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Re: [Erlug] Hard disk da buttare?

To: "ERlug - Lista Pubblica" <erlug@xxxxxxxxxxxxxx>
Subject: Re: [Erlug] Hard disk da buttare?
From: "Guido Bolognesi [Zen]" <zen@xxxxxxxxx>
Date: Wed, 22 Oct 2008 15:43:09 +0200
2008/10/22 Fabio Muzzi <kurgan@xxxxxxxxxx>:
>> A me, ZFS piace sempre piu`. :)
> In che modo zfs dovrebbe poter aiutare in questo senso?
argh. gmail non mi fa mettere a posto il subject. (e ap mi ha
battuto sul tempo) anyway:

"RAID-Z is a data/parity scheme like RAID-5, but it uses dynamic stripe width."
[...]
"This means that every RAID-Z write is a full-stripe write."
[...]
[ricostruzione del raid]
Because the stripes are all different sizes, there's no simple formula
like "all the disks XOR to zero."
You have to traverse the filesystem metadata to determine the RAID-Z geometry.
Note that this would be impossible if the filesystem and the RAID
array were separate products,
which is why there's nothing like RAID-Z in the storage market today.
You really need an integrated view of the logical and physical
structure of the data to pull it off.
[e alla domanda: ma cosi` non e` lento da far paura?]
Actually, it's a trade-off. If your storage pool is very close to
full, then yes, it's slower.
But if it's not too close to full, then metadata-driven reconstruction
is actually faster because
it only copies live data; it doesn't waste time copying unallocated disk space.
[...]
In addition to handling whole-disk failure, RAID-Z can also detect and
correct silent data corruption.
Whenever you read a RAID-Z block, ZFS compares it against its
checksum. If the data disks didn't
return the right answer, ZFS reads the parity and then does
combinatorial reconstruction to figure out
which disk returned bad data. It then repairs the damaged disk and
returns good data to the application.
ZFS also reports the incident through Solaris FMA so that the system
administrator knows that one of
the disks is silently failing.

Da: http://blogs.sun.com/bonwick/entry/raid_z
(che e` il Chief Technical Officer (CTO) of Storage Technologies di Sun.
Ah, e anche: Jeff leads the team which developed ZFS for Solaris.)

-- 
guido . zen@xxxxxxxxxxxxxxx . skype://zenmobile

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