On Wed, Nov 11, 2009 at 3:35 AM, David Vasek <va...@fido.cz> wrote:
> On Tue, 10 Nov 2009, Nick Guenther wrote:
>
>> [ext3 data= / FFS]
>> journal ~= sync (ensures consistency of both metadata and file data)
>> ordered ~= softdep (ensures consistency of metadata both internally
>> and with file data)
>> writeback ~= default (ensures consistency of metadata internally but
>> real file data may not agree, e.g. my empty file)
>> Additionally FFS has the async flag which turns off the internal
>> consistency of the metadata structures; I guess there's no equivalent
>> for this in ext?
>
> Isn't it rather
> default ~= async ?
>
> For ext2, at least.
>
Well I'm not sure because no one seems to really know. Linux's
mount(1) has this to say:
writeback
Data ordering is not preserved - data may be written
into
the main file system after its metadata has been
commitb
ted to the journal. This is rumoured to be the
highest-
throughput option. It guarantees internal file
system
integrity, however it can allow old data to appear
in
files after a crash and journal recovery.
which seems to imply that metadata is written synchronously (because
it only talks about data appearing in files, not about the whole
filesystem getting trashed).
And BSD's mount(1) says:
async Metadata I/O to the file system should be done asyn-
chronously. By default, only regular data is read/writ-
ten asynchronously.
This is a dangerous flag to set since it does not
guaran-
tee to keep a consistent file system structure on the
disk. You should not use this flag unless you are pre-
pared to recreate the file system should your system
crash. The most common use of this flag is to speed up
restore(8) where it can give a factor of two speed in-
crease.
