Re: [gentoo-user] Software RAID Advice Needed

[Jim Burwell <jimb@jsbc.cc>]

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jarry@gmx.net wrote:

>>>If a disk fails,
>>>your system would likely crash (due to the swap device), but would
>>>reboot in a degraded mode (no swap, slow performance, etc).
>>>      
>>>
>>You could avoid that by not using RAID for swap. Instead, use four
>>separate swap partitions, one on each drive. As long as they all have
>>the same priority, the kernel will share swap duties between them
>>equally.
>>    
>>
> 
>If you make more swap partitions on more physical drives with the same
>priority, it is the same as swap on raid0: system strips swap across
>drives. And if some drive crashes and swap partition on that drive has
>been used, very probably system crashes too. But then reboots at least
>with remaining swap partitions...
>  
>

Yes, although there's a posibility of an 'endless crash/reboot' scenereo 
here, if the errors are 'soft' (e.g. not drive just vanishing).  For 
instance, a few bad sectors develop on one of your swap partitions, the 
kernel can't read them, and panic/reboots.  The system comes back up, 
the same swaps are used, and it happens again, over and over until you 
edit the bad partition out of the fstab.

In a redundant RAID situation, I'm presuming that a bad sector or two 
would result in the RAID driver detaching the bad drive, and chugging 
along in degraded mode, where if this happened in a distrubuted swap 
situation, it's already 'too late', since the sectors are lost and the 
kernel would probably panic.

>>There's no real benefit to using RAID for swap, unless you are
>>limited on RAM and use swap a lot, when RAID0 may help.
>>    
>>
>
>There is some benefit, if you use raid1 for swap. In such a case
>even drive failure does not cause system crash, because swap space
>is mirrored too. But raid1 slightly degrades swap performance...
>  
>

This is exactly why I'm doing RAID1 on swap.  If one drive goes poof, my system stays up.  Based on what this server is going to do, it should rarely use much swap, so swap performance isn't a priority for me.  Plus, as you say, I believe the performance hit on swap writes (reads should actually be faster) should wind up being only a bit slower than if you were swapping to a single drive.


- Jim

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