Comment by barrkel | Hacker News Reader

Comment by barrkel | Hacker News Reader

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    $ head -1 /proc/meminfo
    MemTotal:        4046880 kB

    $ grep model.name /proc/cpuinfo | head -1
    model name      : Intel(R) Core(TM)2 Quad CPU    Q6600  @ 2.40GHz

    $ sudo zpool list
    NAME   SIZE  ALLOC   FREE  EXPANDSZ   FRAG    CAP  DEDUP  HEALTH  ALTROOT
    tank  36.2T  27.2T  9.09T         -    10%    74%  1.00x  ONLINE  -

That's 10x 4TB WD Red drives in raidz2 configuration.

I've been running ZFS for the past 6 years, a little over 2 years on ZoL, and 4 years before that on Nexenta (Ubuntu userland with OpenSolaris kernel).

I've had 5 HDD failures over the past few years, never lost any data.

I've found ZoL to have less jitter when streaming video files in particular over SMB than the Solaris implementation did. ZoL is tighter on memory though.

raidz2 isn't particularly fast for random I/O (since it acts like it has a single spindle), but I don't do much random I/O. It's mostly video and local backup cache.

I'll be thinking about expanding my current setup soon enough. I'll probably be aiming for a raid10 setup for better random access, and of course the new system will have more RAM, and ECC.

Thanks. Beefier cpu than the beefier I got here, but RAM is a great hint that for personal needs ZFS can run on normal machines.

Interesting that you had no issues without ECC in the first place. Some ZFS boards are pretty against trying ZFS without.

ZFS boards really encourage you to use ECC because one of the nominal features of ZFS, "end-to-end checksumming", is not quite as useful if you have a point between ends where it won't notice silent data corruption (e.g. a bad stick of non-ECC RAM).

ZFS will notice, depending on _when_ the munging happened (if it was mangled after it got to RAM but before ZFS checksummed it, it obviously can't), but that's (primarily) why ZFS boards often tell you not to do that.

Not only that, but I believe a bad checksum can also cause a cascade to "fix the data", causing further corruption.

For that to happen, you would need a bad checksum and the corresponding data block to be corrupted in such a way that the corrupted data block checksums to the corrupted checksum. Given that the checksum is 256-bit, the scariest I'd call that is "possible in theory".

> Interesting that you had no issues without ECC in the first place. Some ZFS boards are pretty against trying ZFS without.

ZFS without ECC is no better or worse than any other filesystem without ECC, in that silent data corruption is possible. It is for this reason just a lot worse than ZFS with ECC, introducing a fault condition normally not seen on ZFS.

But that fault condition does not go away because you chose to skip ZFS due to no ECC. And you will get a lot of other problems on top that ZFS would have helped against, even without ECC.

Lastly, you'll end up back in the horrible land of static partitioning schemes with questionable tooling. Why suffer through that if you do not have to?

ZFS don't really need ECC that much, just FUD.

Most likely someone got bitten by faulty RAM, got corrupted data, which stopped after getting ECC RAM. => "ZFS wants ECC" myth, but in reality you just need stable hardware.

At the same time, if you are running like 30-40 TB pool, you probably want that ECC RAM, it will cost only small fraction of total storage box price and will save you from rebuilding a pool if shit happens.

It's not so much that ZFS needs ECC anymore than any other filesystem, but rather when you do have RAM errors, you are going to find out very quickly.

Other filesystems will happily let you write corrupt data blocks...

See my other comment a bit up in the thread. ZFS does data checksumming which relies on the ram not lying to you.

But at least with ZFS checksums you have a statistical chance of detecting bad RAM because it will sometimes manifest as checksum errors, whereas with non-checksumming filesystems you just get silent data corruption.

I don't know about anyone else, but when I'm seeking advice about software from people on the internet whom I don't know personally, I tend to weight the words of the authors of the software in question more highly than that of people who didn't write it. There are probably exceptions to that I'm currently forgetting.

And the authors of ZFS, from the Sun days on, have been consistently and repeatedly saying that if you care about your data, use ECC.

I personally don't get why people seem so resistant to using ECC. It isn't that much more expensive and we know these errors happen in around 8% of DIMMS[1]. It reminds me of people I know who refuse to buy a decent power supply and then complain about hardware failures because of shoddy power.

Personally, paying a slight premium to avoid an almost one in 10 likelihood of silent data corruption seems like a no-brainer to me. But then, I care about my data.

Edit: to be clear, ZFS will perform less dangerously with non-ECC ram than a filesystem that doesn't checksum with non-ECC, because it will detect Bad Things happening and tell you about it. ECC helps avoid the problems in the first place.

[1] http://research.google.com/pubs/pub35162.html

ECC isn't that much more expensive, if you already have a processor and motherboard that give you that option. If you're trying to use ZFS on surplus consumer Intel equipment, ECC is a major expense.

And aside from the cost issue, I don't think people are resistant to using ECC. You have probably just misinterpreted people who are justifiably shooting down ZFS/ECC scaremongering: Advising people to never use ZFS without ECC RAM is bad advice, because the advice should simply be to use ECC RAM if you want enterprise grade reliability, whether or not ZFS is part of the picture. Some people aren't in need of that level of reliability but can still benefit from ZFS, and they shouldn't be misled into thinking that ZFS has some particular need for ECC RAM.

I agree on the last part, and I think the particular weirdness here is a result of ZFS historically mentioning the value of ECC a lot more than other FSes. I think this probably makes people think that ZFS in some way depends on it more than others, rather than simply pointing out that there's a disturbingly high chance of encountering a problem without it that applies to everything.

And sure, if you're building a frankenbox, ECC is probably not an option, or the sort of expense that takes it out of the frankenbox category. I do hope that folks wouldn't store important data on dodgy hardware, but that is about more than RAM, and also none of my business.

I don't believe I'm misinterpreting people's reactions - I've witnessed people assert that ECC is a scam, waste of money and similar. Even after I point them to that link I posted above, they still seem to believe that It Won't Happen To Them. ("I don't have millions of machines.")

In any case, I still find it frankly bizarre that people run the risk with important data. If you asked people if they wanted to buy a CPU that had an ~8% chance of undetectably lying to them, I'm pretty sure the vast majority would at least want to spring for the premium one that allows at the least detecting the lie.

> I've witnessed people assert that ECC is a scam, waste of money and similar. Even after I point them to that link I posted above, they still seem to believe that It Won't Happen To Them. ("I don't have millions of machines.")

If by your own citation 92% of DIMMs operate with zero errors per year, and a consumer machine has at most four DIMMs, then it is actually pretty likely that any given consumer machine will operate without RAM errors even without ECC. And when you multiply by the low probability that a DRAM error will cause catastrophic data loss, then it is very easy to come to a reasonable conclusion that ECC is not worth the expense.

Hey, it is your data. I personally don't like gambling with mine, but yours is none of my business.

Again, I don't know how many people out there buy other products with a nearly 1 in 10 chance of undetectably not performing the function they are supposed to perform, but it seems nutty to me.

Although that line of thought does go some way towards explaining the vitamin business...

It's disingenuous of you to keep putting the error probabilities in terms like "nearly 1 in 10" without acknowledging the context that you're talking about the probability of a transient error occurring at any time over the course of a full year of continuous operation. Most people actually are comfortable with the idea that their equipment will have occasional downtime or faults, but you're trying to paint a very different picture.

Buddy, I would heartily encourage you to believe whatever you makes you happy. Your insult is false and petty, and I think you're pretty wrong about the rest of that.

I personally consider it pretty disingenuous to call a fault that damages data on disk 'transient'. The root cause may have been transient, but the damage doesn't go away if/when a stuck bit functions normally again. Or do you consider a stroke leading to paralysis a transient injury?

I'd also like to see a cite that "most people actually are comfortable with the idea that..." a 'transient' fault that scrambles random data they've chosen to keep. Where, exactly, are you getting this?

Finally, assuming you have some actual basis for that claim, how many of those people run ZFS? You wouldn't conflate nontechnical people who buy the cheapest box at Best Buy with folks who take the time to configure software RAID across several disks using a nonstandard filesystem, would you?

>And when you multiply by the low probability that a DRAM error will cause catastrophic data loss, then it is very easy to come to a reasonable conclusion that ECC is not worth the expense.

If your data is worth nothing, then ECC isn't worth the expense.

That's part of the difference between more enterprise level and consumer level equipment. I do some work on my home computer, but I don't have ECC in it. I probably push a few terabytes of work related information over it a year. The rest is many more terabytes of movies and music, and other things that will never notice a bit error. At work where I move 10s of terabytes of information a day, and that information may have cost many hundreds of manhours to create, I use enterprise level memory, disks, and other parts.

I've seen both servers and desktops develop bad ram. You want to know what the difference is when it happens? I get MCE logs from the server and we replace the equipment before anything bad happens. When it happens on the desktop you can end up with crashing programs, reboots, and even worse, corrupt data written to disk.

ZFS does depend on ECC more than other filesystems, in that it does data checksumming, which other filesystems (mostly) do not do. Those checksums can and will lie without ECC, in worst case rendering a data protective measure into a data losing measure.

"if you care about your data, use ECC."

This is great advice, but it has nothing to do with ZFS. It's true no matter what filesystem you use.

It's a good sense thing. Probably means "Don't take steps to use ZFS for data integrity if you don't use simpler, lower level precautions".

I don't think I've seen ZFS use much more than a single core. Apart from it being left over from upgrading desktop machines, I put that CPU in in there for video transcoding.

It's well worth while creating a bunch of sub-filesystems (something ZFS makes really easy) with different settings: block sizes, compression, etc. - and copying sample data to it, to tune things in. The defaults are not necessarily a good fit for everybody. There's no reason not to turn on lz4 compression, for example.

If you create zvols for use with different filesystems or sharing out over iSCSI, watch out for 100+% space consumption (e.g. 500G volume takes up 1+T space out of the pool, which is more than you would expect even with 8+2 raid overhead). I understand it relates to block vs stripe mismatch. This makes it a little less handy for things like virtual machine backing store than I had originally hoped.

I think the ECC myth has long ago been debunked no?

The ECC myth?

Issues related to not having ECC RAM are rare but if they happen, you might not notice the corrupted data, unless you try to access it.

Wow! That is a lot of porn!

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