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Date:      Thu, 24 Nov 2011 19:26:55 +0000
From:      Matthew Seaman <m.seaman@infracaninophile.co.uk>
To:        APseudoUtopia <apseudoutopia@gmail.com>
Cc:        freebsd-questions@freebsd.org
Subject:   Re: Setting up ZFS - Filesystem Properties and Installing on Root
Message-ID:  <4ECE9A7F.9000107@infracaninophile.co.uk>
In-Reply-To: <CAKOHg=PWcpqHdyvaS+oRwghA5m-A-rMxF-wqw3ZAPERzmO+pRw@mail.gmail.com>
References:  <CAKOHg=PEqeMPN_2dfQ+4msi1j7CMUu=iO+pSuezRoa_CELzQtw@mail.gmail.com> <88f3d8e819b3420f8e61723bee90ba5e.squirrel@www.magehandbook.com> <CAKOHg=N4yt0vdac8aZMUTK+8EnoSwxhK+yWmy7MsOXvKrWEmyw@mail.gmail.com> <4ECB580E.20203@infracaninophile.co.uk> <CAKOHg=PWcpqHdyvaS+oRwghA5m-A-rMxF-wqw3ZAPERzmO+pRw@mail.gmail.com>

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On 24/11/2011 19:19, APseudoUtopia wrote:
> On Tue, Nov 22, 2011 at 3:06 AM, Matthew Seaman
> <m.seaman@infracaninophile.co.uk> wrote:
>> On 22/11/2011 02:09, APseudoUtopia wrote:
>>> Another quick question about swap: If I have 4 drives, with 512MB
>>> swap, the system uses all 4 swap partitions, correct? So it's not lik=
e
>>> it'd be going to waste? I'd have a total of 2 GB swap?
>>
>> Well, yes.  If you just declare those raw partitions to be swap areas,=

>> that will be the case.  However, doing this is asking for trouble: you=

>> subvert any resilience features obtained by using ZFS with raidz1.  If=

>> any one of the drives fails, your swap area will break and your system=

>> will probably crash.
>>
>> Better to set up two pairs of gmirrors for swap -- the procedure is
>> described here: http://wiki.freebsd.org/RootOnZFS/GPTZFSBoot/Mirror
>> in section3 "Finish Install."  This will effectively give you a raid10=

>> for your swap, with a total size of 1GB.
>>
>=20
> I'm not sure I understand this. How would that negatively affect the
> raidz1? The swap isn't in the zpool. I understand the system may crash
> if the OS was using the swap space and the drive failed. But would you
> not be able to reboot into a degraded zpool state and still have a
> usable system?
>=20

No -- it means a failed disk can cause your system to crash.  That's not
resilient behaviour.  Yes, the data on the ZFS raidz1 should survive the
crash and the reboot, but the point is ZFS raidz1 should be able to
survive a disk failure like that /without/ a system crash.

	Cheers,

	Matthew

--=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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