Re: Lazy block allocation and block_prepare_write?

[Nikita Danilov <nikita@clusterfs.com>]

Badari Pulavarty <pbadari@us.ibm.com> writes:

> On Tue, 2005-04-19 at 04:22, Nikita Danilov wrote:
>> Badari Pulavarty <pbadari@us.ibm.com> writes:
>> 
>> [...]
>> 
>> >
>> > Yes. Its possible to do what you want to. I am currently working on
>> > adding "delayed allocation" support to ext3. As part of that, We
>> 
>> As you most likely already know, Alex Thomas already implemented delayed
>> block allocation for ext3.
>
> Yep. I reviewed Alex Thomas patches for delayed allocation. He handled
> all the cases in his code and did NOT use any mpage* routines to do
> the work. I was hoping to change the mpage infrastructure to handle
> these, so that every filesystem doesn't have to do their thing.
>

Just keep in mind that filesystem != ext3. :-) Generic support makes
sense only when it is usable by multiple file systems. This is not
always possible, e.g., there is no "generic block allocator" for
precisely the same reason: disk space allocation policies are tightly
intertwined with the rest of file system internals.

>
>> 
>> >
>> > In order to do the correct accounting, we need to mark a page
>> > to indicate if we reserved a block or not. One way to do this,
>> > to use page->private to indicate this. But then, all the generic
>> 
>> I believe one can use PG_mappedtodisk bit in page->flags for this
>> purpose. There was old Andrew Morton's patch that introduced new bit
>> (PG_delalloc?) for this purpose.
>
> That would be good. But I don't feel like asking for a bit in page
> if there is a way to get around it.

Clarification: PG_mappedtodisk is already here, it seems you can reuse
this already existing bit to implement delayed allocation support.

>

[...]

>> >
> Need to think some more. I guess you thought about this more than you
> do :)
>
> Thanks,
> Badari
>

Nikita.