How to speed up slow zpool scrub?

[Eric A. Borisch]

On Fri, Apr 29, 2016 at 9:28 AM, Ronald Klop <ronald-lists at klop.ws> wrote:
> On Fri, 29 Apr 2016 15:47:22 +0200, Karl Denninger <karl at denninger.net>
>> ZFS makes the *assumption*, fair or not, that everything in its
>> RAM-based caches is correct.  If that assumption is violated you will
>> eventually be a very sad Panda.  Use ECC memory or don't use ZFS.
>>
>
> Just like UFS makes an assumption about correct memory and correct disks?
>
> ECC helps ZFS as much as ECC helps UFS.
> And without ECC ZFS provides more failsafes than UFS. But nothing is
> perfect.
> You guys make it sound like ZFS has no added benefits if you don't use ECC,
> which is not true.
>
> UFS < ZFS < ZFS+ECC
> And UFS+ECC is somewhere in between probably.
> As long as people understand the risks/benefits things are ok.

I a key distinction is that UFS has fsck for attempting to repair
inconsistencies on the drive, where ZFS does not have a similar tool,
because "[t]he only way for inconsistent data to exist on disk in a
ZFS configuration is through hardware failure [...] or when a bug
exists in the ZFS software." [1]

So if ZFS fails, it is more likely to fail hard; enter the ECC (avoid
hardware failure) "requirement". I personally have one system running
without ECC, but it is a tiny system at home that serves as the
firewall the cable modem runs into. It is backed up and stores nothing
of real value on the media, but I love having ZFS on it because I can
do things like beadm for upgrades, or diffs of /etc files with
previous (automated) snapshots. (It's also running with less than 4G
of RAM, tsk-tsk...)

If you are storing data you care about* on a ZFS system without ECC,
you are doing it wrong. If that system *is* the backup, you are in a
gray area depending on your risk profile. (Is it OK if the backups
fail, because I still have the source and am willing to risk having
only one copy while I rebuild the backup?)

So I'd temper Karl's statement to "Use ECC memory -- or really
understand the risks -- or don't use ZFS."

 - Eric

[1] http://docs.oracle.com/cd/E19253-01/819-5461/6n7ht6r6p/index.html
* can't afford to lose / can't recreate