Re: Special Kernel Modification

[Andrea Arcangeli]

On Mon, Nov 05, 2001 at 12:52:51AM -0500, Jeff Dike wrote:
> [e-mail] said:
> > >  Mounting it synchronous will  disable caching in the VM.  
> > Who told
> > you that? Synchronous mounting turns off write buffering. Even with
> > "-o sync" writes will still end up in the page cache, they'll just be
> > commited immediately.
> 
> Ummm, how about O_DIRECT instead of O_SYNC (or maybe as well, my googling
> hasn't been clear on whether O_DIRECT bypasses the cache on writes as well)?

O_DIRECT is synchronous but only in terms of data, if you want the
metadata to be synchronous as well you need to open with
O_SYNC|O_DIRECT, and even in such case all the metadata reads will came
from cache.

For example if you only care about being able to reach the data after a
crash (not about the inode info) in a file with all its logical blocks
mapped to physical blcoks (no holes) and then you fsync, later you can
as well avoid O_SYNC and you still don't risk to lose data after a crash
because the block mappings never changes, if you grow/shrink the file
you definitely need O_SYNC to be sure the O_DIRECT data is still there
after a crash instead.

Andrea