RAIDZ read speed half of write?

[Todd Takehana]

Before setting up a new NAS, mainly for Plex, I wanted to try FreeNAS 11.1 and some hardware I have sitting around.

Strangely I am getting about half the read speed as write speed when testing with dd or bonnie++

Setup is 4x3TB Hitachi 7200 ALA640 SATA HDDs connected via a Dell H200 Perc adapter in IT mode (HBA, no RAID) on a Supermicro X11-DPH-i motherboard with 2 x Xeon CPUs and 16 GB of ECC DDR4 RAM. Boot disk is a 32GB Sandisk USB drive. The hard drives are set up as a RAIDZ1 (~7.6TB).

With dd (where bs x counts > installed RAM), I get about 575 MB/s write speed. After clearing cache and reading back the the written file, I get about 230 MB/s read speed. On sequential transfers, each drive is around 160MB/s in either write or read. Thus, 575 MB/s write seems reasonable, but the read speed seems way off.

Any ideas on what might be causing this?

 

[Chris Moore]

Any ideas on what might be causing this?

Function of how ZFS works. You determine the performance level you need and reach that performance by adding vdevs.
How fast do you need / want the pool to read data because that is already faster than 1Gb networking?

Read the following resources:

Slideshow explaining VDev, zpool, ZIL and L2ARC
https://forums.freenas.org/index.ph...ning-vdev-zpool-zil-and-l2arc-for-noobs.7775/

Terminology and Abbreviations Primer
https://forums.freenas.org/index.php?threads/terminology-and-abbreviations-primer.28174/

Why not to use RAID-5 or RAIDz1
https://www.zdnet.com/article/why-raid-5-stops-working-in-2009/

 

[Chris Moore]

PS. Lots of links under the "Useful Links" button in my signature.

 

[Todd Takehana]

Function of how ZFS works. You determine the performance level you need and reach that performance by adding vdevs.
How fast do you need / want the pool to read data because that is already faster than 1Gb networking?

Thanks for the links. I'll take a look at vdevs. For networking I have 10GbE, so faster would be better :)

 

[Chris Moore]

For networking I have 10GbE, so faster would be better

Very roughly, four vdevs (drive groups) would be needed in the pool, depending on the number of drives in each vdev, and the performance level of the individual drives being used.