize: 16 > unique: 8, opcode: RELEASE (18), nodeid: 140290677542048, insize: 64, pid: 0 > unique: 8, success, outsize: 16 > > > However, when disabling readahead, the read size decreases to 4k: > > > $ example/passthrough_ll -d mnt > FUSE library version: 3.9.1 > unique: 1, opcode: INIT (26), nodeid: 0, insize: 56, pid: 0 > INIT: 7.27 > flags=0x003ffffb > max_readahead=0x00020000 > INIT: 7.31 > flags=0x0000f439 > max_readahead=0x00000000 > max_write=0x00020000 > max_background=0 > congestion_threshold=0 > time_gran=1 > unique: 1, success, outsize: 80 > unique: 2, opcode: LOOKUP (1), nodeid: 1, insize: 44, pid: 20911 > lo_lookup(parent=1, name=bin) > 1/bin -> 140509922200528 > unique: 2, success, outsize: 144 > unique: 3, opcode: LOOKUP (1), nodeid: 140509922200528, insize: 45, pid: 20911 > lo_lookup(parent=140509922200528, name=bash) > 140509922200528/bash -> 140510056418784 > unique: 3, success, outsize: 144 > unique: 4, opcode: OPEN (14), nodeid: 140510056418784, insize: 48, pid: 20911 > lo_open(ino=140510056418784, flags=32768) > unique: 4, success, outsize: 32 > unique: 5, opcode: FLUSH (25), nodeid: 140510056418784, insize: 64, pid: 20911 > unique: 5, success, outsize: 16 > unique: 6, opcode: READ (15), nodeid: 140510056418784, insize: 80, pid: 20911 > lo_read(ino=140510056418784, size=4096, off=0) > unique: 6, success, outsize: 4112 > unique: 7, opcode: READ (15), nodeid: 140510056418784, insize: 80, pid: 20911 > lo_read(ino=140510056418784, size=4096, off=4096) > unique: 7, success, outsize: 4112 > unique: 8, opcode: READ (15), nodeid: 140510056418784, insize: 80, pid: 20911 > lo_read(ino=140510056418784, size=4096, off=8192) > unique: 8, success, outsize: 4112 > unique: 9, opcode: READ (15), nodeid: 140510056418784, insize: 80, pid: 20911 > lo_read(ino=140510056418784, size=4096, off=12288) > unique: 9, success, outsize: 4112 > unique: 10, opcode: FLUSH (25), nodeid: 140510056418784, insize: 64, pid: 20911 > unique: 10, success, outsize: 16 > unique: 11, opcode: RELEASE (18), nodeid: 140510056418784, insize: 64, pid: 0 > unique: 11, success, outsize: 16 > > > > Is that intentional? If so, why? There are two cached read methods that linux filesystems can implement: readpage (mandatory) and readpages (optional). The second one is called by the readahead code, and that's the one which can generate multi page read requests. The first one is called in all other cases, so that's the one that will be called with readahead disabled. > > Is there any way to get larger read requests without also enabling > readahead? Only using direct I/O. Thanks, Miklos
