Hi,
On Mon, Jun 05, 2000 at 10:51:03AM -0600, Andreas Dilger wrote:
> It is mostly true that ext3 transactions need enough space reserved at the
> parent transaction for all nested transactions to complete. However, some
> transactions like write and truncate may be split into multiple transactions
> if they grow too large. This problem will disappear when ext3 moves to
> metadata-only journalling.
No it won't, even metadata-only journaling can created unbounded operations
for truncate(). That's OK, though --- essentially, you just treat truncate
as a series of smaller, atomic operations all of which leave the filesystem
in a completely consistent state.
To make truncate(2) atomic, the plan is to use the orphaned file list:
recovery can then complete the truncate based on the new value of the
file length.
> It is my understanding also that XFS and ext3 JFS are basically working in
> the same way. The real question would be if the XFS developers want to
> re-work their code to use ext3 JFS and/or if JFS is sufficiently abstract
> to be a plug-in replacement for the XFS journalling code.
XFS does its own, rather advanced, journaling, supporting things like
striping the journal over multiple disks. The entire point of having
a journaling abstraction is so that if we add that sort of functionality
to Linux journaling, all of the filesystems can take advantage of it
at once. There's no way it would be meant to be an *enforced* journaling
core, just a way of making sure that the hard problems only get solved
once.
> a bit of rework. AFAIK, the only real feature that JFS doesn't yet implement
> is allowing journalling of multiple filesystems to the same journal, although
> I know this is on Stephen's TODO list.
Yep. It's non-trivial due to the amount of user-space management code you
need to get it right in all cases (such as what you do if one of the
filesystems' disk dies --- do you suspend the whole journal, or carry
on but lose the ability to recover the dead filesystem?)
Cheers,
Stephen
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