A possible ZFS ZIL Drive Solution

Patrick

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Thanks to a very kind reader and an awesome ebay deal, I have been able to acquire now 7x Pliant/ Sandisk Lightning SLC SSDs.

4x 200GB Plaint/ Sandisk Lightning
3x 400GB Pliant/ Sandisk Lightning

These are the drives used by NetApp, HP, Dell (+Equalogic) and etc. They use Micron 34nm SLC NAND so in terms of endurance, these things are tanks.

Rated endurance: Unlimited over the 5 year warranty period

There is no write cache so there is nothing stored in RAM in the event of a power failure.

From what I have seen thus far, these are not the fastest drives. On the other hand, with healthy over provisioning and SLC NAND (almost 22% of NAND is for wear leveling) they seem to be fairly durable drives.

Another benefit is that they are SAS drives so one does not worry about SATA-SAS tunnels.

Here is the kicker - they are not that much more expensive than SATA drives. For example these are $350 OBO for the 400GB drives (3 years old): Pliant Lightning Enterprise Class SSD 400GB LB406S 6GB s SAS 2 5 | eBay

That is $0.875/GB (usable.) These may seem expensive but are in the ballpark of higher end consumer SATA drives.
 

gea

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Hard to say, if I would use them as a ZIL, probably not due to the missing supercap that I would not like to miss.

As a ZIL it must compete to a Intel S3700 that is newer with a supercap, write optimized and the reference for a SSD ZIL at the same pricelevel - with a lower capacity (100 or 200 GB) but this does not matter for a ZIL where 10 GB is enough even for a 10 Gb network.

I would mainly prefer this SSD for SSD only datapools where the SAS connector is a plus especially in environments with an expander or in HA environments due its dual connectivity.
 

Patrick

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Hard to say, if I would use them as a ZIL, probably not due to the missing supercap that I would not like to miss.
Well, these are drives that do not need one. The super cap protects you in the case you are going from host -> SSD RAM -> NAND. A lot of companies started using this model to aggregate smaller writes into something that looks more sequential. These Sandisk/ Pliant drives do not have that write cache in RAM. So writes are host -> NAND.

The analogy using host controllers is this:
  • A drive that has write cache but no super cap is a RAID controller with write caching and no FBWC/ BBWC (flash backed write cache/ battery backed write cache)
  • A drive that has write cache and a super cap is a RAID controller with write caching backed by FBWC/ BBWC
  • A drive with no write cache is like a HBA

In that example, a Samsung 850 Pro is like the first. An Intel S3700 and Crucial M500/M550/MX100 would be examples of the second. These Sandisk/ Pliant drives are an example of the third.

The big difference with the 206s and 406s are that the NAND has much higher write endurance from the 34nm process plus being SLC. You get a SAS interface and the other things that come with that (longer cables, better error correcting, full-duplex and etc)
 

gea

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As I understand the problem

A SSD is a highly complex and highly parallelized system with a firmware/ intelligence that does a lot of background reorganisations and optimisations all the time.

A ZIL the other hand is a very special kind of storage device with very special demands. The most important is, that when a write is commited it must be safe even in case of a power outage and even when the SSDs is doing something with the data afterwards in the background. The usual way to ensure this is a supercap. In the specs of the SSDs there is nothing about such a special use case.

There may be SSDs with a supercap where this is not guaranteed and there is the possibility that a SSD without a supercap can guarantee this as well. All SSDs that are usually recommended for a ZIL have a supercap - even the SLC ones.
 
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Jeggs101

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There may be SSDs with a supercap where this is not guaranteed and there is the possibility that a SSD without a supercap can guarantee this as well. All SSDs that are usually recommended for a ZIL have a supercap - even the SLC ones.
I think @Patrick and @gea are talking about different parts of the problem.

@gea - FYI - the Lightning drives have no need for the super cap since they are a write directly to NAND. They are not the highest performance drives around, but they eliminate the DRAM cache as a point of failure.

@Patrick - the reason that these were the standard enterprise SSDs before Hitachi rolled though the market was because of the SAS interface and the write consistency, even getting chunks via RAID arrays. Performance with single drives was nothing spectacular but Pliant had the technology to write across shelves of disks. That's why these were and are popular SAN system disks but you rarely see them in web hosts.

OK and maybe the other reason is that 2 of these new in RAID 1 cost more than an entire dual Xeon system with lower end enterprise drives.

I think the S3700 is more of an enterprise drive where you hook a few up to SATA ports. These Sandisk-pliants are enterprise drives where you put 1000 of them in a system and they can cope with that kind of environment. If you look at the insides via SR you can see why these are huge. We had a big NetApp array with over 400 of these drives and they are big, heavy and reliable.
 

gea

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No doubt, they are perfect data SSDs.

The question is, are they good for a log device for sync writes (ZIL) where you need to look at general behaviour together with other aspects like background tasks example wear leveling and garbage collection on a power failure.

Real question is if they are not much better suited as data SSDs. This is what they are build for. For normal storage use, you do not need a ZIL as a ZIL is used only on sync writes. In such a case, you can pair them with ZIL like a 8GB ZeusRAM or a 100 GB Intel S3700.
 
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Patrick

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OK so some initial benchmarks with the 200GB SLC drives single drive configs:
Sandisk Pliant LB206s - AS SSD.JPG Sandisk Pliant LB206s - ATTO benchmark.JPG Sandisk Pliant LB206s - CrystalDiskMark.JPG

No clue why the single drive CDM tests are so bad.

And two drive RAID 0
Sandisk Pliant LB206s RAID 0 - AS SSD.JPG Sandisk Pliant LB206s RAID 0 - ATTO benchmark.JPG Sandisk Pliant LB206s RAID 0 - CrystalDiskMark.JPG

I have the 400GB SLC versions and the Seagate Pulsar.2 200GB drives in the testbed right now.
 

Chuntzu

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Thanks for the benchmarks, these were of the lb206m drives right? My hope is that the pulsar.2 drives will be similar, fingers crossed!
 

anosh123

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Real question is if they are not much better suited as data SSDs. This is what they are build for. For normal storage use, you do not need a ZIL as a ZIL is used only on sync writes. In such a case, you can pair them with ZIL like a 8GB ZeusRAM or a 100 GB Intel S3700.