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Your internship is going great. You have gained experience with C programming,
you have experienced your first segmentation faults, and youve come out on top.
You are brimming with confidence and ready to handle your next challenge.
Implementing mdadmwrite
Your next job is to implement write functionality for mdadm and then thoroughly
test your implementation. Specifically, you will implement the following function:
int mdadmwrite
uintt startaddr, uintt writelen, const uintt writebuf
Recall that the command for writing is JBODWRITEBLOCK see
ReadMeLabpdf from Lab
As you can tell, it has an interface that is similar to that of the mdadmread
function,which you have already implemented. Specifically, it writes
writelen bytes from the usersupplied writebuf buffer to your storage
system, starting at address startaddr. You may notice that the writebuf
parameter now has a const specifier. We put the const there to emphasize that it is
an in parameter; that is mdadmwrite should only read from this parameter
and not modify it It is a good practice to specify const specifier for your in
parameters that are arrays or structs. Similar to mdadmread, writing to an
outofbound linear address should fail. Write larger than bytes should fail; in
other words, writelen can be at most. On success return the amount of
bytes written.
There are a few more restrictions that you will find out as you try to pass the tests.
Once you implement the above function, you have the basic functionality of your
storage system in place. We have expanded the tester to include new tests for the
write operations, in addition to existing read operations. You should try to pass
these write tests first.
Implementing mdadmwrite permissions
The cryptocurrency startup that you are interning at is also very concerned about
the security of their storage systems. To ensure that data is written by the correct
people and at the correct times, you will also be required to write the following
functions to set the write permissions of the storage system:
int mdadmwritepermissionvoid;
int mdadmrevokewritepermissionvoid;
As you can see, there are no parameters for these functions. Each of these will tell
your storage device whether writing is allowed at that point.
mdadmwritepermission will tell the system that writing is now allowed.
mdadmrevokewritepermission will turn off write permissions to the
system; that is it will tell the system that writing is no longer allowed.
As you might expect, you must turn write permission ON before writing to the
system. You should also always check if this permission is on each time you write
to the system.
Just as all other functions that you have been required to implement, you will need
to use jbodoperationopblock to interact with the storage system.
The following ENUM commands are provided to you:
JBODWRITEPERMISSION: sets the write permission to so that writing will
be allowed. When the command field of op is set to this, all other fields in op are
ignored by JBOD driver. The block argument is passed as NULL.
JBODREVOKEWRITEPERMISSION: sets the write permission to so that
writing will no longer be allowed. When the command field of op is set to this, all
other fields in op are ignored by JBOD driver. The block argument is passed as
NULL.
HINT: check your write permission in your code before writing.
Testing using trace replay
As we discussed before, your mdadm implementation is a layer right above JBOD,
and the purpose of mdadm is to unify multiple small disks under a unified storage
system with a single address space. An application built on top of mdadm will
issue a mdadmmount, mdadmwritepermission and then a series of
mdadmwrite and mdadmread commands to implement the required
functionality, and eventually, it will issue mdadmunmount command. Those
readwrite commands can be issued at arbitrary addresses with arbitrary payloads
and our small number of tests may have missed corner cases that may arise in
practice.
Therefore, in addition to the unit tests, we have introduced trace files, which
contain the list of commands that a system built on top of your mdadm
implementation can issue. We have also added to the tester a functionality to replay
the trace files. Now the tester has two modes of operation. If you run it without any
arguments, it will run the unit tests:
$ tester
If you run it with w pathname arguments, it expects the pathname to point to a
trace file that contains the list of commands. In your repository, there are three
trace files under the traces dir
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