Software Raid Cache

Software Raid Cache

[SoulBlazer]

I was wondering if it would be possible to set aside a large (larger) 
amount of system memory to act as cache for software raid.
I have a few xeon systems here with 4-8gb of memory in them and based on 
watching the performance of some mylex raid cards
jump up when we did a memory upgrade from 64mb -> 128mb I was wondering 
if the same would occur in linux.

If this is even something doable (tweakable) Im wondering what the perf 
gain from allocating say 2 or 4gb just to raid cache would be.

Comments ?

Re: Software Raid Cache

[Neil Brown]

On Friday February 21, soul@lamp-post.net wrote:
> I was wondering if it would be possible to set aside a large (larger) 
> amount of system memory to act as cache for software raid.
> I have a few xeon systems here with 4-8gb of memory in them and based on 
> watching the performance of some mylex raid cards
> jump up when we did a memory upgrade from 64mb -> 128mb I was wondering 
> if the same would occur in linux.
> 
> If this is even something doable (tweakable) Im wondering what the perf 
> gain from allocating say 2 or 4gb just to raid cache would be.
> 
> Comments ?

How do you imagine this extra 'cache' would be used.

Cache on raid cards tends to be battery backed (I believe) and to
operate as a write-behind cache thereby increasing performance.  As
main system memory is not battery backed, you cannot use it as a
write-behind cache.

NeilBrown

Re: Software Raid Cache

[Maurice Hilarius]

With regards to your message at 11:04 PM 2/23/03, Neil Brown. Where you stated:
> > If this is even something doable (tweakable) Im wondering what the perf
> > gain from allocating say 2 or 4gb just to raid cache would be.
> >
> > Comments ?
>
>How do you imagine this extra 'cache' would be used.
>
>Cache on raid cards tends to be battery backed (I believe) and to
>operate as a write-behind cache thereby increasing performance.  As
>main system memory is not battery backed, you cannot use it as a
>write-behind cache.
>
>NeilBrown


True, and the cache on a RAID controller is not clobbered when the OS or 
computer crashes.
Mind you the same problem exists to some degree with PCI based RAID 
controllers, so the most robust are host independent SCSI/SCSI RAID 
controllers.

Too bad these cost about as much as a typical server!



With our best regards,

Maurice W. Hilarius       Telephone: 01-780-456-9771
Hard Data Ltd.               FAX:       01-780-456-9772
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Edmonton, AB, Canada      http://www.harddata.com/
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