Re: [Xen-users] Cheap IOMMU hardware and ECC support importance

[Gordan Bobic <gordan@xxxxxxxxxx>]

On 07/05/2014 11:33 AM, Kuba wrote:

> > > > > any other FS. I have the same feeling about ZFS as Gordan - once you
> > > > > start using it, you cannot imagine making do without it.
> > > > Why exactly is that?  Are you modifying your storage system all the
> > > > time
> > > > or making snapshots all the time?
> > > Yes, I take snapshots all the time. This way it's easy for me to
> > > revert VMs to previous states, clone them, etc. Same goes with my
> > > regular data. And I replicate them a lot.
> > Hm, what for?  The VMs I have are all different, so there's no point in
> > cloning them.  And why would I clone my data?  I don't even have the
> > disk capacity for that and am glad that I can make a backup.
> I tend to clone "production" VMs before I start fiddling with them, so
> that I can test potentially dangerous ideas without any consequences.
> Clones are "free" - they only start using more space when you introduce
> some difference between the clone and the original dataset. You can
> always 'promote' them so they become independent from the original
> dataset (using more space as required). Cloning is just a tool that you
> might or might not find useful.
This is, indeed, a most excellent point about a good, useful use of 
cloning in zfs. Thank you for pointing it out. :)
> > > Once is often enough for me and it happened more then once. If I
> > > hadn't done the checksumming myself, I probably wouldn't even have
> > > known about it. Since I started using it, ZFS detected data corruption
> > > several times for me (within a few years). But I don't own a data
> > > center :) Actual error rates might depend on your workload, hardware,
> > > probabilities and lots of other things. Here's something you might
> > > find interesting:
> > Sure, the more data about failures detected by checksumming we would
> > collect, the more we might be able to make conclusions from it.  Since
> > we don't have much data, it's still interesting to know what failure
> > rates you have seen.  Is it more like 1 error in 50TB read or more like
> > 1 error in 500TB or like 20 in 5TB?
> I don't count them, I'd say 1 in 10TB. But that's not professional
> research-grade statistical data, you shouldn't make decisions on it.
>
> > That there's a statistical rate of failure doesn't mean that these
> > statistical failures are actually seen in daily applications.
> >
> > > http://www.zdnet.com/blog/storage/dram-error-rates-nightmare-on-dimm-street/638
> > Yes, I've seen that.  It's for RAM, not disk errors detected through ZFS
> > checksumming.
> And RAM has nothing to do with the data on the disks.
Unless the bit flip happens in the write-out buffer before it is 
committed to disk.

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