[PATCH v6 0/3] ubi: Add block interface

[Artem Bityutskiy]

On Mon, 2014-02-17 at 07:48 -0300, Ezequiel Garcia wrote:
> On Mon, Feb 17, 2014 at 12:19:52PM +0200, Artem Bityutskiy wrote:
> > Thanks!
> > 
> > On Sun, 2014-02-16 at 17:03 -0300, Ezequiel Garcia wrote:
> > > The main target of this patch is supporting lighter, read-only
> > > filesystem,
> > > by putting a squashfs on top of ubiblock. And since squashfs already
> > > provides
> > > a file cache (which is even configurable), a first uncached
> > > implementation
> > > it's good enough for now.
> > 
> > Doesn't Linux provide a fake inode for _every_ block device, and uses
> > page-cache to cache pages?
> > 
> > There is even entire special-purpose file-system for managing these fake
> > block device inodes, see fs/block_dev.c
> > 
> > static struct file_system_type bd_type = {
> >         .name           = "bdev",
> >         .mount          = bd_mount,
> >         .kill_sb        = kill_anon_super,
> > };
> > 
> > I am really not sure which other cache do we need. The only thing coming
> > to my mind is about emulating a HW disk cache in software. But I would
> > not go that far without a really good reason.
> > 
> 
> If we ever re-add the write support, it would be natural to implement it
> using a 1-LEB cache. Block sectors are much much smaller than LEB's
> (around three orders of magnitude), so we would want to read an entire
> LEB into a 1-LEB buffer, update the buffer, and then write it back.
> 
> In this scheme, it's almost straightforward to keep this buffer around
> as a cache, and only write-back to disk when the new read/write requests
> asks for a different LEB.
> 
> IIRC, this is the main reason why we had the 1-LEB cache.

May be. But you are submitting R/O driver. And yet talk about caches and
write support in several places. Caches are only relevant to write
support. Write support is absent. Let's focus on what is present.

Feel free to put all the thoughts about a possible write support
implementation to a single big comment in the code. That's fine. I do
not object. I just want to focus on what is present.

-- 
Best Regards,
Artem Bityutskiy