Messages in this thread | | | Date | Fri, 27 Apr 2007 11:31:17 -0700 | From | Andrew Morton <> | Subject | Re: [ext3][kernels >= 2.6.20.7 at least] KDE going comatose when FS is under heavy write load (massive starvation) |
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On Fri, 27 Apr 2007 08:18:34 -0700 (PDT) Linus Torvalds <torvalds@linux-foundation.org> wrote:
> echo 5 > /proc/sys/vm/dirty_background_ratio > echo 10 > /proc/sys/vm/dirty_ratio
That'll help a lot.
ext3's problem here is that a single fsync() requires that ext3 sync the whole filesystem. Because
- a journal commit can contain metadata from multiple files, and if we want to journal one file's metadata via fsync(), we unavoidably journal all the other file's metadata at the same time.
- ordered mode requires that we write a file's data blocks prior to journalling the metadata which refers to those blocks.
net result: syncing anything syncs the whole world.
There are a few areas in which this could conceivably be tuned up: if a particular file doesn't currently have any metadata in the commit, we don't actually need to sync its data blocks: we could just transfer them into next commit. Hard, unlikely to be of benefit.
Arguably, we could get away without syncing overwritten data blocks. Users would occasionally see older data than they otherwise would have after a crash. Could help a bit in some circumstances.
But none of this explains a 20-minute hang, unless a *lot* of fsyncs are being performed, perhaps.
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