From: Hugo Mills <hugo@carfax.org.uk>
To: Martin <m_btrfs@ml1.co.uk>
Cc: linux-btrfs@vger.kernel.org
Subject: Re: ditto blocks on ZFS
Date: Sat, 17 May 2014 15:24:58 +0100 [thread overview]
Message-ID: <20140517142457.GB2537@carfax.org.uk> (raw)
In-Reply-To: <ll7lvd$ulk$1@ger.gmane.org>
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On Sat, May 17, 2014 at 01:50:52PM +0100, Martin wrote:
> On 16/05/14 04:07, Russell Coker wrote:
> > https://blogs.oracle.com/bill/entry/ditto_blocks_the_amazing_tape
> >
> > Probably most of you already know about this, but for those of you who haven't
> > the above describes ZFS "ditto blocks" which is a good feature we need on
> > BTRFS. The briefest summary is that on top of the RAID redundancy there...
> [... are additional copies of metadata ...]
>
>
> Is that idea not already implemented in effect in btrfs with the way
> that the superblocks are replicated multiple times, ever more times, for
> ever more huge storage devices?
Superblocks are the smallest part of the metadata. There's a whole
load of metadata that's not in the superblocks that isn't replicated
in this way.
> The one exception is for SSDs whereby there is the excuse that you
> cannot know whether your data is usefully replicated across different
> erase blocks on a single device, and SSDs are not 'that big' anyhow.
>
>
> So... Your idea of replicating metadata multiple times in proportion to
> assumed 'importance' or 'extent of impact if lost' is an interesting
> approach. However, is that appropriate and useful considering the real
> world failure mechanisms that are to be guarded against?
>
> Do you see or measure any real advantage?
This. How many copies do you actually need? Are there concrete
statistics to show the marginal utility of each additional copy?
Hugo.
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next prev parent reply other threads:[~2014-05-17 14:25 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-05-16 3:07 ditto blocks on ZFS Russell Coker
2014-05-17 12:50 ` Martin
2014-05-17 14:24 ` Hugo Mills [this message]
2014-05-18 16:09 ` Russell Coker
2014-05-19 20:36 ` Martin
2014-05-19 21:47 ` Brendan Hide
2014-05-20 2:07 ` Russell Coker
2014-05-20 14:07 ` Austin S Hemmelgarn
2014-05-20 20:11 ` Brendan Hide
2014-05-20 14:56 ` ashford
2014-05-21 2:51 ` Russell Coker
2014-05-21 23:05 ` Martin
2014-05-22 11:10 ` Austin S Hemmelgarn
2014-05-22 22:09 ` ashford
2014-05-23 3:54 ` Russell Coker
2014-05-23 8:03 ` Duncan
2014-05-21 23:29 ` Konstantinos Skarlatos
2014-05-22 15:28 Tomasz Chmielewski
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