ZFS...

Tue Apr 30 14:05:49 UTC 2019

Michelle Sullivan
http://www.mhix.org/
Sent from my iPad

> On 01 May 2019, at 00:01, Alan Somers <asomers at freebsd.org> wrote:
> 
>> On Tue, Apr 30, 2019 at 7:30 AM Michelle Sullivan <michelle at sorbs.net> wrote:
>> 
>> Karl Denninger wrote:
>>> On 4/30/2019 05:14, Michelle Sullivan wrote:
>>>>>> On 30 Apr 2019, at 19:50, Xin LI <delphij at gmail.com> wrote:
>>>>>> On Tue, Apr 30, 2019 at 5:08 PM Michelle Sullivan <michelle at sorbs.net> wrote:
>>>>>> but in my recent experience 2 issues colliding at the same time results in disaster
>>>>> Do we know exactly what kind of corruption happen to your pool?  If you see it twice in a row, it might suggest a software bug that should be investigated.
>>>>> 
>>>>> All I know is it’s a checksum error on a meta slab (122) and from what I can gather it’s the spacemap that is corrupt... but I am no expert.  I don’t believe it’s a software fault as such, because this was cause by a hard outage (damaged UPSes) whilst resilvering a single (but completely failed) drive.  ...and after the first outage a second occurred (same as the first but more damaging to the power hardware)... the host itself was not damaged nor were the drives or controller.
>>> .....
>>>>> Note that ZFS stores multiple copies of its essential metadata, and in my experience with my old, consumer grade crappy hardware (non-ECC RAM, with several faulty, single hard drive pool: bad enough to crash almost monthly and damages my data from time to time),
>>>> This was a top end consumer grade mb with non ecc ram that had been running for 8+ years without fault (except for hard drive platter failures.). Uptime would have been years if it wasn’t for patching.
>>> Yuck.
>>> 
>>> I'm sorry, but that may well be what nailed you.
>>> 
>>> ECC is not just about the random cosmic ray.  It also saves your bacon
>>> when there are power glitches.
>> 
>> No. Sorry no.  If the data is only half to disk, ECC isn't going to save
>> you at all... it's all about power on the drives to complete the write.
> 
> ECC RAM isn't about saving the last few seconds' worth of data from
> before a power crash.  It's about not corrupting the data that gets
> written long before a crash.  If you have non-ECC RAM, then a cosmic
> ray/alpha ray/row hammer attack/bad luck can corrupt data after it's
> been checksummed but before it gets DMAed to disk.  Then disk will
> contain corrupt data and you won't know it until you try to read it
> back.

I know this... unless I misread Karl’s message he implied the ECC would have saved the corruption in the crash... which is patently false... I think you’ll agree..

Michelle

> 
> -Alan
> 
>>> 
>>> Unfortunately however there is also cache memory on most modern hard
>>> drives, most of the time (unless you explicitly shut it off) it's on for
>>> write caching, and it'll nail you too.  Oh, and it's never, in my
>>> experience, ECC.
> 
> Fortunately, ZFS never sends non-checksummed data to the hard drive.
> So an error in the hard drive's cache ram will usually get detected by
> the ZFS checksum.
> 
>> 
>> No comment on that - you're right in the first part, I can't comment if
>> there are drives with ECC.
>> 
>>> 
>>> In addition, however, and this is something I learned a LONG time ago
>>> (think Z-80 processors!) is that as in so many very important things
>>> "two is one and one is none."
>>> 
>>> In other words without a backup you WILL lose data eventually, and it
>>> WILL be important.
>>> 
>>> Raidz2 is very nice, but as the name implies it you have two
>>> redundancies.  If you take three errors, or if, God forbid, you *write*
>>> a block that has a bad checksum in it because it got scrambled while in
>>> RAM, you're dead if that happens in the wrong place.
>> 
>> Or in my case you write part data therefore invalidating the checksum...
>>> 
>>>> Yeah.. unlike UFS that has to get really really hosed to restore from backup with nothing recoverable it seems ZFS can get hosed where issues occur in just the wrong bit... but mostly it is recoverable (and my experience has been some nasty shit that always ended up being recoverable.)
>>>> 
>>>> Michelle
>>> Oh that is definitely NOT true.... again, from hard experience,
>>> including (but not limited to) on FreeBSD.
>>> 
>>> My experience is that ZFS is materially more-resilient but there is no
>>> such thing as "can never be corrupted by any set of events."
>> 
>> The latter part is true - and my blog and my current situation is not
>> limited to or aimed at FreeBSD specifically,  FreeBSD is my experience.
>> The former part... it has been very resilient, but I think (based on
>> this certain set of events) it is easily corruptible and I have just
>> been lucky.  You just have to hit a certain write to activate the issue,
>> and whilst that write and issue might be very very difficult (read: hit
>> and miss) to hit in normal every day scenarios it can and will
>> eventually happen.
>> 
>>>   Backup
>>> strategies for moderately large (e.g. many Terabytes) to very large
>>> (e.g. Petabytes and beyond) get quite complex but they're also very
>>> necessary.
>>> 
>> and there in lies the problem.  If you don't have a many 10's of
>> thousands of dollars backup solutions, you're either:
>> 
>> 1/ down for a looooong time.
>> 2/ losing all data and starting again...
>> 
>> ..and that's the problem... ufs you can recover most (in most
>> situations) and providing the *data* is there uncorrupted by the fault
>> you can get it all off with various tools even if it is a complete
>> mess....  here I am with the data that is apparently ok, but the
>> metadata is corrupt (and note: as I had stopped writing to the drive
>> when it started resilvering the data - all of it - should be intact...
>> even if a mess.)
>> 
>> Michelle
>> 
>> --
>> Michelle Sullivan
>> http://www.mhix.org/
>> 
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