Shaun,
running these commands:
#sysctl vm.dirty_ratio
vm.dirty_ratio = 40
# sysctl vm.dirty_background_ratio
vm.dirty_background_ratio = 10
shows that these values are even higher by default. When you said RAID buffer size, you meant the controllers cache memory size?
Regards,
Strahinja
On Wed, Oct 10, 2012 at 3:09 PM, Shaun Thomas
<sthomas@optionshouse.com> wrote:
On 10/10/2012 02:12 AM, Strahinja Kustudić wrote:
total used free shared buffers cached
Mem: 96730 96418 311 0 71 93120
vm.dirty_background_ratio = 1
vm.dirty_ratio = 5
Again, these would go in sysctl.conf, or /etc/sysctl.d/10-dbserver.conf or something. If you have a newer kernel, look into vm.dirty_background_bytes, and vm.dirty_bytes.
The why of this is brought up occasionally here, but it comes down to your vast amount of memory. The defaults for even late Linux kernels is 5% for dirty_background_ratio, and 10% for dirty_ratio. So if you multiply it out, the kernel will allow about 4.8GB of dirty memory before attempting to flush it to disk. If that number reaches 9.6, the system goes synchronous, and no other disk writes can take place until *all 9.6GB* is flushed. Even with a fast disk subsystem, that's a pretty big gulp.
The idea here is to keep it writing in the background by setting a low limit, so it never reaches a critical mass that causes it to snowball into the more dangerous upper limit. If you have a newer kernel, the ability to set "bytes" is a much more granular knob that can be used to match RAID buffer sizes. You'll probably want to experiment with this a bit before committing to a setting. So it did a little swapping, but only minor, still I should probably
decrease shared_buffers so there is no swapping at all.
--
Shaun Thomas
OptionsHouse | 141 W. Jackson Blvd. | Suite 500 | Chicago IL, 60604
312-444-8534
sthomas@optionshouse.com
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