Am 21.04.2013 06:35, schrieb openindiana-discuss-requ...@openindiana.org:
------------------------------
Message: 3
Date: Sat, 20 Apr 2013 21:13:09 -0700
From: Richard Elling<richard.ell...@richardelling.com>
To: Discussion list for OpenIndiana
<openindiana-discuss@openindiana.org>
Subject: Re: [OpenIndiana-discuss] vdev reliability was:
Recommendations for fast storage
Message-ID:<0b43e9ea-10fd-41af-81ef-31644ff49...@richardelling.com>
Content-Type: text/plain; charset=windows-1252
Terminology warning below?
On Apr 18, 2013, at 3:46 AM, Sebastian Gabler<sequoiamo...@gmx.net> wrote:
>Am 18.04.2013 03:09, schriebopenindiana-discuss-requ...@openindiana.org:
>>Message: 1
>>Date: Wed, 17 Apr 2013 13:21:08 -0600
>>From: Jan Owoc<jso...@gmail.com>
>>To: Discussion list for OpenIndiana
>> <openindiana-discuss@openindiana.org>
>>Subject: Re: [OpenIndiana-discuss] Recommendations for fast storage
>>Message-ID:
>> <cadcwueyc14mt5agkez7pda64h014t07ggtojkpq5js4s279...@mail.gmail.com>
>>Content-Type: text/plain; charset=UTF-8
>>
>>On Wed, Apr 17, 2013 at 12:57 PM, Timothy Coalson<tsc...@mst.edu> wrote:
>>> >On Wed, Apr 17, 2013 at 7:38 AM, Edward Ned Harvey (openindiana) <
>>> >openindi...@nedharvey.com> wrote:
>>> >
>>>> >>You also said the raidz2 will offer more protection against failure,
>>>> >>because you can survive any two disk failures (but no more.) I would
argue
>>>> >>this is incorrect (I've done the probability analysis before). Mostly
>>>> >>because the resilver time in the mirror configuration is 8x to 16x faster
>>>> >>(there's 1/8 as much data to resilver, and IOPS is limited by a single
>>>> >>disk, not the "worst" of several disks, which introduces another factor
up
>>>> >>to 2x, increasing the 8x as high as 16x), so the smaller resilver window
>>>> >>means lower probability of "concurrent" failures on the critical vdev.
>>>> >> We're talking about 12 hours versus 1 week, actual result of my
machines
>>>> >>in production.
>>>> >>
>>> >
>>> >Did you also compare the probability of bit errors causing data loss
>>> >without a complete pool failure? 2-way mirrors, when one device completely
>>> >dies, have no redundancy on that data, and the copy that remains must be
>>> >perfect or some data will be lost. On the other hand, raid-z2 will still
>>> >have available redundancy, allowing every single block to have a bad read
>>> >on any single component disk, without losing data. I haven't done the math
>>> >on this, but I seem to recall some papers claiming that this is the more
>>> >likely route to lost data on modern disks, by comparing bit error rate and
>>> >capacity. Of course, a second outright failure puts raid-z2 in a much
>>> >worse boat than 2-way mirrors, which is a reason for raid-z3, but this may
>>> >already be a less likely case.
>>Richard Elling wrote a blog post about "mean time to data loss" [1]. A
>>few years later he graphed out a few cases for typical values of
>>resilver times [2].
>>
>>[1]https://blogs.oracle.com/relling/entry/a_story_of_two_mttdl
>>[2]http://blog.richardelling.com/2010/02/zfs-data-protection-comparison.html
>>
>>Cheers,
>>Jan
>
>Notably, Richard's models posted do not include BER. Nevertheless it's an
important factor.
[..] /snip
> From the back of my mind it will impact reliability in different ways in ZFS:
>
>- Bit error in metadata (zfs should save us by metadata redundancy)
>- Bit error in full stripe data
>- Bit error in parity data
These aren't interesting from a system design perspective. To enhance the model
to deal
with this, we just need to determine what percentage of the overall space
contains copied
data. There is no general answer, but for most systems it will be a small
percentage of the
total, as compared to data. In this respect, the models are worst-case, which
is what we want
to use for design evalulations.
As others already pointed out, the case where disk-based read errors are
getting into focus when you read from an array/vdev that has no more
redundancy. Indeed, there is no difference between parity and stripe
data. There is however a difference to metadata when those are provided
redundantly, even in a non-redundant vdev layout. That is the case in ZFS.
NB, traditional RAID systems don't know what is data and what is not data, so
they could
run into uncorrectable errors that are not actually containing data. This
becomes more
important for those systems which use a destructive scrub, as opposed to ZFS's
readonly
scrub. Hence, some studies have shown where scrubbing can propagate errors in
non-ZFS
RAID arrays.
AFAIK, traditional RAID constructs may have two additional issues
compared to ZFS.
1. As you mention, usually the whole stripe set needs to be rebuild,
whereas resilver only rebuilds active data.
2. The error may or may not be detected by the controller. In ZFS, even
if a read error goes silent down the whole food-chain, there are still
block-based checksums separating chaff from wheat.
>
>AFAIK, a bit error in Parity or stripe data can be specifically dangerous when
it is raised during resilvering, and there is only one layer of redundancy left.
OTOH, BER issues scale with VDEV size, not with rebuild time. So, I think that Tim
actually made up a valid point about a systematically weak point of 2-way mirrors
or raidz1 on in vdevs that are large in comparison to the BER rating of their
member drives. Consumer drives have a BER of 1:10^14..10^15, Enterprise drives
start at 1:10^16.
>I do not think that zfs will have better resilience against rot of parity data
than conventional RAID. At best, block level checksums can help raise an error, so
you know at least that something went wrong. But recovery of the data will
probably not be possible. So, in my opinion BER is an issue under ZFS as anywhere
else.
Yep, which is why my MTTDL model 2 explicitily (MTTDL[2]) considers this case;-)
Me personally had only one single zpool failing on resilver in 4 years
of using ZFS. The underlying issue was WD Green drives not being
compatible with LSI 1068. (Simplified, that is some kind of BER
scenario, including disks, link, firmware, etc., because what the users
sees in zpool status is that the data don't read correctly.) However, I
can not give proper account of what the error scenario indeed was,
because I was acting too chaotic myself in order to get the pool back.
In the end, I ended up using Hitachi drives and got the data from a
fresh backup. If I remember correctly, I had both scenarios, individual
file corruption, and loss of the complete pool among the several
attempts of resilvering. Now, how should be the behaviour in case of a
single block (or a small number of them) that comes back unreadable?
Will I loose the complete pool, or will damage be limited to the
affected blocks, resp. files referencing these blocks? (Given, that all
I will use is zpool.)
>
>Best,
>
>Sebastian
>PS: I occurred to me that WD doesn't publish BER data for some of their drives
(at least all I have searched for while writing this). Anybody happens to be in
possession of full specs for WD drives?
The trend seems to be that BER data is not shown for laptop drives, which is a
large part of
the HDD market. Presumably, this is because the load/unload failure mode
dominates in
this use case as the drives are not continuously spinning. It is a good idea to
use components
in the environment for which they are designed, so I'm pretty sure you'd never
consider using
a laptop drive for a storage array.
I was looking at RE-4 drives. The issue was probably the BER/UER mixup.
They have it in the data sheet as "Non-recoverable read errors per bits
read", which is pretty obvious now. The spec is 1:10^15, for the record.
BR
Sebastian
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