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

[Stephen Baker]

On Tue, 2018-05-29 at 19:48 -0500, Rajil Saraswat wrote:
> Hello,
> 
> I have setup a new setup with root on ZFS. Unfortunately, mariadb is
> performing quite poorly on ZFS compared with an Ext4 file system.  I
> have filed a issue about this at
> https://github.com/zfsonlinux/zfs/issues/7573. If anybody else using
> mythtv with ZFS successfully please can you share your mysql config
> (/etc/mysql/my.cnf).
> 
> Thanks

I have no direct experience with ZFS on Linux. All of my experience is
on Solaris.

I assume that you have a separate dataset for the database where the
block size matches the I/O block size of the database.

There are a number of reasons that ZFS will perform poorly. The top two
reasons that I experienced are zpool >80% full and elevator seeks by
the disk driver. In the first case, the free space allocation switches
from track allocation to block allocation, which causes high
fragmentation in the storage; sequential reads of a file become random
reads on the disk. There is no need to belabor the performance
differences between sequential and random I/O. In the second case, high
locality of disk operations will cause I/O starvation for the locality
outliers. The recommendations that I have are keep zpool <80% full a/o
change the appropriate kernel tuneable, and change the disk optimizer
from elevator to FIFO.

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