Make the kernel start write-out earlier

Similar to the recent post about setting the vm.min_free_kbytes value there’s another sysctl that might improve the behaviour: the dirty ratio.

To cite a bit of an lwn.net article:

Imagine a machine with lots of memory – say 100Gig. […]
Suppose this filesystem is being written to steadily so that the maximum amount of memory is always dirty. With the default vm.dirty_ratio of 40%, this could be 40Gig.[…]
Waiting for 40gig to flush for an atime update to complete is clearly unsatisfactory.

Now this isn’t entirely correct anymore; recent kernels do some tuning of these values, but you get the gist.

Even if you have a super-fast storage subsystem, you’ll have to process that data – and if you use DRBD for that filesystem, DRBD has to push that data through your network to the other node(s) – which will take some time, even with 10Gbit Ethernet …
Another advantage is that you will not as easily get into OOM situations.

So, the simple conclusion: for (possibly) heavily I/O loaded systems, set some sane value for the vm.dirty_background_bytes sysctl; again, see the recent post for more details.

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