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

[lee <lee@xxxxxxxxxxxxxxx>]

Gordan Bobic <gordan@xxxxxxxxxx> writes:

>> On 07/02/2014 11:45 PM, lee wrote:
>>> On Linux there is for all intents and purposes one implementation.
>>
>> Where is this implementation?  Is it available by default?  I only saw
>> that there's a Debian package for ZFS which involves fuse.
>
> http://lmgtfy.com/?q=zfs+linux&l=1

Funny --- how many findings do you get?  A couple million?

'apt-cache search zfs' is *much* more relevant.

>> Does ZFS do that?  Since it's about keeping the data safe, it might have
>> a good deal of protection against user errors.
>
> I don't think it's possible to guard against user errors. If you're
> concerned about user errors, get someone else to manage your machines
> and not give you the root password.

It is possible to guard.  It's not possible to prevent them.

> You don't have to rebuild a pool. The existing pool is modified in
> place and that usually takes a few seconds. Typically the pool version
> headers get a bump, and from there on ZFS knows it can put additional
> metadata in place.
>
> Similar happens when you toggle deduplication on a pool. It puts the
> deduplication hash table headers in place. Even you remove the volume
> that has been deduplicated and don't have any deduplicated blocks
> afterwards, the headers will remain in place. But that doesn't break
> anything and it doesn't require rebuilding of a pool.

Then it should be easy to turn features off later.

>> You might enable a new feature and find that it causes
>> problems, but you can't downgrade ...
>
> You could have to _use_ a feature that causes problems just because
> it's available. And features that broken are rare, and non-critical.

And what do I do then?  Rebuild the pool to somehow downgrade to a
previous version of ZFS?

>>>> It seems that ZFS isn't sufficiently mature yet to use it.  I haven't
>>>> learned much about it yet, but that's my impression so far.
>>>
>>> As I said above - you haven't done your research very thoroughly.
>>
>> I haven't, yet all I've been reading so far makes me very careful.  When
>> you search for "zfs linux mature", you find more sources saying
>> something like "it is not really mature" and not many, if any, that
>> would say something like "of course you should use it, it works
>> perfectly".
>
> There's a lot of FUD out there, mostly coming from people who have
> neither tried it nor know what they are talking about. Whatever next?
> "It must be true because I read it on the internet"?

Perhaps nothing of what you're saying about ZFS is true ;)

>> IIRC, when I had the WD20EARS in software RAID-5, I got messages about
>> barriers being disabled.  I tried to find out what that was supposed to
>> tell me, and it didn't seem to be too harmful, and there wasn't anything
>> I could do about it anyway.  What if I use them as JBOD with ZFS and get
>> such messages?
>
> No idea, I don't see any such messages. It's probably a feature of
> your RAID controller driver.

I didn't even have a RAID controller when that happened.

>> So the wikipedia article about SATA is wrong?  Or how does that work
>> when any of the involved devices does not support some apparently
>> substantial parts of the SATA protocol?
>
> You misunderstand. When I say "FIS" I am talking about FIS based
> switching, as opposed to command based switching. Perhaps a lack of
> clarity on my part, apologies for that.

Np --- the article doesn't say much.

>> As far as I've seen, that doesn't happen.  Instead, the system goes
>> down, trying to access the unresponsive disk indefinitely.
>
> I see a disk get kicked out all the time. Most recent occurrence was 2
> days ago.

You seem to have a lot of disks failing.

> "zfs status" shows you the errors on each disk in the pool. This
> should be monitored along with regular SMART checks. Using ZFS doesn't
> mean you no longer have to monitor for hardware failure, any more than
> you can not monitor for failure of a disk in a hardware RAID array.

At how many errors do you need to replace a disk?  Are sectors that had
errors being re-used, or does ZFS keep a list of sectors not to use
anymore?

>>>> Or how unreliable is a disk that spends significant amounts of time on
>>>> error correction?
>>>
>>> Exactly - 7 seconds is about 840 read attempts. If the sector read
>>> failed 840 times in a row, what are the chances that it will ever
>>> succeed?
>>
>> Isn't the disk supposed not to use the failed sector once it has been
>> discovered, meaning that the disk might still be useable?
>
> When a sector becomes unreadable, it is marked as "pending". Rad
> attempts from it will return an error. The next write to it will cause
> it to get reallocated from the spare sectors the disk comes with. As
> far as I can tell, some disks try to re-use the sector when a write
> for it arrives, and see if the data sticks to the sector within the
> ability of the sector's ECC to recover. If it sticks, it's kept, if it
> doesn't, it's reallocated.

That would mean that a disk which has been failed due to error
correction taking too long may still be fine.

>>>> You seem to like the HGST ones a lot.  They seem to cost more than the
>>>> WD reds.
>>>
>>> I prefer them for a very good reason:
>>> http://blog.backblaze.com/2014/01/21/what-hard-drive-should-i-buy/
>>
>> Those guys don't use ZFS.  They must have very good reasons not to.
>
> I don't know what they use.

They're using ext4.


-- 
Knowledge is volatile and fluid.  Software is power.

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