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Is there any way to make the system more consistent when using LUKS? (or slow storage in general) as it is everything is snappy until the write buffer is full, then everything grinds to a halt as the kernel blocks writes. Same issue on my laptop with the slow SSD - its fine, then I have to wait for 30 seconds while it flushes, meanwhile I can do nearly nothing. I'm hoping to tune the disk cache system? Alternatively, if I can get things to not completely cease while blocked, instead only blocking the write that's actually being blocked?

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I ran into similar issues, turns out my hdd has bad blocks –  warl0ck Sep 11 '12 at 7:05
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3 Answers

There are a couple options. You can use ionice to set priorities for certain things. You can also try a different elevator, deadline would probably make more sense in your case:

http://www.redhat.com/magazine/008jun05/features/schedulers/

http://wlug.org.nz/LinuxIoScheduler

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Try:

echo 100 > /proc/sys/vm/dirty_ratio

This should postpone the blocking until all your free memory is used for write cache.

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I wouldn't set it to 100 - this will eat into your buffer cache for caching blocks read from disk, making the system even slower if memory is full of dirty blocks and every read means a trip to the disk (which is already busy trying to write dirty pages). Also, you don't want to fill all available memory making it necessary to swap some pages to disk if an application needs more RAM, adding to the I/O load. The default is 10, but used to be 40 - try 40. There's generally diminishing returns - 80 won't usually be twice as good as 40. –  Johnny Apr 10 '13 at 21:44
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Add

elevator=as

to your kernel line in /boot/grub/grub.conf.

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A short explanation of what that does and how that could help here would be nice. –  Mat Jan 2 at 18:23
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