Walter Cramer wfc at mintsol.com
Tue Apr 30 16:08:03 UTC 2019

Brief "Old Man" summary/perspective here...

Computers and hard drives are complex, sensitive physical things.  They, 
or the data on them, can be lost to fire, flood, lightning strikes, theft, 
transportation screw-ups, and more.  Mass data corruption by faulty 
hardware or software is mostly rare, but does happen.  Then there's the 
users - authorized or not - who are inept or malicious.

You can spent a fortune to make loss of the "live" data in your home 
server / server room / data center very unlikely.  Is that worth the time 
and money?  Depends on the business case.  At any scale, it's best to have 
a manager - who understands both computers and the bottom line - keep a 
close eye on this.

"Real" protection from data loss means multiple off-site and generally 
off-line backups.  You could spend a fortune on that, too...but for your 
use case (~21TB in an array that could hold ~39TB, and what sounds like a 
"home power user" budget), I'd say to put together two "backup servers" - 
cheap little (aka transportable) FreeBSD systems with, say 7x6GB HD's, 
raidz1.  With even a 1Gbit ethernet connection to your main system, savvy 
use of (say) rsync (net/rsync in Ports), and the sort of "know your data / 
divide & conquer" tactics that Karl mentions, you should be able to 
complete initial backups (on both backup servers) in <1 month.  After that 
- rsync can generally do incremental backups far, far faster.  How often 
you gently haul the backup servers to/from your off-site location(s) 
depends on a bunch of factors - backup frequency, cost of bandwidth, etc.

Never skimp on power supplies.


[Credits:  Nothing above is original.  Others have already made most of my 
points in this thread.  It's pretty much all decades-old computer wisdom 
in any case.]

On Tue, 30 Apr 2019, Michelle Sullivan 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.
>> 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.
> 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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