Help test softupdates journaling (SUJ)
jroberson at jroberson.net
Sat Jan 9 10:09:50 UTC 2010
On Sat, 9 Jan 2010, Alexander Motin wrote:
> Jeff Roberson wrote:
>> I have been augmenting softupdates with a small journal that will be
>> processed in lieu of fsck in the event of a crash. I have written some
>> about this project here: http://jeffr_tech.livejournal.com/
> Sounds cool, but I have one question. Excuse my possible ignorance.
> I was looking for BIO_FLUSH consumers and haven't found UFS there.
> Unbacked write caching probably can make SoftUpdates unreliable, but it
> is bearable while foreground fsck is used. As I understand, journaled
> recovery is more dependent on data coherency, and so needs either
> unbacked write caching to be disabled, or BIO_FLUSH to be used in
> respective points by FS code. Am I right? So what's about BIO_FLUSH?
Softupdates definitely relies on proper disk ordering. People who want
reliability in the face of power failure need to buy nice disks and buy
battery backup systems. Many cheap disks lie about flush and this has
bitten ZFS. SU+J will still work with foreground fsck if you want to be
absolutely certain of your data in the event of a power outage.
It would be possible to implement a flush barrier in between writing the
journal and permitting the meta-data modifications, and again after
metadata modifications and before journal free. SU+J would be more
tolerant to out of order filesystem operations following the journal write
than vanilla softupdates. However, I'm not sure how much it will help,
and it is not part of my current plans. It is probably worthwhile to
> Alexander Motin
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