Bristol Digest, Vol 528, Issue 5

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Today's Topics:

1. Re: RAID options (Steve King)
2. Re: RAID options (Martin Moore)


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

Message: 1
Date: Thu, 5 Dec 2013 09:06:54 -0000
From: "Steve King" <debian@invux.com>
To: "Bristol and Bath Linux User Group" <bristol@mailman.lug.org.uk>
Subject: Re: [bristol] RAID options
Message-ID:
<e4637548cf05b017989925c45ce99b66.squirrel@dazzle.invux.com>
Content-Type: text/plain;charset=iso-8859-1

> 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
>

I would follow Alex's practice. Disk space is cheap and if a disk fails
with an active swap partition on it, you are going to need a reboot, and
may have lost some data. RAID is for resilience and data security, this
applies to swap as it does the rest of you data.

--
Steve





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

Message: 2
Date: Thu, 5 Dec 2013 10:30:15 -0000
From: "Martin Moore" <martinm@it-helps.co.uk>
To: <debian@invux.com>, "'Bristol and Bath Linux User Group'"
<bristol@mailman.lug.org.uk>
Subject: Re: [bristol] RAID options
Message-ID:
<!&!AAAAAAAAAAAYAAAAAAAAAFLxZtQqo65Oo+1jhlUB9DvCgAAAEAAAAO8u7x1hJQNHhhhM6qUs79kBAAAAAA==@it-helps.co.uk>

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

Cheers all. I went for the RAIDed option.


Martin.

-----Original Message-----
From: bristol-bounces@mailman.lug.org.uk
[mailto:bristol-bounces@mailman.lug.org.uk] On Behalf Of Steve King
Sent: 05 December 2013 09:07
To: Bristol and Bath Linux User Group
Subject: Re: [bristol] RAID options

> 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
>

I would follow Alex's practice. Disk space is cheap and if a disk fails with
an active swap partition on it, you are going to need a reboot, and may have
lost some data. RAID is for resilience and data security, this applies to
swap as it does the rest of you data.

--
Steve



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End of Bristol Digest, Vol 528, Issue 5
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