ZFS - poor performance with "large" directories

[Freddie Cash]

On Thu, Nov 26, 2015 at 2:19 AM, krad <kraduk at gmail.com> wrote:

> true, but in my experience usb pen drives are variable in terms of
> performance across different sticks and different areas of the same stick.
> This can complicate things a little, and is often not worth the effort. You
> obviously run the ssd over usb though, and I still do on one server I run
> as I haven't been able to sort the down time yet.
>

​Nowadays, USB 3.x-based sticks in USB 3.x ports should be fast enough that
they'll be helpful.  You won't get the full 5 Gbps from one (unless you
spend as much or more than an SSD), but it will be much better than the
measly 0.5 Gbps of a USB 2.x stick/port.

Don't bother trying with a USB 2.x stick, or with anything plugged into a
USB 2.x port.  Invariably, it will just slow things down.

I used to use 8 GB USB2 sticks in USB2 ports for L2ARC (with a separate one
for the root filesystem).  When I had 4x IDE disks in a raidz1 vdev​, they
helped.  When I migrated to 4x SATA1 disks in a raidz1 vdev, they helped.
When I migrated to 4x SATA3 disks in dual-mirror vdevs (with root-on-ZFS),
suddenly the USB stick became the bottleneck.  Removing it actually made
the whole system faster (better throughput, more IOps, lower latency,
smoother system overall).

​As always, YMMV, and test it with your own setup.  :)​

-- 
Freddie Cash
fjwcash at gmail.com