6Gb/s SAS with cache vs 12Gb/s without for JBOD software RAID

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ynari

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Sep 21, 2016
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I've an SAS drive, but no controller (yet), looking to change some SATA drives to SAS. (SSDs will likely stay SATA). Looking around there's a variety of options.

All my disks are currently SAS 6Gb/s or SATA III. I'm thinking it's probably better to get a controller with cache, than go for a 12Gb/s one without. The H330 is a cheap-ish 12Gb, but I imagine the cache will help things more.

I'll be running in JBOD mode, currently using mdraid with an SSD bcache, but moving to ZFS with L2ARC SSD cache.

Any advice? I know hardware RAID with a cache is a lot faster, I'm presuming it's exactly the same when using software RAID on JBOD, with a cache.
 

pricklypunter

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Nov 10, 2015
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A 12Gbps HBA will prove useful to you if you ever plan on running 12gbps SAS/ SATA SSD's, it's much less fruitful for spinning disks. If you get the chance of one going cheap, or you actually need the performance benefit then grab it. ZFS really wants direct hardware access and control, the less "other" hardware that gets in the way of that the better. ZFS will use an "on pool" cache if no other is available, or you can add in your own devices, like a SLOG or L2ARC to perform caching duties. You will want your SLOG device to be protected against power loss, so look for SSD's that have that feature. A UPS won't hurt either, but a proper PLP scheme in your SLOG, that guarantees end to end data integrity, is what you're after. Also, if you can, bump up your RAM first, before adding an L2ARC, adding one may actually hit your performance rather than boost it if you get the numbers wrong :)
 

T_Minus

Build. Break. Fix. Repeat
Feb 15, 2015
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If you're going with ZFS don't go with a RAID card with cache go with a plain HBA like LSI 3008 based cards, $60 to 150$ on ebay many 're-branded' with LSI chip.

I wouldn't worry about SAS3 HDD or SAS3 HBA for HDD but for SAS3 SSD they are nice :)

A SAS SSD will perform > than a SATA SSD of 'same spec' too.
 

_alex

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Jan 28, 2016
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sas3 also gives higher bandwidth if you use extender (Backplanes). so you can put a lot more sas or sata devices on the same number of lanes and maybe go with fewer hba's and fewer pcie slots occupied.
for zfs, don't use hw-raid with your drives in raid0 or such, go for an hba in jbod/it-mode.
 
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ynari

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Sep 21, 2016
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Great, clear answer then, thank you. SAS3, no cache!

I may get an SAS SSD at some stage, but they're a bit expensive at reasonable capacities, even second hand!

Now to confirm the HBA, there's some reports the PERC H330 isn't ideal for JBOD under FreeBSD, I'll have a search around..

At the moment I have a five disk SAS/SATA Icybox IB-555SK and a four 2.5" disk IB-2222SK on the way, but that's then the chassis completely full, unless I set up an external one with expanders etc as you say. I can live with eight disks as I'm looking to eventually move to a five disk RAIDZ2 with an L2ARC mirror, leaving one spare drive on an 8 channel SAS controller (without expanders), the other spare channel I can leave on SATA for testing and alternate boots..