Looking for recommendations on msata ssd for zfs slog

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pricklypunter

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Nov 10, 2015
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I have turned my attention to using a slog for my array. It's not that I really need one, things seem to be running reasonably well so far, but I am interested to see just how much of a difference I can make by adding it.

Here's what my setup currently looks like:

I am using a Debian 8 VM with ZoL and have created a RaidZ2 array consisting of 8 x 3TB HGST Ultrastars. I have created 3 ZVols on the array, one for my Plex server's media (7TB), and a couple for my Windows domain (4TB & 1.5TB). All 3 ZVols are being accessed via iscsi, because it's easy, from the other VM's. I am not currently using nfs for an ESXi VM datastore, or was planning to, but I may in the future with another array. The array total is 21.8 TB or thereabouts. The VM datastore resides on a 250GB Samsung 850Evo and ESXi boots from a USB pen drive. I have not set sync=always yet, but rather left it as standard/ inherited until I have a way to move the zil away from the array.

I only have space in the chassis for mSATA devices, unless I go metal bashing, and would love to find something with power loss protection. So far I have looked for Intel 320/ 730 in mSATA but coming up empty. Is there anything else available for reasonable money?
 

Biren78

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Not many mSATA have good write endurance and PLP. They're designed as laptop drives which don't need those features. I wonder if it'd be better to turn the mSATA into a SATA port then Velcro a 2.5" drive somewhere.
 

pricklypunter

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I hear ya. I have spent the better part of 2 days digging about and looking for something suitable that won't upset the rest of the build and is affordable. Trouble is, there really isn't anywhere in the chassis that is not either interfering with airflow or where I would feel safe having one stuck on, although in saying that, there might just be room to plunk one on top of the power supply, but heat might then possibly be an issue.

So far, the best fit I have found for the money is the 256GB Micron/ Crucial M600. It has async PLP, seems decent fast and has much better, allegedly, write endurance that all of it's predecessors. Well, according to this review anyway: Micron M600 SSD Review (256GB/1TB) - The SSD Review

Also, as I understand it from Micron's blurb, the drive does a sneaky SLC thingy on the die and begins converting data to an MLC layout once the disk becomes more than 50% full. So I figure, as it's only use will be a slog, I need probably a max of about 8-10GB, maybe a smidge more, so I can massively underprovision it and never really need to worry about write endurance becoming an issue. It has a 3 year warranty as well, not the best, but I doubt I'll need to make use of it :)
 

pricklypunter

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Cheers for that @Patrick

I very nearly pulled the trigger on one of these just a little while ago, but decided to wait on some more comments from the guys here before I commited myself. I have just looked at the datasheet from Micron for the M600 and although the sneaky buggers would have you believe that it has PLP, it's right there as a feature in the marketing brochure, the datasheet actually says it has power loss protection for data at rest, or in other words, anything not in flight. I don't see how they can claim the disk has PLP, seems like a real switcharoo on the end user :)

So back to the drawing board then...
 

cw823

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Jan 14, 2014
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80gb s3500 are not hard to find on ebay and shouldn't generate enough heat so make locating it in a creative fashion an issue.
 
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pricklypunter

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That's an ok drive for something like Intel Rapid Start in a laptop, but definitely has no place as a zil/ slog I'm afraid. The lack of PLP could, in the wrong circumstances, blow away your array, a risk I'm not willing to take with my data, plus it's also tiny so that limits my ability to underprovision it for write endurance and then there's the 3Gbps, and the IOPS, it's really just too slow for what I need imho :)
 

pricklypunter

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In the end, I went with an Intel S3500 80GB. Snagged it for $90 brand new in the box. I'll just need to be creative with the chassis placement :)
 
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_alex

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Jan 28, 2016
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hi, sorry for getting this old thread up ...
i'm currently also considering msata-ssd for zil, and looking around what the options are.

For PLP, here is stated that it's not necessary that inflight data is protected, and the micron are mentioned to be sufficient as slog/zil as they protect flushed data.

Hardware - OpenZFS

Does anyone have more insight if this can be considered to be right / does not put Data at risk on a power-failure?

also, any other recommendations for slog/zil in msata formfactor?
 

nitrobass24

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Dec 26, 2010
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Let me open up my NUC, I have been meaning to part it out since I did my new build. I know it has an M550 in it, will confirm PLP, shortly.
 

Patrick

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@Deslok the M550 does not have traditional power loss protection. Those capacitors do not back writes cached in SSD DRAM.
 
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_alex

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@Deslok the M550 does not have traditional power loss protection. Those capacitors do not back writes cached in SSD DRAM.
That it doesn't back the DRAM is known, i wonder what exactly is protected by the caps then.

As ZFS obviously does flush data written to ZIL / before data is assumed to be safe on the ZIL the question is if they can be assumed safe to use as SLOG or not.
 

Patrick

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That it doesn't back the DRAM is known, i wonder what exactly is protected by the caps then.

As ZFS obviously does flush data written to ZIL / before data is assumed to be safe on the ZIL the question is if they can be assumed safe to use as SLOG or not.
I believe they cannot be. If data is written to the M550 the drive will acknowledge the write when it hits the SSD DRAM but the data will not be protected from power loss until that data goes from SSD DRAM to NAND.
 

_alex

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Well, this is the question. If the drive honors a SATA Flush that should be issued by the ZIL Write.
So, it should acknowledge the write only when data is persisted to NAND and not DRAM.
This is at least as i understand it.

Also, if it would be acknowledged before a Flush happens, data written to ZIL that resides in the main pool and not an SLOG would never be safe, as it would be in the HDD`s Cache that is volatile.
 

cookiesowns

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That it doesn't back the DRAM is known, i wonder what exactly is protected by the caps then.

As ZFS obviously does flush data written to ZIL / before data is assumed to be safe on the ZIL the question is if they can be assumed safe to use as SLOG or not.
The caps probably provide enough protection to flush the mapping table from DRAM cache to the NAND in case of power failure during a rw event.

That said my one cent on mSATA SSD FOR ZIL...

If you are in a situation where you need a ZIL, then that means you care about performance. I highly don't recommend a non DC mSATA or regular SATA SSD as ZIL. Latency is key here.. You're basically shooting yourself in the foot with a non up to par SSD for ZIL.
 

_alex

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did some Research, these small caps obviously make sure the nand Programming is consistent in case of power loss. this might be 'good enough' for slog if the drive doesn't lie when acknowleding a flush.

https://www.micron.com/~/media/docu.../ssd_power_loss_protection_white_paper_lo.pdf

anyway, guess the m500 are not sufficient for zil in terms of endurance but haven't found any other in msata formfactor yet that would make sense as slog.
 

_alex

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guess these can be found, with a bit luck also in EU, but write iops are maybe far from optimal for slog?

assuming i put the array on 2x QDR, capacity to hold 5sec between txg would be ca. max 40gb - before overprovisioning.

edit: assuming the SSD could hold up with writing 2x QDR / 40GBit/s lol