[svlug] summary of nvidia/sil soft raid & Linux soft raid

[Ivan Sergio Borgonovo]

OK I think I've been misunderstood about what I've learnt and what are still my doubt about "software raid controller" aka fakeraid.

- While it is clear that fakeraid ctls use the cpu to do raid stuff it is not clear if *all* the job is done by the cpu.
http://www.gamepc.com/labs/view_content.asp?id=eoyraid&page=6&cookie%5Ftest=1
this table *implicitly suggests* that eg. in sil all raid levels supported excluding raid 5 are hardware and raid 5 is software, in AMCC all the raid levels including raid 5 are hardware.

- looking at the c code in the sata_sil and sata_nv drivers there is no reference to raid. That means that even if there was a bit of hardware support for raid in those ctls Linux is not able to exploit it at the moment.

- the (only available?) benchmarks of the ctls in raid mode may be hardly influenced by the drivers

- the benchmarks seems to suggest that in non raid mode nvidia should have a bit advantage over sli + looking at the drivers code and at this page: http://linux-ata.org/sata-status.html nvidia linux driver seems to support queueing, sli drivers shouldn't.

- since nvidia proprietary subdrivers seems to use the open source sata_nv and a proprietary subdriver for raid (thx Rick), using nvidia chipset seeems a better choice over sil in raid 0 using Linux soft raid (md).

- once dmraid will be part of debian install my preference may change in favour of the most open driver that at this moment seems to be sil.

- BIOS setting seems to confuse grub (nvidia) or kernel (sil) if you enable raid in the BIOS, unless I suppose you use dmraid.

- dmraid
to make a BIOS soft raid work you've to
- tell Linux not to mess up with data stored by the BIOS
- read BIOS data to "simulate" raid sets.
- mark invalid hd accordingly with the BIOS standard
This is useful if you dualboot or if you want to reconstruct sets in BIOS.


Since in Debian sid amd64, and I suppose other debain versions, dmraid is not supported in the installation process, being able to put up a system that can be recovered without a rescue disk is a bit painfull. There may be a chance that BIOS recovery could work without installing dmraid just leaving unpartitioned some sectors at the end of the disks (untested), but I wonder how linux can signal to the BIOS which HD became invalid. Since I'm going to use RAID 0 I can patiently wait dmraid installation will be supported since disabling raid in the bios makes installation process smooth using sata_nv and Linux soft raid (md) in sid amd64.


-- 
Ivan Sergio Borgonovo
http://www.webthatworks.it