BLKZEROOUT and the buffer cache

[Sitsofe Wheeler <sitsofe@xxxxxxxxx>]

Hi,

When using the BLKZEROOUT ioctl what should be the impact on buffer
cache data? Is the caller expected to invalidate the cache and if so
how?

The following program demonstrates the issue:

#!/usr/bin/python2
import fcntl
import os
import struct

def ioctl(fd, req, fmt, *args):
    buf = struct.pack(fmt, *(args or [0]))
    buf = fcntl.ioctl(fd, req, buf)
    return struct.unpack(fmt, buf)[0]

def file_is_null(f):
    CHUNKSIZE = 1 * 1024 * 1024
    null_chunk = "\0" * CHUNKSIZE
    f.seek(0)
    while True:
        chunk = f.read(CHUNKSIZE)
        if not(chunk):
            return True
        elif chunk != null_chunk:
            return False

def write_ones(f):
    CHUNKSIZE = 1 * 1024 * 1024
    ones_chunk = "\xff" * CHUNKSIZE
    f.seek(0)
    try:
        while True:
            f.write(ones_chunk)
    except IOError as e:
        if e.errno != 28:
            raise
    f.flush()

IOCTL_BLKFLSBUF = 0x1261
IOCTL_BLKGETSIZE64 = 0x80081272
IOCTL_BLKZEROOUT = 0x127f

# /dev/sde is a 2GByte fixed size Hyper-V VHDX file
fd = open('/dev/sde', 'w')
f = open('/dev/sde', 'r')
size = ioctl(fd, IOCTL_BLKGETSIZE64, 'L')
write_ones(fd)
os.fsync(fd)
print ioctl(fd, IOCTL_BLKZEROOUT, 'LL', 0, size)
os.fsync(fd)
print file_is_null(f) # Returns False
write_ones(fd)
print ioctl(fd, IOCTL_BLKFLSBUF, 'LL', 0, 0)
print ioctl(fd, IOCTL_BLKZEROOUT, 'LL', 0, size)
print file_is_null(f) # Returns True

# grep . /sys/block/sde/queue/* 2> /dev/null
/sys/block/sde/queue/add_random:1
/sys/block/sde/queue/discard_granularity:33554432
/sys/block/sde/queue/discard_max_bytes:4294966784
/sys/block/sde/queue/discard_zeroes_data:0
/sys/block/sde/queue/hw_sector_size:512
/sys/block/sde/queue/iostats:1
/sys/block/sde/queue/logical_block_size:512
/sys/block/sde/queue/max_hw_sectors_kb:512
/sys/block/sde/queue/max_integrity_segments:0
/sys/block/sde/queue/max_sectors_kb:512
/sys/block/sde/queue/max_segments:2048
/sys/block/sde/queue/max_segment_size:4096
/sys/block/sde/queue/minimum_io_size:4096
/sys/block/sde/queue/nomerges:0
/sys/block/sde/queue/nr_requests:128
/sys/block/sde/queue/optimal_io_size:0
/sys/block/sde/queue/physical_block_size:4096
/sys/block/sde/queue/read_ahead_kb:128
/sys/block/sde/queue/rotational:1
/sys/block/sde/queue/rq_affinity:1
/sys/block/sde/queue/scheduler:[noop] deadline cfq 
/sys/block/sde/queue/write_same_max_bytes:0

-- 
Sitsofe | http://sucs.org/~sits/
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