[CentOS] Re: RAID Hot Spare

Scott Silva ssilva at sgvwater.com
Sat Feb 2 00:48:53 UTC 2008


on 2/1/2008 4:33 PM Dean Maluski spake the following:
> On Fri, 2008-02-01 at 16:11 -0800, nate wrote:
>> Dean Maluski wrote:
>>> I've googled this question without a great deal of information.
>>> Of couse will be creating RAID0 swap but leaving that out of the
>>> question for obvious reasons.
>> You really should use anything but RAID 0 for swap. If you need
>> to swap and that device is dead then your system is hosed.
>>
>> At one point I read that you can get RAID0-"like" performance
>> by having multiple swap partitions on multiple devices and mounting
>> them with the same priority(mount option pri=(some number)). It
>> (was/is) supposed to stripe the swap partitions. Not sure if that
>> ever worked, though I have configured systems over the years to
>> use matching swap priorities, never really looked to see if it
>> was doing what I expected though.
>>
>> Yeah, from swapon(2):
>> [..]
>> If two or more areas have the same priority, and it is the
>> high-est priority available, pages are allocated on a
>> round-robin basis between them.
>>
>> nate
> OK, not really an answer to my hot spare question.
> What I read sounds similar to what you state that if you create multiple
> swap partions the system will create a raid0 of it.
> So what is the recommendation? create 1 swap partition on one drive?
And for your hot spare question, you create the raid arrays the normal way, 
with raid type, number of drives set to 3, set the number of spares to 1, and 
have the 4 partitions on the command line.
mdadm --create  --level=5 --raid-devices=3 --spare-devices=1 /dev/part1 
/dev/part2 /dev/part3 /dev/part4

It is all in the man page if you want other options.

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