I attached a USB key to the USB to try the L2ARC cache and also to use the drive that otherwise useless because it shows very bad write performances.
The model is KINGSTON DATATRAVELER 2.0 and some benchmark are found here:
http://usbflashspeed.com/search/?size=& ... AVELER+2.0
I have a 8 GB model. From the graphs, for 4 KiB reads the flash drive should achieve at least 2.5 MiB/s, probably even 4 MiB/s. Compared to the my WDC green drives, able to get about 0.35 MiB/s on 4 KiB random reads (actually 2x0.35 MiB/s, since I use a mirror setup), the improvement is significant.
The usual zpool iostat output is
- Code: Select all
capacity operations bandwidth
pool alloc free read write read write
--------------------------------------------- ------ ------ ------ ------ ------ ------
Thor 1024Gi 1.73Ti 6 2 162Ki 68.5Ki
mirror 1024Gi 1.73Ti 6 2 162Ki 68.5Ki
GPTE_34B525B3-6497-49AC-B8C7-8F82A9FF7B3C - - 1 1 81.5Ki 68.6Ki
GPTE_2C939C1F-D9AA-4675-A633-C1DF2180E750 - - 2 1 82.2Ki 68.6Ki
cache - - - - - -
GPTE_A59A8BEB-DA91-457C-81DD-DC82A2F6947C 6.72Gi 430Mi 0 0 1.93Ki 72.3Ki
--------------------------------------------- ------ ------ ------ ------ ------ ------
but if I try to work for example with Lightroom and I tag photos they don't change much, as expected: the database is probably already in memory and the job involves writes, not random reads.
What I would like to know or test is the effectiveness of the L2ARC: even if it is useless, I would like to be sure that it doesn't worsen performances.
Obviously I don't need it, my load on the HD is very light, I'm just experimenting.