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SubjectRe: Ext4 and the "30 second window of death"
On Wed, Apr 01, 2009 at 04:12:21PM +0100, Matthew Garrett wrote:
> On Wed, Apr 01, 2009 at 06:20:50AM +0100, Sitsofe Wheeler wrote:
>
> > Just out of curiosity, when laptop mode is happening is there a
> > guarantee that writes to other files won't be reordered to before the
> > fsync?
>
> laptop-mode does two things - tweak the dirty page semantics slightly
> (not in an interestingly relevant way) and call sys_sync() a few seconds
> after something hits disk rather than cache. In contrast to Ted's
> suggestion that laptop-mode reduces data integrity, it actually enhances
> it by opportunistically ensuring that data hits disk. It's the
> lengthening of the commit intervals that usually accompanies it that
> increases the risk of data loss.

It *can* reduce data integrity; it really depends on how it's tuned
and what scenario you're talking about. To the extent that it uses
sys_sync(), it could help in some cases as well, since filesystems
that do delayed allocation will wake up when the commit interval
fires, and then force out all writes to the disk, yes. But before the
commit interval, there is an increased risk of data loss --- which the
user requested.

The other subtlety comes if we add fsync() suppression to laptop mode
--- which is something that Bart Samwel is very interested in doing
and I talked to him at FOSDEM about this. As Jeff Garzik recently
pointed out, however, if we let the system reorder writes across
fsync() boundaries, or if we combine two writes to the same block
separated by an fsync(), and the system crashes in the middle of
pushing all of these blocks out to the disk, we can end up trashing
the consistency guarantees of a database such as mysql or postgres.
It's a good point, but it only applies if we add fsync() suppression
to laptop mode --- which we haven't done yet.
- Ted


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