simplefs - a ridiculosly simple file system from the scratch

[Greg Freemyer]

On Wed, Aug 7, 2013 at 6:07 PM, Greg Freemyer <greg.freemyer at gmail.com> wrote:
> On Tue, Aug 6, 2013 at 11:06 AM, Sankar P <sankar.curiosity at gmail.com> wrote:
>>> One recommendation: add support for page-cache. Start with read-cache
>>> only, and then mmap support, you need that to allow binary execution.
>>>
>>
>> oh okay. You suggest that I should do this before I start implementing
>> the support for extents ?
>>
>> Sankar
>
> Sankar,
>
> Do a strace on just about any program and you will see mmap is used
> heavily.  Extents is an internal nicety, but mmap is core
> functionality that you would need even on a simpleFS for an embedded
> system.

I just read your todo's.

Multi-block files should come before extents and/or mmap support.  I
guess you realize that ext2 and ext3 are NOT extent based filesystems.
 ext4 is the first extent based one.

So its:
ext2 - block based filesystem
ext3 - block based with a journal
ext4 - extent based with a journal

In reality the current ext4 driver has a matrix of supported features.
 There are flag bits which tell you which features a specific
filesystem instance supports.  You may want to atleast look at those
feature flags to get an idea of the kinds of features ext4 supports.

If you don't have lseek() support for read, you need to get it added.
lseek() is pretty trivial for reads so there is not much reason not to
have it as the very next step.

For write, it means you have to implement a read/modify/write cycle in
your filesystem.  I gather you don't have that.

With just those todo's I go in this order:

- lseek() support with read (trivial)
- partial block write support (requires read/modify/write logic, so not trivial)
- lseek() support for write (trivial now you have partial block write support)
- multi-block support, requires a major refactor most likely.

Greg