SLC SSD really needed for SLOG?

[Thomymaster]

Hi

I was just wondering if i really need a SLC based SSD for a SLOG?

If you look for example on a Samsung Evo 840 SSD (which has MLC chips), it has:

-a MTBF of 1.500.00 hours
- 32.000/62000/70000 IOPS write (120/250/500GB)

Or you can even use a Evo 840 Pro, which has this SLC-Simulation.

So what is the key argument for using SLC-based SSDs as a SLOG device?


Cheers
Thomy

[Onichan]

You can use MLC or even TLC for a SLOG, the reason SLC is recommended is because of the amount of writes it has. MLC SSD will have at least 3000 writes which for a 120GB SSD is ~350TB (though tests show they last quite a bit longer than that). While it's quite a lot, you have to remember all writes will first go to the SLOG so for a busy NAS that really won't last all that long. While SLC has in the tens or hundreds of thousands for writes.

If this is a home NAS I don't see the point in even having a SLOG and in an small/medium business even 350TB might not last that long.

[Thomymaster]

No this is not a home NAS, but a business one. Ok i agree, but if i take wear-levelling into account here (which i think almost every modern SSD can do in hardware), do i still have this problem?

Is there a recommendation for a SLC SSD, i only find last-generation SSDS (250MB/s read/write, 7k IOPS @4K) and i think the SSD should have maximum IOPS @4K Writes, or is only the sequential write important here=

[Onichan]

My first post took into account wear leveling, which as far as I know the OS must send TRIM commands to the SSD otherwise it can't know when data is deleted as normally just the file table is erased for file deletes and not the actual space on the drive. TRIM tells the ssd to also erase the blocks themselves. Anyways even if you are properly sending TRIM it still doesn't change the fact the flash has a limited number of writes. Which is going to be a problem with SSDs, it's just a matter of when.

I haven't looked into server grade SSD's in a while, but a quick google makes it seem like they have started using MLC for server grade SSD's except they have a much higher number of writes then consumer grade ones and of course come with a higher price tag. Looking at newegg http://www.newegg.com/Enterprise-SSDs/S ... ?Tid=11691 I know Intel is good and have heard good things about Kingston, http://www.newegg.com/Product/Product.a ... 3ER1855853 with quick math it seems like it gets over 5000 writes which is quite a bit better than the consumer version. Then again with as high as the price is you could probably just buy a consumer one and when it wears out buy another consumer one and still end up being cheaper then that, but that is something you would need to decide.

Anyways any SSD you buy will be faster by a large amount than any HDD you could get. It really depends on what the drive is used for, but for a SLOG I would think high IO with decent writes is what you want. That drive has 2.4K for writes which is comparable to a 7 disk raid5 with 15k disks so I would say that is extremely good. It does have lower IO and read/writes then consumer ones, but it's designed for longer endurance. Though no matter if you go enterprise or consumer SSD I don't think it would be a bottleneck unless you are using a 10G NIC, which even then I don't think IO will be the issue, but the write/read speeds.

[ku-gew]

ALL writes go to SLOG or only syncs?

[Onichan]

You know what I initially thought it was all, but according to http://www.nexenta.com/corp/zfs-educati ... or-dummies and http://forums.freenas.org/threads/some- ... nas.13633/ it seems link only syncs, sorry about that. I am not that familiar with the ZIL/SLOG, I just know for a business you do need to take into account the amount of sync writes you will be doing and how much endurance the SSD you buy has. Of course as the articles say loosing the SLOG/ZIL isn't an issue as you only loose performance, but it most likely will be noticeable and will require you replacing the SSD.

Personally with the cost of enterprise SSD's I saw on newegg I would be tempted to just try consumer ones as you could end up buying 2-3 of them for the cost of one enterprise one (if you go this route I would just buy one now and wait till your SMART on the SSD says it has 25% or something endurance left then buy a new one. That way it should be cheaper when buying the 2nd one.) Though keep in mind not all SSDs are equal especially when under heavy load. The chart at the bottom of http://www.anandtech.com/show/6489/playing-with-op shows how different and erratic performance can be on different SSDs under long term load.

[b0ssman]

why do you think you need a SLOG for a home nas?

for most home nas usecases this is not required because you just dont get the kind of load that would require a SLOG

[ku-gew]

It is a business NAS.

ZIL can be added later, that means you can install everything, then use the zilstat.ksh script available somewhere online to check how many writes actually go to the ZIL and you decide.

[STAMSTER]

I'm also considering 16nm Synchronous MLC NAND SSD device - 240GB Patriot Blaze 555/535MB/s.
I think for a big enterprise only you are welcomed to use SLC based or so-called 'enterprise grade' drives...
For a decent load of your nas box (SMB) we could go with an MLC based drive...

[Lee Sharp]

I created a mirror for my ZIL device. Two cheaper SSDs and I check the SMAT warnings often. I had to replace one after about a year, but the other is fine.

[STAMSTER]

Why did you had to replace it? SMART errors or actually SSD had an failure?

[Lee Sharp]

Re-allocated sectors were climbing fast in SMART. Samsung stood behind it on the SMART errors.

[Onichan]

For a business I would say it's a good idea to preemptively replace it, but from the endurance testing http://techreport.com/review/27436/the- ... -petabytes it seems like a good brand SSD will last far longer than the SMART says it will. So you probably could just reuse it for something else non critical.

[STAMSTER]

"A good brand SSD" is very difficult to state these days.... even Intel breaks during warranty lifetime.

[Onichan]

Well nothings perfect that's the point of backups, but I would consider Intel, Samsung, Crucial as good brands and see no problem using them even past their SMART warning.

[Lee Sharp]

I replaced it because the vendor said they would replace it. New and free is good.