Date: Sun, 09 Jan 2011 10:03:12 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: Jean-Yves Avenard <jyavenard@gmail.com> Cc: freebsd-stable@freebsd.org Subject: Re: ZFS - moving from a zraid1 to zraid2 pool with 1.5tb disks Message-ID: <4D2987E0.7060701@infracaninophile.co.uk> In-Reply-To: <AANLkTikn2G_23M3PbERTo8KR3sDqxkhWr=OntA4cVwh9@mail.gmail.com> References: <4D1C6F90.3080206@my.gd> <ifsia5$5ub$2@dough.gmane.org> <4D21E679.80002@my.gd> <84882169-0461-480F-8B4C-58E794BCC8E6@my.gd> <BEBC15BA440AB24484C067A3A9D38D7E0149F32D13E3@server7.acsi.ca> <m262ty39th.wl%randy@psg.com> <4D297587.4030108@infracaninophile.co.uk> <AANLkTikn2G_23M3PbERTo8KR3sDqxkhWr=OntA4cVwh9@mail.gmail.com>
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On 09/01/2011 09:01, Jean-Yves Avenard wrote:
> Hi
>=20
> On 9 January 2011 19:44, Matthew Seaman <m.seaman@infracaninophile.co.u=
k> wrote:
>> Not without backing up your current data, destroying the existing
>> zpool(s) and rebuilding from scratch.
>>
>> Note: raidz2 on 4 disks doesn't really win you anything over 2 x mirro=
r
>> pairs of disks, and the RAID10 mirror is going to be rather more perfo=
rmant.
>=20
> I would have thought that the probability of failure to be slightly dif=
ferent.
> Sure you out of 4 disks, 2 can fail in both conditions.
>=20
> *But*, in raidz2, any two of the four can fail.
> In RAID10, the two disks that failed must be in different block
> otherwise you loose it all
>=20
> As such the resilience for failure in a RAIDz2 is far greater than in
> a RAID10 system
So you sacrifice performance 100% of the time based on the very unlikely
possibility of drives 1+2 or 3+4 failing simultaneously, compared to the
similarly unlikely possibility of drives 1+3 or 1+4 or 2+3 or 2+4
failing simultaneously?[*] That's not a trade-off worth making IMHO.
If the data is that valuable, you should be making copies of it to some
independent machine all the time and backing up at frequent intervals,
which backups you keep off-site in disaster-proof storage.
Cheers,
Matthew
[*] All of this mathematics is pretty suspect, because if two drives
fail simultaneously in a machine, the chances are the failures are not
independent, but due to some external cause [eg. like the case fan
breaking and the box toasting itself.] In which case, the comparative
chance of whatever it is affecting three or four drives at once renders
the whole argument pointless.
--=20
Dr Matthew J Seaman MA, D.Phil. 7 Priory Courtyard
Flat 3
PGP: http://www.infracaninophile.co.uk/pgpkey Ramsgate
JID: matthew@infracaninophile.co.uk Kent, CT11 9PW
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