[mythtv-users] Poor performance with ZFS (ZFS on Linux)

Mark Wedel mwedel at sonic.net
Fri Jun 1 05:25:28 UTC 2018

On 05/31/2018 04:14 PM, Rajil Saraswat wrote:
> On 05/31/2018 04:38 PM, Mike Holden wrote:
>> If you aren't using dedup, then you can very easily get good performance with 
>> much lower memory.
>> Some tips on mysql tuning on zfs here, as well as other nuggets:
>> http://open-zfs.org/wiki/Performance_tuning
>> and here:
>> https://wiki.freebsd.org/ZFSTuningGuide
> Thanks, I did have these included in my setup but the performance was poor.

  I don't think you ever included a zpool status of your system, but if you 
assign the entire disk for a zpool (instead of using partitions), ZFS will 
recognize that and try to be faster.  From my recollection, these were the 2 
main things you get by doing that:

1) If ZFS is using the entire disk, it will enable write caching on the SSD, 
because it can then know the status of the cache, if it synchronizes the cache 
that data it expects is written out, etc.  If the disk is partitioned, ZFS can't 
be sure that something else isn't also accessing that disk and thus messing with 
those settings.

2) For SSD, ZFS will try to align its blocks with the memory cells of the drive. 
  Otherwise, especially if you have partitioned the disk, each time ZFS writes, 
it may end up accessing 2 memory cells, requiring 2-rewrites of those cells, etc.

  Since it sounds like you are running a mirror, you could try removing one 
drive from the mirror, making a pool on that entire disk and putting your 
databased on that and see if it performs better (it may not - I don't know).

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