supporting laptops fs-semantic changes (was Re: Ext4 and the "30second window of death")

[Linda Walsh]

Matthew Garrett wrote:
> > The other subtlety comes if we add fsync() suppression to laptop mode
-----
   Perhaps this has already been suggested, but rather than
adding all these semantics to the core file-system / kernel routines,
wouldn't it be preferable to allow some 'layering' of a pseudo,
memory-based file-system, OVER some 'real' file system (OR), definable
set of files (under a subdir...or same device...or whatever).

The semantics of when the virtual-fs would sync to the physical-fs/files
controlled via mount options.  Physical disk writes would be controlled by
selectively ignoring or honoring various "sync" events (time expired,
sync, fsync).

This could allow file-systems with different 'needs' (DB, or otherwise)
to be treated differently. 

The advantage of another layer, is you could define _how much_ buffering
you wanted to allocate to a filesystem (or file-set).  Maybe it's tolerable
losing a audio-recording of a talk, so large buff + don't sync 'cept when
full is fine.  Sensitive filesystems(or sets) (i.e. db's), could be set
with buffers to hold largest 'single-writes', but sync/fsyncs are what
they are.

An optimization could provide for read/writes through the user-mem 
controlled
buffered 'fs', to do direct I/O rather than into normal file-buffs where
possible, since presumably all accesses to a file would go through the
layer or not.

Wouldn't require application changing, and wouldn't require changing
well defined, lower-level kernel-filesystem operations.

Just a thought.
Linda

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