Bristol Digest, Vol 528, Issue 3

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Today's Topics:

1. RAID options (Martin Moore)
2. Re: RAID options (Shane McEwan)
3. Re: RAID options (Martin Moore)
4. Re: RAID options (Alex Butcher)


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Message: 1
Date: Wed, 4 Dec 2013 09:40:48 -0000
From: "Martin Moore" <martinm@it-helps.co.uk>
To: "'Bristol and Bath Linux User Group'" <bristol@mailman.lug.org.uk>
Subject: [bristol] RAID options
Message-ID:
<!&!AAAAAAAAAAAYAAAAAAAAAFLxZtQqo65Oo+1jhlUB9DvCgAAAEAAAABgADh/iN/lPhk77iLOoBOIBAAAAAA==@it-helps.co.uk>

Content-Type: text/plain; charset="us-ascii"

Mornin' all.

I'm waiting for Santa to deliver a couple of new servers today which will be
running Wheezy with MDADM RAID 1.

I've always ignored the question before, but would like to get it right!

So, Swap partition - (history shows it's probably not needed, but will be
there anyway). Raided or not? Presumably it will slow things down if it is,
but if the active disk dies then the system would have a big problem?

Boot stuff - how to ensure the system will boot from either disk (using
GRUB).


Cheers,

Martin.
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Message: 2
Date: Wed, 04 Dec 2013 10:17:16 +0000
From: Shane McEwan <shane@mcewan.id.au>
To: bristol@mailman.lug.org.uk
Subject: Re: [bristol] RAID options
Message-ID: <529F012C.2030501@mcewan.id.au>
Content-Type: text/plain; charset=UTF-8; format=flowed

On 04/12/13 09:40, Martin Moore wrote:
> So, Swap partition - (history shows it's probably not needed, but will be
> there anyway). Raided or not? Presumably it will slow things down if it is,
> but if the active disk dies then the system would have a big problem?

If you've got tons of RAM then swap is usually unnecessary. I usually
configure a little bit of swap (a gig or two, at most) to give me a
little bit of headroom but basically if you're regularly swapping then
your machine is going to be pretty unresponsive. You don't want to have
too much swap because the Linux Out Of Memory killer won't run until
swap is full. That means your machine could be unresponsive for hours
while it's trashing away filling swap before OOM killer finally kills
the offending process.

As far as putting swap on a RAID disk . . . with RAID-1 I don't see the
point. If the active disk fails you're gonna need a reboot anyway to
boot off the mirror. That being said, I don't think you'll see much
performance difference between RAID and non-RAID and, as I said above,
if you're swapping you're already screwed.

Shane.





------------------------------

Message: 3
Date: Wed, 4 Dec 2013 10:27:35 -0000
From: "Martin Moore" <martinm@it-helps.co.uk>
To: "'Bristol and Bath Linux User Group'" <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] RAID options
Message-ID:
<!&!AAAAAAAAAAAYAAAAAAAAAFLxZtQqo65Oo+1jhlUB9DvCgAAAEAAAAJ+7ojqPJbhNsKiIKD087A0BAAAAAA==@it-helps.co.uk>

Content-Type: text/plain; charset="us-ascii"

Cheers, makes sense. Gone for hot swap discs so shouldn't need a reboot!

Currently have 2G RAM and usage never goes above 3G. New boxes have 8G so
should be fine. I'll put a small swap on but not RAID it.

-----Original Message-----
From: bristol-bounces@mailman.lug.org.uk
[mailto:bristol-bounces@mailman.lug.org.uk] On Behalf Of Shane McEwan
Sent: 04 December 2013 10:17
To: bristol@mailman.lug.org.uk
Subject: Re: [bristol] RAID options

On 04/12/13 09:40, Martin Moore wrote:
> So, Swap partition - (history shows it's probably not needed, but will
> be there anyway). Raided or not? Presumably it will slow things down
> if it is, but if the active disk dies then the system would have a big
problem?

If you've got tons of RAM then swap is usually unnecessary. I usually
configure a little bit of swap (a gig or two, at most) to give me a little
bit of headroom but basically if you're regularly swapping then your machine
is going to be pretty unresponsive. You don't want to have too much swap
because the Linux Out Of Memory killer won't run until swap is full. That
means your machine could be unresponsive for hours while it's trashing away
filling swap before OOM killer finally kills the offending process.

As far as putting swap on a RAID disk . . . with RAID-1 I don't see the
point. If the active disk fails you're gonna need a reboot anyway to boot
off the mirror. That being said, I don't think you'll see much performance
difference between RAID and non-RAID and, as I said above, if you're
swapping you're already screwed.

Shane.



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------------------------------

Message: 4
Date: Wed, 4 Dec 2013 10:30:40 +0000 (GMT)
From: Alex Butcher <lug@assursys.co.uk>
To: Bristol and Bath Linux User Group <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] RAID options
Message-ID: <alpine.LFD.2.03.1312041027160.7082@nffheflf.pb.hx>
Content-Type: TEXT/PLAIN; charset=US-ASCII; format=flowed

On Wed, 4 Dec 2013, Shane McEwan wrote:

> On 04/12/13 09:40, Martin Moore wrote:
>> So, Swap partition - (history shows it's probably not needed, but will be
>> there anyway). Raided or not? Presumably it will slow things down if it is,
>> but if the active disk dies then the system would have a big problem?
>
> If you've got tons of RAM then swap is usually unnecessary. I usually
> configure a little bit of swap (a gig or two, at most) to give me a little
> bit of headroom but basically if you're regularly swapping then your machine
> is going to be pretty unresponsive. You don't want to have too much swap
> because the Linux Out Of Memory killer won't run until swap is full. That
> means your machine could be unresponsive for hours while it's trashing away
> filling swap before OOM killer finally kills the offending process.
>
> As far as putting swap on a RAID disk . . . with RAID-1 I don't see the
> point. If the active disk fails you're gonna need a reboot anyway to boot off
> the mirror.

Eh? My expectation would be that if your swap is unRAIDed, and a disc
providing it fails, then all the swapped-out pages will be unreadable, and
the machine will panic the first time it needs to bring them back into
memory and fails. RAIDing the discs that provide swap should prevent that
(at the cost of disc space, and, presumably, slower swapping-out as writes
have to be at least duplicated) and allow you to down the machine and
replace the failed disc at your convenience. RAID levels >=1 are primarily
about availability, not performance.

> That being said, I don't think you'll see much performance difference
> between RAID and non-RAID and, as I said above, if you're swapping you're
> already screwed.
>
> Shane.

Best Regards,
Alex



------------------------------

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End of Bristol Digest, Vol 528, Issue 3
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