[ros-kernel] AW: Re: AW: Re: making vfat asynchronous

[Hartmut B. <har...@te...>]

> > The current cache manger can only handle small memory segments. This
> > results in many calls to the fsd, when there is a read or write with 
large
> > datas, which are not in the cache.
> >
> This is not true; the fileystem can select the size of the segments which 

> are used.
The filesystem know the size. For a read operation the kernel seclect the 
correct size. When the datas are in the cache, there is a simple copy 
operation within memory. When the datas are not in the cache, the cache 
manger makes for each cache segment a call to the FSD for reading the 
segment. On FAT32 it is a call for a 4k page. This is result from moving 
the cache manager function from vfat to the kernel.

> > The cooperation betwen the cache and the memory manger isn't good. Each 

> > missing page results in a single call to the fsd for 4K page. The cache 

> > manager has no read ahead or delayed write  function.
> >
> I'm working on fixing this. Could you give more detail about what is
> missing in the cache manager to support asynchronous i/o.

The missing functions are not for asynchronous i/o. For a 'big' rewrite of 
the vfat driver, there needs some more of the Cc-functions (CcFlushCache, 
CcCanIWrite, CcDefereWrite, CcPurgeCacheSection, ...). Vfatfs must also 
implement some callback functions (AcquireForLazyWrite, 
ReleaseFromLazyWrite, AcquireForReadAhead, ReleaseFromReadAhead).

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