simplefs - a ridiculosly simple file system from the scratch

[Sankar P]

On Thu, Aug 8, 2013 at 3:55 AM, Greg Freemyer <greg.freemyer at gmail.com> wrote:
> 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.
>

Thank you so much for the detailed response.

I will go in this order as this seem to be logically progressive nicely iiuc.

When do you think I should bring in support for mmap and page-caching
that Rajat in the above plan ?

Thanks.

-- 
Sankar P
http://psankar.blogspot.com