Usage
=====

.. contents::

How to use f3write and f3read
-----------------------------

If you prefer watching a video than reading the text below, check out
Spatry's Cup of Linux's video demo of F3 on YouTube
`here <https://www.youtube.com/watch?v=qnezKfCTO7E>`__.

My implementation of H2testw is composed of two applications:
``f3write``, and ``f3read``. ``f3write`` fills a file system up with 1GB
files named N.h2w, where N is a number. Whereas, ``f3read`` validates
those files. If the content of all N.h2w files is valid, the drive is
fine. The last file may be less than 1GB since ``f3write`` takes all
available space for data. Below the result on my fake card:

::

    $ ./f3write /media/michel/5EBD-5C80/
    Free space: 28.83 GB
    Creating file 1.h2w ... OK!
    Creating file 2.h2w ... OK!
    Creating file 3.h2w ... OK!
    Creating file 4.h2w ... OK!
    Creating file 5.h2w ... OK!
    Creating file 6.h2w ... OK!
    Creating file 7.h2w ... OK!
    Creating file 8.h2w ... OK!
    Creating file 9.h2w ... OK!
    Creating file 10.h2w ... OK!
    Creating file 11.h2w ... OK!
    Creating file 12.h2w ... OK!
    Creating file 13.h2w ... OK!
    Creating file 14.h2w ... OK!
    Creating file 15.h2w ... OK!
    Creating file 16.h2w ... OK!
    Creating file 17.h2w ... OK!
    Creating file 18.h2w ... OK!
    Creating file 19.h2w ... OK!
    Creating file 20.h2w ... OK!
    Creating file 21.h2w ... OK!
    Creating file 22.h2w ... OK!
    Creating file 23.h2w ... OK!
    Creating file 24.h2w ... OK!
    Creating file 25.h2w ... OK!
    Creating file 26.h2w ... OK!
    Creating file 27.h2w ... OK!
    Creating file 28.h2w ... OK!
    Creating file 29.h2w ... OK!
    Free space: 0.00 Byte
    Average Writing speed: 2.60 MB/s

    $ ./f3read /media/michel/5EBD-5C80/
                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ...       0/  2097152/      0/      0
    Validating file 2.h2w ...       0/  2097152/      0/      0
    Validating file 3.h2w ...       0/  2097152/      0/      0
    Validating file 4.h2w ...       0/  2097152/      0/      0
    Validating file 5.h2w ...       0/  2097152/      0/      0
    Validating file 6.h2w ...       0/  2097152/      0/      0
    Validating file 7.h2w ...       0/  2097152/      0/      0
    Validating file 8.h2w ...       0/  2097152/      0/      0
    Validating file 9.h2w ...       0/  2097152/      0/      0
    Validating file 10.h2w ...       0/  2097152/      0/      0
    Validating file 11.h2w ...       0/  2097152/      0/      0
    Validating file 12.h2w ...       0/  2097152/      0/      0
    Validating file 13.h2w ...       0/  2097152/      0/      0
    Validating file 14.h2w ...       0/  2097152/      0/      0
    Validating file 15.h2w ...       0/  2097152/      0/      0
    Validating file 16.h2w ...       0/  2097152/      0/      0
    Validating file 17.h2w ...       0/  2097152/      0/      0
    Validating file 18.h2w ...       0/  2097152/      0/      0
    Validating file 19.h2w ...       0/  2097152/      0/      0
    Validating file 20.h2w ...       0/  2097152/      0/      0
    Validating file 21.h2w ...       0/  2097152/      0/      0
    Validating file 22.h2w ...       0/  2097152/      0/      0
    Validating file 23.h2w ...       0/  2097152/      0/      0
    Validating file 24.h2w ... 1916384/   180768/      0/      0
    Validating file 25.h2w ...  186816/  1910336/      0/      0
    Validating file 26.h2w ...       0/  2097152/      0/      0
    Validating file 27.h2w ...       0/  2097152/      0/      0
    Validating file 28.h2w ...       0/  2097152/      0/      0
    Validating file 29.h2w ...   28224/  1705280/      0/      0

      Data OK: 1.02 GB (2131424 sectors)
    Data LOST: 27.81 GB (58322336 sectors)
               Corrupted: 27.81 GB (58322336 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average Reading speed: 9.54 MB/s
      

This report shows that my flash card is pretty much garbage since it can
only hold 1.02GB. ``f3write`` only writes to free space, and will not
overwrite existing files as long as they aren't named N.h2w. However, as
the previous report shows, files from 1.h2w to 23.h2w were written
before 24.h2w and yet had all their content destroyed. Therefore, it is
not wise to test nonempty cards because if the card has a problem, it
may erase the old files.

When ``f3read`` reads a sector (i.e. 512 bytes, the unit of
communication with the card), ``f3read`` can check if the sector was
correctly written by ``f3write``, and figure out in which file the
sector should be and in which position in that file the sector should
be. Thus, if a sector is well formed, or with a few bits flipped, but
read in an unexpected position, ``f3read`` counts it as overwritten.
Slightly changed sectors, are sectors at right position with a fews bits
flipped.

Notice that ``f3write`` doesn't overwrite sectors by itself, it's done
by the drive as a way to difficult a user to uncover its fault. The way
the drive overwrites sectors is arbitrary. From the point of view of a
file system, what ``f3read`` sees, the way the drive wraps around seems
often contrived, but, from the drive's view, it is just an address
manipulation.

The last lines of the output of ``f3write`` and ``f3read`` provide good
estimates of the writing and reading speeds of the tested card. This
information can be used to check if the claimed class of the card is
correct. Check this
`link <https://en.wikipedia.org/wiki/Secure_Digital#Speeds>`__ out for
more information about classes. Note that the speeds provided by F3 are
estimates, don't take them as perfect since they suffer influence even
from other processes in your machine. Also, be aware that your card
reader and USB port can limit the throughput of the drive.

Later I bought a second card that works just fine; I got the following
output running F3 on it:

::

    $ ./f3write /media/michel/6135-3363/
    Free space: 29.71 GB
    Creating file 1.h2w ... OK!
    Creating file 2.h2w ... OK!
    Creating file 3.h2w ... OK!
    Creating file 4.h2w ... OK!
    Creating file 5.h2w ... OK!
    Creating file 6.h2w ... OK!
    Creating file 7.h2w ... OK!
    Creating file 8.h2w ... OK!
    Creating file 9.h2w ... OK!
    Creating file 10.h2w ... OK!
    Creating file 11.h2w ... OK!
    Creating file 12.h2w ... OK!
    Creating file 13.h2w ... OK!
    Creating file 14.h2w ... OK!
    Creating file 15.h2w ... OK!
    Creating file 16.h2w ... OK!
    Creating file 17.h2w ... OK!
    Creating file 18.h2w ... OK!
    Creating file 19.h2w ... OK!
    Creating file 20.h2w ... OK!
    Creating file 21.h2w ... OK!
    Creating file 22.h2w ... OK!
    Creating file 23.h2w ... OK!
    Creating file 24.h2w ... OK!
    Creating file 25.h2w ... OK!
    Creating file 26.h2w ... OK!
    Creating file 27.h2w ... OK!
    Creating file 28.h2w ... OK!
    Creating file 29.h2w ... OK!
    Creating file 30.h2w ... OK!
    Free space: 0.00 Byte
    Average Writing speed: 4.90 MB/s

    $ ./f3read /media/michel/6135-3363/
                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ... 2097152/        0/      0/      0
    Validating file 2.h2w ... 2097152/        0/      0/      0
    Validating file 3.h2w ... 2097152/        0/      0/      0
    Validating file 4.h2w ... 2097152/        0/      0/      0
    Validating file 5.h2w ... 2097152/        0/      0/      0
    Validating file 6.h2w ... 2097152/        0/      0/      0
    Validating file 7.h2w ... 2097152/        0/      0/      0
    Validating file 8.h2w ... 2097152/        0/      0/      0
    Validating file 9.h2w ... 2097152/        0/      0/      0
    Validating file 10.h2w ... 2097152/        0/      0/      0
    Validating file 11.h2w ... 2097152/        0/      0/      0
    Validating file 12.h2w ... 2097152/        0/      0/      0
    Validating file 13.h2w ... 2097152/        0/      0/      0
    Validating file 14.h2w ... 2097152/        0/      0/      0
    Validating file 15.h2w ... 2097152/        0/      0/      0
    Validating file 16.h2w ... 2097152/        0/      0/      0
    Validating file 17.h2w ... 2097152/        0/      0/      0
    Validating file 18.h2w ... 2097152/        0/      0/      0
    Validating file 19.h2w ... 2097152/        0/      0/      0
    Validating file 20.h2w ... 2097152/        0/      0/      0
    Validating file 21.h2w ... 2097152/        0/      0/      0
    Validating file 22.h2w ... 2097152/        0/      0/      0
    Validating file 23.h2w ... 2097152/        0/      0/      0
    Validating file 24.h2w ... 2097152/        0/      0/      0
    Validating file 25.h2w ... 2097152/        0/      0/      0
    Validating file 26.h2w ... 2097152/        0/      0/      0
    Validating file 27.h2w ... 2097152/        0/      0/      0
    Validating file 28.h2w ... 2097152/        0/      0/      0
    Validating file 29.h2w ... 2097152/        0/      0/      0
    Validating file 30.h2w ... 1491904/        0/      0/      0

      Data OK: 29.71 GB (62309312 sectors)
    Data LOST: 0.00 Byte (0 sectors)
               Corrupted: 0.00 Byte (0 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average Reading speed: 9.42 MB/s
      

Since ``f3write`` and ``f3read`` are independent, ``f3read`` can be used
as many times as one wants, although ``f3write`` is needed only once.
This allows one to easily repeat a test of a card as long as the N.h2w
files are still available.

As a final remark, if you want to run ``f3write`` and ``f3read`` with a
single command, check out the shell script
```log-f3wr`` <https://github.com/AltraMayor/f3/blob/master/log-f3wr>`__.
This script runs ``f3write`` and ``f3read``, and records their output
into a log file. Use example:
``log-f3wr log-filename /media/michel/5EBD-5C80/``

.. raw:: html

   <div id="users_notes">

.. raw:: html

   </div>

Users' notes
~~~~~~~~~~~~

Randy Champoux has brought to my attention that ``f3read`` could
eventually read data from the system cache instead of from the flash
card. Since version 2.0, F3 eliminates this possibility as long as the
kernel honors the system call ``posix_fadvise(2)`` with advice
``POSIX_FADV_DONTNEED``. Linux has and honor
``posix_fadvise(2)/POSIX_FADV_DONTNEED``, the Mac does not have the
system call, and I don't know if Windows/Cygwin, or FreeBSD honors it.
In doubt about this issue, just disconnect and connect back the device
after ``f3write`` runs and before calling ``f3read``.

Notice that the issue pointed by Randy Champoux is entirely an OS
matter, that is, it doesn't change if the drive being tested is fake or
not. In 2014, I've run into a "smarter" fake card that tries hard to
behave like a good one using its internal cache to fool the test. In
practice, these newer cards can only mislead ``f3read`` with a limited
number of blocks, but those looking for a precise, repeatable report
should follow the advice of disconnecting and connecting back the device
before ``f3read`` runs. Consider the real example of a fake drive that
presents this behavior. The drive announces a size of 128GB but its real
capacity is less than 8GB:

::

    $ ./f3write --end-at=16 /media/michel/DISK_IMG/ && ./f3read /media/michel/DISK_IMG/
    Free space: 124.97 GB
    Creating file 1.h2w ... OK!                               
    Creating file 2.h2w ... OK!                           
    Creating file 3.h2w ... OK!                           
    Creating file 4.h2w ... OK!                           
    Creating file 5.h2w ... OK!                           
    Creating file 6.h2w ... OK!                           
    Creating file 7.h2w ... OK!                         
    Creating file 8.h2w ... OK!                         
    Creating file 9.h2w ... OK!                         
    Creating file 10.h2w ... OK!                         
    Creating file 11.h2w ... OK!                         
    Creating file 12.h2w ... OK!                         
    Creating file 13.h2w ... OK!                         
    Creating file 14.h2w ... OK!                         
    Creating file 15.h2w ... OK!                         
    Creating file 16.h2w ... OK!                        
    Free space: 108.97 GB
    Average writing speed: 2.87 MB/s
                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ... 2097152/        0/      0/      0
    Validating file 2.h2w ... 2097152/        0/      0/      0
    Validating file 3.h2w ... 2097152/        0/      0/      0
    Validating file 4.h2w ... 2097152/        0/      0/      0
    Validating file 5.h2w ... 2097152/        0/      0/      0
    Validating file 6.h2w ... 2097152/        0/      0/      0
    Validating file 7.h2w ... 2097152/        0/      0/      0
    Validating file 8.h2w ...  266176/  1830976/      0/      0
    Validating file 9.h2w ...       0/  2097152/      0/      0
    Validating file 10.h2w ...       0/  2097152/      0/      0
    Validating file 11.h2w ...       0/  2097152/      0/      0
    Validating file 12.h2w ...       0/  2097152/      0/      0
    Validating file 13.h2w ...       0/  2097152/      0/      0
    Validating file 14.h2w ...       0/  2097152/      0/      0
    Validating file 15.h2w ...       0/  2097152/      0/      0
    Validating file 16.h2w ...  523920/  1573232/      0/      0

      Data OK: 7.38 GB (15470160 sectors)
    Data LOST: 8.62 GB (18084272 sectors)
               Corrupted: 8.62 GB (18084272 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average reading speed: 12.73 MB/s
      

After disconnecting the drive and connecting it back, ``f3read``
produced the following output:

::

    $ ./f3read /media/michel/DISK_IMG/
                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ... 2097152/        0/      0/      0
    Validating file 2.h2w ... 2097152/        0/      0/      0
    Validating file 3.h2w ... 2097152/        0/      0/      0
    Validating file 4.h2w ... 2097152/        0/      0/      0
    Validating file 5.h2w ... 2097152/        0/      0/      0
    Validating file 6.h2w ... 2097152/        0/      0/      0
    Validating file 7.h2w ... 2097152/        0/      0/      0
    Validating file 8.h2w ...  266176/  1830976/      0/      0
    Validating file 9.h2w ...       0/  2097152/      0/      0
    Validating file 10.h2w ...       0/  2097152/      0/      0
    Validating file 11.h2w ...       0/  2097152/      0/      0
    Validating file 12.h2w ...       0/  2097152/      0/      0
    Validating file 13.h2w ...       0/  2097152/      0/      0
    Validating file 14.h2w ...       0/  2097152/      0/      0
    Validating file 15.h2w ...       0/  2097152/      0/      0
    Validating file 16.h2w ...       0/  2097152/      0/      0

      Data OK: 7.13 GB (14946240 sectors)
    Data LOST: 8.87 GB (18608192 sectors)
               Corrupted: 8.87 GB (18608192 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average reading speed: 12.50 MB/s
      

Notice that file 16.h2w, that last file ``f3write`` wrote, has no longer
good sectors. What happened is that the last sectors of 16.h2w were in
the internal cache of the drive when ``f3read`` ran right after
``f3write``, but were not there after the forced reset. The internal
cache will fool any test that doesn't write beyond the real capacity of
the drive plus the size of the internal cache, and does not hard reset
the drive. One can estimate the size of this cache as follows: 523920 \*
512B ~ 256MB.

Tom Metro once ran ``f3write`` on a 16GB flash drive formatted with ext2
file system, and obtained puzzling free space at the end of
``f3write``'s output:

::

    % ./f3write /media/Kodi/
    Free space: 14.50 GB
    Creating file 1.h2w ... OK!
    Creating file 2.h2w ... OK!
    Creating file 3.h2w ... OK!
    Creating file 4.h2w ... OK!
    Creating file 5.h2w ... OK!
    Creating file 6.h2w ... OK!
    Creating file 7.h2w ... OK!
    Creating file 8.h2w ... OK!
    Creating file 9.h2w ... OK!
    Creating file 10.h2w ... OK!
    Creating file 11.h2w ... OK!
    Creating file 12.h2w ... OK!
    Creating file 13.h2w ... OK!
    Creating file 14.h2w ... OK!
    Free space: 755.80 MB
    Average writing speed: 13.77 MB/s
      

This happened because ext2 and some other file systems reserve space for
special purposes. So they don't allow ``f3write`` to use that reserved
space. It's mostly safe to ignore that free space. If one wants to use
all space possible, there're two options: (1) using a file system that
doesn't reserve space (e.g. FAT), or (2) reducing the reserved space. How
to go for the second option depends on the used file system. The
`page <http://www.microhowto.info/howto/reduce_the_space_reserved_for_root_on_an_ext2_ext3_or_ext4_filesystem.html>`__
explains how to reduce the reserved space on ext2, ext3, and ext4 file
systems.

Elliot Macneille has ran into an application that reports the size of
one of its good flash cards as 15.71GB, whereas ``f3read`` only finds
14.63GB. Details on how space is accounted varies with operating system,
applications, file system used to format the drive, etc. However, there
is a common source for this problem that often explains most of the
difference: part of the computer industry (including F3) takes 1GB as
being 2^30 bytes, whereas the rest of the industry assumes that 1GB is
equal to 10^9 bytes. Some people use GiB for the first definition, but
its use is not universal, and some users even get confused when they see
this unit. With this information in mind, the mystery is easily solved:
14.63GiB \* 2^30 / 10^9 = 15.71GB.

When Art Gibbens tested a flash card hosted in a camera connected to his
Linux box, at some point F3 didn't show progress, and could not be
killed. After a reboot, the card was read only. Using an adapter to
connect his card directly to his machine, he recreated the partition of
the card, and successfully ran F3 with the card in the adapter. Thus,
Art's experience is a good warning if you're testing your card in a
device other than an adapter. Please, don't take it as a bug of F3. I'm
aware of only two things that can make a process "survive" a kill
signal: hardware failures, and/or bugs in the kernel. F3 doesn't run in
kernel mode, so Art's camera is likely the root of the problem.

Darrell Johnson has reported that a flash card he got stopped working
after he filled it up. This could be that the only memory chip the card
had died, but it is just speculation since Darrell was not able to
obtain more information. The important message here is that if you test
your card with F3, or just copy files into it, and it stops working,
it's not your fault because writing files to a card shouldn't damage it
if it is a legitimate card.

Username Kris,
`here <https://fixfakeflash.wordpress.com/2010/08/20/linux-h2testw-alternative-program-called-f3-by-michel%C2%A0machado/#comment-2234>`__,
asked what's the difference between "dosfsck -vtr /dev/sda1" and F3.
dosfsck(8) makes two assumptions that F3 does not: (1) one needs write
access to the device being tested, not the file system in it; (2)
hardware may fail, but it won't lie. The first assumption implies that
one likely needs root's rights to run dosfsck, what is just a small
inconvenience for simple uses. The second assumption is troublesome
because a fake card may be able to persuade dosfsck(8) to report it's
fine, or not report the whole problem, or give users the illusion the
memory card was fixed when it wasn't. I singled dosfsck(8) out because
of the question about it, but those two assumptions are true for fsck
software for other file systems and badblocks(8) as well.

Mac user Athanasios Tourtouras noticed that Spotlight of OS X, which
runs in the background, also indexes the content of removable drives.
Although Spotlight does not interferes with ``f3write/f3read``, its
indexing takes away around 2MB/s of bandwidth, so ``f3write/f3read``
will run slower as well as their speed measurements will underestimate
the real capacity of the drive. Not to mention that you likely don't
want to index test files. You can disable the indexing of removable
drives including the flash drive to Spotlight's exclude list by going to
System Preferences / Spotlight / Privacy.

.. raw:: html

   <div id="comp_h2testw">

.. raw:: html

   </div>

On the compatibility with H2testw's file format
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Starting at version 3.0, F3 generates files following H2testw's file
format. This feature should help users that use both applications and/or
interact with users that use the other application. However, there are
two caveats explained in this section that users should be aware.

**Verifying files created by H2testw with F3read.** The caveat here is
that H2testw only writes files whose size is a multiple of 1MB, whereas
F3write fills up the memory card. Thus, verifying files created by
H2testw with F3read works, but, likely, will not test approximately 1MB
of space that H2testw does not write.

**Verifying files created by ``f3write`` with H2testw.** The caveat here
is that H2testw requires that all files have sizes that are multiple of
1MB. When it is not the case for a single file, H2testw rejects all
files, and issues the message "Please delete files \*.h2w." This problem
often comes up with the last file ``f3write`` generates to fill up the
space available in the memory card being tested. The solution is to
truncate the size of the last file ``f3write`` generates to the closest
multiple of 1MB. Assuming the last file is 30.h2w, the following command
does exactly that:

::

    $ truncate --size=/1M /media/michel/6135-3363/30.h2w
      

If you want to exchange files with H2testw users often, check out the
shell script
```f3write.h2w`` <https://github.com/AltraMayor/f3/blob/master/f3write.h2w>`__.
This script calls ``truncate`` after ``f3write`` runs successfully.

f3probe - the fastest drive test
--------------------------------

H2testw's algorithm has been the gold standard for detecting fake flash
(see
`here <https://fightflashfraud.wordpress.com/2008/11/24/h2testw-gold-standard-in-detecting-fake-capacity-flash/>`__
and
`here <https://sosfakeflash.wordpress.com/2008/09/02/h2testw-14-gold-standard-in-detecting-usb-counterfeit-drives/>`__)
because it is robust against all counterfeits. However, as drives'
capacity grows, the time to test these newer drives becomes so painful
that one rarely runs H2testw's algorithm on a whole drive, but only a
fraction of it. See question "Why test only 25% or 32GB?" on `this
FAQ <https://web.archive.org/web/20180318154936/https://www.ebay.com/gds/All-About-Fake-Flash-Drives-2013-/10000000177553258/g.html>`__
for a defense of this approach.

The problem with this approach is that drives are still getting bigger,
and counterfeiters may, in the future, be able to profit with fake drives
whose real capacity are large enough to fool these partial tests. This
problem is not new. For example, Steve Si implemented
`FakeFlashTest.exe <https://www.rmprepusb.com/tutorials/-fake-usb-flash-memory-drives>`__,
which has successfully reduced the amount of time to test drives, and is
still able to give a good estimate of the real size of fake drives. Yet,
`FakeFlashTest.exe's
algorithm <https://rmprepusb.blogspot.com/2013/10/a-faster-test-for-fake-sd-cards-and-usb.html>`__
is not a definitive answer to the problem because FakeFlashTest.exe's
algorithm still needs to write to at least all good memory of tested
drives.

When a drive is fake, ``f3probe`` writes the size of the cache of the
drive a couple times, and a small number of blocks as the example in the
next section shows. From the example in the next section, the fake drive
has 7.86GB (16477879 blocks) of usable memory, but advertises itself as
being a 15.33GB (32155648 blocks) drive. It is worth noticing that given
that 7.86GB / 15.33GB ~ 51.2%, this fake drive already violates the 25%
recommendation mentioned above. ``f3probe`` wrote 2158 blocks to find
the real size of the drive, whereas FakeFlashTest.exe would have written
at least 16477879 blocks. That is, ``f3probe`` wrote no more than 2158 /
16477879 ~ 0.01% of all that FakeFlashTest.exe would have written. Even
if FakeFlashTest.exe wrote only 1% of the real size of the drive,
``f3probe`` would still write only 1% of what FakeFlashTest.exe would
write under this hypothetical, two-order improvement! Moreover,
``f3probe`` provides the exact geometry of the drive, what allows one to
"fix" the drive using the highest capacity possible.

When a drive is not fake, ``f3probe`` writes about half of its size, or
2GB, whichever is smaller. Thus, if the previous drive were not fake,
``f3probe`` would've written 2GB, and FakeFlashTest.exe 15.33GB. While
the difference 2GB / 15.33GB ~ 13.05% is much smaller, it is still
large. When a fake drive has some cache, ``f3probe`` will slow down, but
given that ``f3probe`` is optimized to deal with these cases it is still
fast. I do not know how FakeFlashTest.exe deal with drives that have
cache. If FakeFlashTest.exe simply ignores a drive's cache, it may over
estimate the size of fake drives.

This breakthrough against counterfeiters was only possible because
``f3probe``'s algorithm assumes a tight model of how fake drives work.
In spite of the fact that I have not found a drive, fake or not, that
confuses ``f3probe``, I've marked ``f3probe`` as experimental for now
because this model has not been battle proven. Although there is a
chance of finding out that the model is incomplete, there is also a
chance that the model can be simplified if one can be sure that not all
types of fake flash the model predicts really exist; the latter chance
holds a promise of even higher testing speeds. Of course, efficient
flash tests like the one implemented in ``f3probe`` may slow down as
fake chips become "smarter". For now, though, ``f3probe`` gives us the
upper hand over counterfeiters.

Finally, thanks to ``f3probe`` being free software, and once ``f3probe``
is battle proven, ``f3probe`` could be embedded on smartphones, cameras,
MP3 players, and other devices to stop once and for all the
proliferation of fake flash.

How to use f3probe
~~~~~~~~~~~~~~~~~~

Different from ``f3write/f3read`` that works on the file system of the
drive, ``f3probe`` works directly over the block device that controls
the drive. In practice, this means three requirements. First, one has to
have root access (i.e. superuser account) on the machine that will run
the test. Second, the user must know how to find the block device of the
drive. Third, you must be careful on the previous requirement to avoid
messing your machine up. If you don't have root access, you can't use
``f3probe``; use ``f3write/f3read`` in this case. The use example below
helps with the second requirement, but don't forget that you are the one
responsible for doing it right!

The command lsblk(8) is handy to find the block device of the drive. In
the example below, which I got running lsblk on my laptop, an experienced
user can quickly identify that my flash drive that is mounted at
"/media/michel/A902-D705" is block device "/dev/sdb". If you don't have
much experience, you may want to run lsblk before connecting the drive
to your computer, and to run lsblk again after connecting the drive to
easily identify what was added to the output of lsblk. Checking out the
content of folder "/media/michel/A902-D705" to confirm that it's the
correct drive is a good idea as well. The block device "sdb" is the disk
(see column "TYPE"), and "sdb1" is the first and only partition of my
flash drive; your drive may have none or more partitions. You want to
choose the drive, not a partition.

::

    $ lsblk 
    NAME   MAJ:MIN RM   SIZE RO TYPE MOUNTPOINT
    sda      8:0    0 232.9G  0 disk 
    +-sda1   8:1    0   218G  0 part /
    +-sda2   8:2    0     1K  0 part 
    +-sda5   8:5    0    15G  0 part [SWAP]
    sdb      8:16   1  15.3G  0 disk 
    +-sdb1   8:17   1  15.3G  0 part /media/michel/A902-D705
    sr0     11:0    1  1024M  0 rom  

If you get confused between "sdb" and "sdb1", don't worry, ``f3probe``
will report the mistake and point out the proper one. However, I cannot
emphasize it enough, you MUST identify the correct drive. If I had
chosen "sda", ``f3probe`` may have messed my computer. Once the device
is chosen, just prefix it with "/dev/" to obtain its full name.

Once you have carefully identified the device, you run ``f3probe`` like
in the example below (please use the correct device!):

::

    $ sudo ./f3probe --destructive --time-ops /dev/sdb
    [sudo] password for michel: 
    F3 probe 8.0
    Copyright (C) 2010 Digirati Internet LTDA.
    This is free software; see the source for copying conditions.

    WARNING: Probing normally takes from a few seconds to 15 minutes, but
             it can take longer. Please be patient.

    Bad news: The device `/dev/sdb' is a counterfeit of type limbo

    You can "fix" this device using the following command:
    f3fix --last-sec=16477878 /dev/sdb

    Device geometry:
                 *Usable* size: 7.86 GB (16477879 blocks)
                Announced size: 15.33 GB (32155648 blocks)
                        Module: 16.00 GB (2^34 Bytes)
        Approximate cache size: 0.00 Byte (0 blocks), need-reset=yes
           Physical block size: 512.00 Byte (2^9 Bytes)

    Probe time: 1'13"
     Operation: total time / count = avg time
          Read: 472.1ms / 4198 = 112us
         Write: 55.48s / 2158 = 25.7ms
         Reset: 17.88s / 14 = 1.27s

There is a lot in the previous example. First, it took one minute and 13
seconds for ``f3probe`` to identify that this 16GB drive had only
7.86GB. Second, I used command sudo(8) to run ``f3probe`` as root.
Third, I used option "--time-ops" to add the last four lines of the
output; these lines show the time taken to read, write, and reset the
drive during the test. The rest of this section covers the other aspects
of this output.

The option --destructive instructs ``f3probe`` to disregard the content
of the drive to speed up the test. Without option --destructive, one
would see a line "Probe finished, recovering blocks... Done" in the
previous output to let the user know that ``f3probe`` has recovered all
blocks in the drive to their original state. While the conservative mode
is very convenient, you should not rely too much on it. If ``f3probe``
crashes, the conservative mode won't work. Moreover, depending on the
fake drive, the conservative mode may not recover the drive to its exact
original state. In case you are running ``f3probe`` on a
memory-constrained computer (e.g. an old Raspberry Pi board), you can
still run it in conservative mode reducing the amount of memory needed
with option "--min-memory". If you don't have memory to test a large
drive even using option "--min-memory", you need to use option
"--destructive". The conservative mode is the default in the hope that
it will eventually save you from a mistake.

The line "Bad news: The device \`/dev/sdb' is a counterfeit of type
limbo" summarizes the results presented below this line. The types of
drives are good, damaged (seriously failing), limbo (the most common
type of fake drives), wraparound (a rare, if existing at all, type of
fake drives), and chain (a rare type of fake drives). If you ever find
wraparound and chain drives, please consider donating them to my
collection.

The probe time of 1'13" includes the time to run the probe algorithm,
take measurements, and the time to perform all operations on the drive.
But it doesn't include the time to recover the saved blocks (if this
feature is enabled). Therefore, the test would take roughly another
55.48s (i.e. total write time) to write all blocks back to the drive. As
some will notice, the time to perform all operations on the drive is
what dominates the probe time: 472.1ms + 55.48s + 17.88s = 1'13". It's
worth noticing that read and write speed estimates derived from the
times of these operations are not accurate because they mix sequential
and random accesses.

The next example gives you the chance to practice reading ``f3probe``'s
outputs:

::

    $ sudo ./f3probe --time-ops /dev/sdc
    [sudo] password for michel: 
    F3 probe 8.0
    Copyright (C) 2010 Digirati Internet LTDA.
    This is free software; see the source for copying conditions.

    WARNING: Probing normally takes from a few seconds to 15 minutes, but
             it can take longer. Please be patient.

    Probe finished, recovering blocks... Done

    Good news: The device `/dev/sdc' is the real thing

    Device geometry:
                 *Usable* size: 3.77 GB (7913472 blocks)
                Announced size: 3.77 GB (7913472 blocks)
                        Module: 4.00 GB (2^32 Bytes)
        Approximate cache size: 0.00 Byte (0 blocks), need-reset=no
           Physical block size: 512.00 Byte (2^9 Bytes)

    Probe time: 10'06"
     Operation: total time / count = avg time
          Read: 2'22" / 3724018 = 38us
         Write: 7'41" / 3719233 = 124us
         Reset: 379.7ms / 1 = 379.7ms

This second drive is a good one; it has all blocks necessary to hold its
announced size of 3.77GB, what is roughly 4GB.

The next section shows how to fix the 16GB drive using ``f3fix`` as
suggested by ``f3probe``.

Users' notes
~~~~~~~~~~~~

Philip de Lisle has a SD card reader on this laptop that is not backed
by a USB port. So when he tries ``f3probe /dev/mmcblk0``, he gets the
error message: ``Device `/dev/mmcblk0' is not backed by a USB device``.
This happens because the current version of ``f3probe`` only works on
devices that are mounted at a USB port; a future release of ``f3probe``
may lift this restriction. In the meanwhile, one can work around this
restriction using an external USB card reader.

How to "fix" a fake drive
-------------------------

You should not easily settle down for a fake drive, fight back and get
your money back! Doing so will help you and others. If you are still
reading this section, you already realized that you own a fake drive,
and would like to be able to use it without losing data.

As shown in the previous section, my 16GB fake drive can only hold
7.86GB. Moreover, ``f3probe`` suggested how I can use ``f3fix`` to fix
my drive. ``f3fix`` fixes fake drives creating a partition that includes
only the usable memory of the drive. ``f3fix`` takes a few seconds to
finish.

The execution of ``f3fix`` on my fake drive went as follows:

::

    $ sudo ./f3fix --last-sec=16477878 /dev/sdb
    F3 fix 8.0
    Copyright (C) 2010 Digirati Internet LTDA.
    This is free software; see the source for copying conditions.

    Error: Partition(s) 1 on /dev/sdc have been written, but we have been unable to inform the kernel of the change, probably because it/they are in use.  As a result, the old partition(s) will remain in use.  You should reboot now before making further changes.
    Drive `/dev/sdc' was successfully fixed

If ``f3fix`` reports that you need to force the kernel to reload the new
partition table, as shown above, just unplug and plug the drive back.
Once the new partition is available, format it:

::

    $ sudo mkfs.vfat /dev/sdb1
    mkfs.fat 3.0.26 (2014-03-07)

At this point your card should be working fine, just mount the new
partition to access it. However, before using the drive, test all its
blocks with ``f3write/f3read``. The test of my card went as follows:

::

    $ ./f3write /media/michel/8A34-CED2/
    Free space: 7.84 GB
    Creating file 1.h2w ... OK!                         
    Creating file 2.h2w ... OK!                         
    Creating file 3.h2w ... OK!                         
    Creating file 4.h2w ... OK!                         
    Creating file 5.h2w ... OK!                         
    Creating file 6.h2w ... OK!                         
    Creating file 7.h2w ... OK!                        
    Creating file 8.h2w ... OK!                        
    Free space: 0.00 Byte
    Average writing speed: 4.64 MB/s

    $ ./f3read /media/michel/8A34-CED2/
                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ... 2097152/        0/      0/      0
    Validating file 2.h2w ... 2097152/        0/      0/      0
    Validating file 3.h2w ... 2097152/        0/      0/      0
    Validating file 4.h2w ... 2097152/        0/      0/      0
    Validating file 5.h2w ... 2097152/        0/      0/      0
    Validating file 6.h2w ... 2097152/        0/      0/      0
    Validating file 7.h2w ... 2097152/        0/      0/      0
    Validating file 8.h2w ... 1763608/        0/      0/      0

      Data OK: 7.84 GB (16443672 sectors)
    Data LOST: 0.00 Byte (0 sectors)
               Corrupted: 0.00 Byte (0 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average reading speed: 16.54 MB/s

As reported by ``f3write/f3read`` above, the real memory of my fake
drive is in good shape. But it may not be the case for yours. For
example, the following is ``f3read``'s output for another 16GB drive
with real size of 7GB fixed as described in this section:

::

                      SECTORS      ok/corrupted/changed/overwritten
    Validating file 1.h2w ... 2097152/        0/      0/      0
    Validating file 2.h2w ... 2097152/        0/      0/      0
    Validating file 3.h2w ... 2097088/       64/      0/      0
    Validating file 4.h2w ... 2097152/        0/      0/      0
    Validating file 5.h2w ... 2088960/     8192/      0/      0
    Validating file 6.h2w ... 2097152/        0/      0/      0
    Validating file 7.h2w ... 2037632/        0/      0/      0

      Data OK: 6.97 GB (14612288 sectors)
    Data LOST: 4.03 MB (8256 sectors)
               Corrupted: 4.03 MB (8256 sectors)
        Slightly changed: 0.00 Byte (0 sectors)
             Overwritten: 0.00 Byte (0 sectors)
    Average reading speed: 946.46 KB/s

If you get some sectors corrupted, repeat the ``f3write/f3read`` test.
Some drives recover from these failures on a second full write cycle.
However, if the corrupted sectors persist, the drive is junk because
not only is it a fake drive, but its real memory is already failing.

Good luck!
