verify_blocks in PatternStomper should be direct

[akiradeveloper]

Currently, PatternStomper#verify_blocks opens the device with "rb" mode but it results in 4KB reads through page cache even if we use @block_size less than 4KB (e.g. 1 sector).

We should fix. Let's make it direct.

I tried using File::RDONLY | File::BINARY | File::DIRECT as the 2nd arg of File.open but it ends in error "Errno::EINVAL: Invalid argument -". I don't know why. Do you have a idea, Joe?

I am writing a new test to check unlikely path (RAM buffer read). But, it fails because of the issue above. I want to solve this

[akiradeveloper]

The minimum test to reproduce. hvg1-cache2g is just a linear device.

mode = File::RDONLY | File::BINARY | File::DIRECT
File.open("/dev/mapper/hvg1-cache2g", mode) do |io|
  io.read(512).size
end

This is the strace log.
read(5, ...) fails. The buffer (2nd arg) is strange.

open("/dev/mapper/hvg1-cache2g", O_RDONLY|O_DIRECT) = 5
fstat(5, {st_mode=S_IFBLK|S_ISVTX|0660, st_rdev=makedev(251, 3), ...}) = 0
fstat(5, {st_mode=S_IFBLK|S_ISVTX|0660, st_rdev=makedev(251, 3), ...}) = 0
ioctl(5, SNDCTL_TMR_TIMEBASE or TCGETS, 0x7fffa0a47c20) = -1 ENOTTY (Inappropriate ioctl for device)
read(5, 0x16de3a0, 512)                 = -1 EINVAL (Invalid argument)
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
close(5)                                = 0
getrlimit(RLIMIT_STACK, {rlim_cur=8192*1024, rlim_max=RLIM_INFINITY}) = 0
write(2, "fopen-test.rb:4:in `read'", 25fopen-test.rb:4:in `read') = 25
write(2, ": ", 2: )                       = 2
write(2, "Invalid argument - /dev/mapper/h"..., 43Invalid argument - /dev/mapper/hvg1-cache2g) = 43
write(2, " (", 2 ()                       = 2
write(2, "Errno::EINVAL", 13Errno::EINVAL)           = 13