Software vs. Hardware and upgrading later

[Nate Finch]

How much of a performance hit is software RAID likely to cause? Are we talking like 5-10% or 30-50%?

If I build a server with software RAID 5 and later on decide I want to upgrade to a hardware controller.... is that possible? How much of a hassle are we talking?

Finally, are the controller cards in the $200 range pure hardware, or are they still taxing the CPU?

Thanks muchly for your replies... and my apologies for any questions which have been asked and answered (which is probably all of them.... I swear I tried to find the answers myself).

-Nate

 

[hokatichenci]

How much of a performance hit is software RAID likely to cause? Are we talking like 5-10% or 30-50%?

If I build a server with software RAID 5 and later on decide I want to upgrade to a hardware controller.... is that possible? How much of a hassle are we talking?

Finally, are the controller cards in the $200 range pure hardware, or are they still taxing the CPU?

Thanks muchly for your replies... and my apologies for any questions which have been asked and answered (which is probably all of them.... I swear I tried to find the answers myself).

-Nate

On my Gentoo Hardened Linux box using RAID5 wasn't entirely nasty, but I had serious stability issues for some reason (probably due to the comparatively outrageous settings I use). RAID10 takes up like 5-15% CPU usage at full tilt I believe, using an external standard SATA card. Since RAID5 calculates checksums, you will probably be looking at a nastier hit but of course it depends a lot on how fast of a machine you have. AFAIK you can't switch between hardware/software without backing up all the data, since every hardware manuf let alone every OS/software raid system handles the storage differently. I don't think you can really bunch together everything by price, its best just to read reviews on each card to make sure you are getting what you pay for.

 

[unhappy_mage]

I used 100% of a cyrix 166 in getting 15-20 MB/s of write speeds. This is my last experience with sw raid 5, and I'd say it's pretty darn positive. On a modern chip, this might mean you use 300 mHz worth of a chip, and get 30-40 MB/s. On my machine (2.4c p4) this means it'd be using about 12.5% cpu. However, my last experience with sw raid 5 is pretty long ago, so I don't know if they've vectorized everything into sse or something that would improve performance. Fake edit: Yep, it does:

raid5: automatically using best checksumming function: pIII_sse
   pIII_sse  :  2284.000 MB/sec
raid6: int32x1    531 MB/s
raid6: int32x2    777 MB/s
raid6: int32x4    531 MB/s
raid6: int32x8    429 MB/s
raid6: mmxx1     1875 MB/s
raid6: mmxx2     1878 MB/s
raid6: sse1x1    1222 MB/s
raid6: sse1x2    1562 MB/s
raid6: sse2x1    1867 MB/s
raid6: sse2x2    2460 MB/s
raid6: using algorithm sse2x2 (2460 MB/s)
md: raid6 personality registered as nr 8

So in other words, my system can do 2.2 GB/s of raid5 checksumming, or 2.4 of raid 6.

Basically, the raid operations themselves are cheap, but you'll want to find a system that doesn't actively wait on the disks; that's what can draw the real cpu time.

 

[dirtydr]

Finally, are the controller cards in the $200 range pure hardware, or are they still taxing the CPU?

Any controller card is going to use the CPU to some extent. If you base your server around a motherboard that supports zero channel raid you can use one of these cards that does hardware XOR for $200

A list of supported boards is here. I dont have any experience with ZCR but my old DPT raid controller used about 3-5% cpu when setup as a level 5 iirc.

What role will this server be filling?

 

[Nate Finch]

It's going to be primarily a fileserver. Basically, I'm going to use it store disk images, MP3s, backup my other machines to it, and run a source control server on it. I'm debating whether to use it as a web server as well. It would be nice to be able to reuse the hardware, but I'm a little nervous about having a machine with hundreds of gigs of my precious data on it accessible outside the LAN.

-Nate