poke_loop_backed_fs_annotated.sh

# Results for https://gist.github.com/labeneator/6937753

# Show the mountpoint for the loopbacked fs
/tmp/zeros on /tmp/bah type ext2 (rw,loop=/dev/loop0)


# Meminfo stats before ddd

Cached:        1947592 kB
SwapCached:          0 kB
Dirty:               0 kB

**Sleeping for 10.. Waiting for I/O**
## Actual dd command
### 
##   $ dd if=blktrace of=/tmp/bah/blktrace oflag=sync
##   212+1 records in
##   212+1 records out
##   108740 bytes (109 kB) copied, 0.016489 seconds, 6.6 MB/s
##
##

# Meminfo stats after dd. Notice that the Cached value grows by the 'blktrace'  size from above '108k'
# Also note that we now have some dirty blocks. I believe that these are made up of the blktrace content + ext3 metadata + any other OS dirty buffers

Cached:        1947700 kB
SwapCached:          0 kB
Dirty:             160 kB

****

# What does the world look like if we view the loopback fs via VFS (which effectively get it's results from the pagecache)
# This is an ls -alisa /tmp/bah
total 309
      2   1 drwxr-xr-x  3 23142 xxxx   1024 Oct 11 17:30 .
8936929 188 drwxrwxrwt 12 root  root   188416 Oct 11 17:01 ..
     12 108 -rw-r--r--  1 root  root   108740 Oct 11 17:30 blktrace
     11  12 drwx------  2 root  root    12288 Oct  9 14:42 lost+found

# What does the world look like if we view the loopbacking file  via debugfs. 
# This reflects what's actually on disk
# Note that blktrace file is missing....

debugfs 1.39 (29-May-2006)
debugfs:        2   40755 (2)  23142    100    1024 11-Oct-2013 17:30 .
      2   40755 (2)  23142    100    1024 11-Oct-2013 17:30 ..
     11   40700 (2)      0      0   12288  9-Oct-2013 14:42 lost+found

debugfs:

****


# And we can see that it's in the pagecache

filename                                                                                       size        total_pages    min_cached page       cached_pages        cached_size        cached_perc
--------                                                                                       ----        -----------    ---------------       ------------        -----------        -----------
/tmp/zeros                                                                                1,048,576                256                  0                 37            151,552              14.45
---
total cached size: 151,552


# Running fadvise to evict /tmp/zeros from the page cache
****

Going to fadvise /tmp/zeros as mode POSIX_FADV_DONTNEED
offset: 0
length: 1048576
mode: POSIX_FADV_DONTNEED
WIN

# Post fadvise stats
Buffers:        522112 kB
Cached:        1947548 kB
SwapCached:          0 kB
Dirty:              12 kB

###### Running sync to awake pdflush and sleeping for 10 secs

# Page cache stats after eviction

filename                                                                                       size        total_pages    min_cached page       cached_pages        cached_size        cached_perc
--------                                                                                       ----        -----------    ---------------       ------------        -----------        -----------
/tmp/zeros                                                                                1,048,576                256                  0                  4             16,384               1.56
---
total cached size: 16,384

## What the backing fs looks like after pagecache eviction

debugfs 1.39 (29-May-2006)
debugfs:        2   40755 (2)  23142    100    1024 11-Oct-2013 17:30 .
      2   40755 (2)  23142    100    1024 11-Oct-2013 17:30 ..
     11   40700 (2)      0      0   12288  9-Oct-2013 14:42 lost+found
     12  100644 (1)      0      0   108740 11-Oct-2013 17:30 blktrace

debugfs: