Hacker News Reader

Comment by moe | Hacker News Reader

This is a myth started by someone who did not understand how filesystems work.

You are wrong. Don't spread potentially dangerous maladvice on the internet when you have no idea what you're talking about.

The official ZFS documentation[1] tells you to use ECC Ram and why. The first google hit for "zfs ecc ram"[2] further elaborates on the risks of using ZFS without ECC memory.

[1] https://pthree.org/2013/12/10/zfs-administration-appendix-c-...

[2] http://louwrentius.com/please-use-zfs-with-ecc-memory.html

View on HN

RAID is not a backup, zfs especially so. Zfs (and RAID) is designed to protect your data from a small, enumerable and specific set of failures, namely the right-there-in-the-acronymn "inexpensive disks".

If your house burns down ZFS will not save your data. If too many disks fail zfs will not save your data. If your disks' unrecoverable read error rate is too high and your array is rebuilding zfs will not save your data.

If your computer is fundamentally broken (which is what a machine with a persistent memory error is) zfs will not save your data.

Take backups.

If you had a 'stuck bit' anywhere in your memory space you'd be way deep into nasal demon unspecified behavior the first time you tried to dereference a pointer that crossed that bit. When your hardware is that broken you can't count on the OS to not stab your dog much less ZFS to do anything sane.

Note that this is just as true for all storage systems. Regular filesystems might corrupt themselves or do other insane things. Hardware RAID or kernel software raid will happily propogate the error. How many bits separate the kernel record for "this disk is totally cool" from "this is a new disk and should be zeroed"?

While it is possible to have an unimportable pool, it is also possible to have ext4 and XFS filesystems that can neither be mounted nor repaired with fsck. When dealing with undefined behavior caused by bit flips, virtually any failure is possible, especially when you consider bit flips to kernel data structures. No software can save you from bit flips and if they are your concern (as they should be), then you should refuse to use computers that lack ECC RAM.

As for the articles that you link, they are correct to say that you want to use ECC RAM. However, there is nothing specific to ZFS that makes it require ECC any more than any other filesystem. It should also be noted that I wrote the official documentation on this subject. It can be found at the Open ZFS wiki, rather than the pages you linked:

http://open-zfs.org/wiki/Hardware#ECC_Memory

I apologize. I was sure to be dealing with a troll and was sloppy with my sources (i.e. I was sure to have once read it in the official docs, but here I just linked the first official looking google hit that was not the solaris docs).

there is nothing specific to ZFS that makes it require ECC any more than any other filesystem

This statement still troubles me. I was under the assumption that ZFS scrubbing may indeed lead to compounding corruption as described in the blog post I linked. Hence the ongoing(?) debate in the ZFS community.

However, I consider myself disqualified from this discussion and sorry again for my tone. I'll read up on the current state of the argument.

It is easy to get confused and it could happen to anyone.

That said, the only thing in the code remotely similar to "compound corruption" is logic for preventing such a scenario, which has the following comment:

			/*
			 * Don't rewrite known good children.
			 * Not only is it unnecessary, it could
			 * actually be harmful: if the system lost
			 * power while rewriting the only good copy,
			 * there would be no good copies left!
			 */

https://github.com/zfsonlinux/zfs/blob/master/module/zfs/vde...

That touches more than obvious at a glance because the mirror code is used for processing both mirrors and ditto blocks.

Sorry for the very late reply that you probably won't see.

I'm not familiar with ZFS at all, so I don't know what the terminology 'ditto blocks' and 'mirrors' mean, so there's a good chance I'm not talking sense here at all.

The main argument for ZFS on non-ECC using systems being worse than other less sophisticated file systems seems to be that just reading a file (or during periodic 'scrubbing') while you have bad ram can cause good files to be corrupted when ZFS notices checksum errors and tries to correct them. (There are other arguments about not being able to mount pools due to bad ram, but that seems no different to any other FS).

People seem to be quite insistent that this is possible, and at first glance the code you linked above doesn't seem to be relevant since it refers to 'known good' data, but the problem scenario is when data has (erroneously) been determined to be bad due to faulty ram. Will ZFS overwrite good data in that situation in an attempt to correct what it sees as a disk error?

A few links where people are making this claim:

https://forums.freenas.org/index.php?threads/ecc-vs-non-ecc-...

The pthree.org article you linked to is my blog. It is not the official ZFS documentation nor the official ZFS on Linux documentation. It is only a storage administrator (me) explaining what I know of the inner workings of ZFS.

However, thanks for linking to my blog! :)

Reply on news.ycombinator.com