Date: Fri, 29 Apr 2016 08:47:22 -0500 From: Karl Denninger <karl@denninger.net> To: freebsd-fs@freebsd.org Subject: Re: How to speed up slow zpool scrub? Message-ID: <08d59afe-c835-fa8d-0e52-78afcb1cc030@denninger.net> In-Reply-To: <alpine.GSO.2.20.1604290821210.23612@freddy.simplesystems.org> References: <381846248.2672053.1461695277122.JavaMail.yahoo.ref@mail.yahoo.com> <381846248.2672053.1461695277122.JavaMail.yahoo@mail.yahoo.com> <1461736217.1121.17.camel@michaeleichorn.com> <alpine.GSO.2.20.1604290821210.23612@freddy.simplesystems.org>
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On 4/29/2016 08:31, Bob Friesenhahn wrote:
> On Wed, 27 Apr 2016, Michael B. Eichorn wrote:
>>
>> It does not *need* to be ECC ram, ECC is just *highly recommended*. As
>> one of the key features of zfs is bitrot prevention, it makes sense to
>> protect against bitrot everywhere. Zfs (and thus freenas) is just fine
>> with non-ecc ram. Just, like for any filesystem if the bit is flipped
>> in ram it will be recorded as such on disk.
>
> This is not necessarily the case. Zfs does not offer additional
> protections for data in RAM. It assumes that data in RAM is protected
> in other ways. The on-disk checksum only verifies that the data was
> not modified since it was checksummed, but it may already be corrupt.
> The risk factor is pretty high if RAM becomes corrupted since zfs uses
> so much RAM.
>
> It is possible to lose data and even the whole pool due to memory
> corruption.
>
> There are well known cases where users encountered continual/periodic
> pool corruptions due to flaky RAM.
>
> Bob
To amplify what Bob said using ZFS on a system without ECC RAM is just
begging to lose the entire pool at some point due to a random bit-error
in system memory and the fact that it happened may be completely
concealed from you for quite a while until at a random later point in
time you discover the pool is hopelessly corrupt.
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.
--
Karl Denninger
karl@denninger.net <mailto:karl@denninger.net>
/The Market Ticker/
/[S/MIME encrypted email preferred]/
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