On May 28, 2012, at 12:46 PM, Lionel Cons wrote:

> On Mon, May 28, 2012 at 9:06 PM, Iwan Aucamp <aucam...@gmail.com> wrote:
>> I'm getting sub-optimal performance with an mmap based database (mongodb)
>> which is running on zfs of Solaris 10u9.
>> System is Sun-Fire X4270-M2 with 2xX5680 and 72GB (6 * 8GB + 6 * 4GB) ram
>> (installed so it runs at 1333MHz) and 2 * 300GB 15K RPM disks
>> - a few mongodb instances are running with with moderate IO and total rss
>> of 50 GB
>> - a service which logs quite excessively (5GB every 20 mins) is also
>> running (max 2GB ram use) - log files are compressed after some time to
>> bzip2.
>> Database performance is quite horrid though - it seems that zfs does not
>> know how to manage allocation between page cache and arc cache - and it
>> seems arc cache wins most of the time.
>> I'm thinking of doing the following:
>> - relocating mmaped (mongo) data to a zfs filesystem with only metadata
>> cache
>> - reducing zfs arc cache to 16 GB
>> Is there any other recommendations - and is above likely to improve
>> performance.
> The only recommendation which will lead to results is to use a
> different OS or filesystem. Your choices are
> - FreeBSD with ZFS
> - Linux with BTRFS
> - Solaris with QFS
> - Solaris with UFS
> - Solaris with NFSv4, use ZFS on independent fileserver machines
> There's a rather mythical rewrite of the Solaris virtual memory
> subsystem called VM2 in progress but it will still take a long time
> until this will become available for customers and there are no real
> data yet whether this will help with mmap performance. It won't be
> available for Opensolaris successors like Illumos available either
> (likely never, at least the Illumos leadership doesn't see the need
> for this and instead recommends to rewrite the applications to not use
> mmap).

This is a mischaracterization of the statements given. The illumos team
says they will not implement Oracle's VM2 for valid, legal reasons. 
That does not mean that mmap performance improvements for ZFS 
cannot be implemented via other methods.

The primary concern for mmap files is that the RAM footprint is doubled.
If you do not manage this via limits, there can be a fight between the 
page cache and ARC over a constrained RAM resource.
 -- richard

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