jonroberts
> Almost all new motherboards now offer on-board SATA RAID - should
> this always be ignored in favour of the SME software RAID?
First point is that the mb/chips/drivers need to be compatible with CentOS.
If the hardware is compatible, then it should be OK to use.
If you choose to use hardware RAID of any description (on board or separate cards) then you always select "single drive" installation when installing sme OS. You do not select software RAID1.
sme just sees the hardware RAID as if it were one drive. The hardware takes care of mirroring/striping to suit the type of RAID you configure the hardware for.
Personally I would say yes to ignore any on board hardware RAID, and just use software RAID1.
> You make the point that with software RAID, the server is only down for > the time it takes to replace the drive, whereas the hardware RAID may > well want to rebuild the array before rebooting the server (Good point & > one I'd not considered).
I have only used cheaper RAID cards in Windows, and they did require the drive to be replaced and the array to be fully rebuilt before the system would start the OS. That's a few hours wait for modern size drives eg 80Gb plus.
I'm not sure how this works in Linux, but as it is a hardware function, then I assume you have to wait also.
I think more expensive cards work diffently though, perhaps someone else who has used them could comment
Software RAID1 on sme rebuilds the array while you are using the server, without interruption to data. It's a bit slower during that time depending on your system.
> What about if the motherboard fails & the drives need to be moved to
> another server? I guess the on-board RAID would be a problem (unless > identical RAID chipset in use) but would the software array work in this > case?
I don't have that experience to quote for sure, but with hardware RAID the drives are definitely dependant on the same hardware being available.
If a plugin board or motherboard fails, then I imagine you need an identical spare, which may not be so easy to get in a hurry.
There's a good reason to have a spare board !
In my Windows server (I have to use that due to some apps requiring Windows), I have a hot connected spare 3rd drive, to speed up the process of getting back on air in the event of a drive failure, but I still have to wait a few hours for the array to rebuild, even though I don't have to open the box.
By comparison if you use software RAID1, then the drives just plug into the normal hard drive connections on the motherboard. They are not dependant on the exact hardware make & model, they just need a Primary & Secondary channel connection on any motherboard, better to use similar processor types though.
I routinely swap a drive out of a software RAID1 sme server, and that drive functions perfectly well as a standalone server (in RAID1 degraded mode) if I plug it into the same channel on another computer elsewhere. A drive removed from a P4 even runs on a Celeron 333, with a few module errors here and there.
That's not ideal of course, just an example to show it does work on vastly different equipment.
That is a part of my backup strategy, as it neatly backs up all installed apps etc.
> Finally is performance any consideration? Again, with dedicated RAID
> cards this may be the case, but what about on-board RAID, how would > this compare to SME Software RAID?
Again I don't have direct experince to quote you here, but I'm happy to belive Charlie Brady & his recommendations.
Software RAID1 is as capable as hardware RAID.
If you think about it, they are both software RAID ultimately, it's just a software program telling the bits where to write compared to a dedicated chip telling the bits where to write, and doesn't the dedicated chip use software anyway !
Charlie has posted numerous times in these forums various links to performance tests of hardware vs software RAID, which suggest there is little if no difference. Some posters here seem to disagree.
Hardware RAID via onboard the motherboard or via plugin card, I don't see that there could be any significant difference between them although you have an external bus to contend with, so therefore software RAID should still stand up well in comparison.
Perhaps a more knowledgeable user can also comment on performance.
I think that software RAID has a number of pluses when considering maintenance & simplicity & is cheaper to implement if you don't need to buy a card. It seems that possible performance differences would be a non issue for most of us, and any claims otherwise are likely to be incorrect.
19 Replies
6379 Views