# [FreeNAS] ZFS is speed limited by cache disk port?



## SS (Jun 3, 2013)

Hi there,

I am running *F*reeNAS and using ZFS, *I* have 8 x 500 GB disks and 1 x 60 GB SSD for cache. The 8 x 500 GB disks are on a SAS controller with 6 Gbit speed on each SATA port, but the cache disk is on the motherboard_'_s controller which is 3 Gbit on the SATA port.

Does this *a*ffect performance? *W*ould it be recommended to get another small SATA controller with a SATA 6 Gbit port to put the cache drive on (the one *I* have is maxed out at 8 disks)? *O*r does this not *a*ffect performance at all?


Thanks


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## DutchDaemon (Jun 3, 2013)

Disclaimer: PC-BSD, DesktopBSD, *FreeNAS*, NAS4Free, m0N0WALL, pfSense, ArchBSD, kFreeBSD topics.


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## wblock@ (Jun 3, 2013)

The SATA 2 theoretical bandwidth limit is 300 M/second, not much slower than the fastest SSDs.  And those speeds are usually only seen in atypical uses. So it probably makes little difference. If it's a serious concern, connect the SSD to the faster controller, moving one of the hard drives to the motherboard controller. Ordinary hard drives don't come close to the SATA 2 speed limit anyway.


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## mav@ (Jun 4, 2013)

Not a direct answer to the question, but note that not many external controllers may beat on-board Intel AHCI SATA. In terms of IOPS (that may be important for fast caching SSD) I've seen none yet. What's about linear bandwidth, unless you are using a 10 Gbps NIC, 3/6 Gbps SATA interface difference is probably irrelevant any way.


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## SS (Jun 14, 2013)

mav@ said:
			
		

> Not a direct answer to the question, but note that not many external controllers may beat on-board Intel AHCI SATA. In terms of IOPS (that may be important for fast caching SSD) I've seen none yet. What's about linear bandwidth, unless you are using a 10 Gbps NIC, 3/6 Gbps SATA interface difference is probably irrelevant any way.



Thank you for the answer. I think the onboard controller is fast enough with IOPS, but *I* am planing on using a 4 x 1 Gbit network, so at full load *I* will be able to pull 4 Gbit. Do you think that SATA-3 will be a bottleneck/problem*?* (The server won't run on full load from the disks all the time, it is a VMware environment with 11 mixed Linux and Windows VM's)

Is it worth buying a PCI-E SATA-6 controller for the cache?


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## throAU (Jun 17, 2013)

Your disks won't keep up with SAS 6 gigabit.  Even a 60 gig_abit_ SSD will struggle to saturate SATA 2, 4 gigabit.  Bigger SSDs are pushing 6 gigabit, but small ones, no.

Irrespective of being able to pull 4 gigabit in theory, I very much doubt you will be doing that in reality with a VMware environment, as the I/O will be a lot more random and a lot less sequential.


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