Topic: Promise ATA/Raid

[Jim Gooch]

Sorry Fred - been there done that and got the T shirt. If the Raid is a card - not embedded as mine was - then suggest you look at swapping it for something else that has 2.4.18-5 drivers and is supported.

I took the totally unusual step of speaking to the Server manufacturers Tech Suport line who approached Promise for later drivers - none & no plans was the response.

[FredS]

OK thanks for the info

[Mike Kelly]

Promise RAID under Linux comes up a lot.  There are a few things to keep in mind when dealing with Promise "RAID" controllers:

1.  Most Promise RAID controllers (ie:  FastTrack series) are basically standard IDE controllers with a BIOS extension that allows you to define drive arrays and boot from either member of the array.

2.  Being standard IDE controllers, the RAID functionality is actually provided by the Promise driver.  Which makes it...  software RAID, albeit of a different sort than the standard Linux software RAID.

My recommendation is to not bother with the proprietary Promise drivers.  They'll only get you into trouble when a new kernel upgrade comes around and you can't update your system because Promise hasn't released drivers for a given kernel.  Since it's really just software RAID anyhow, just connect your drives to the Promise controller (which Linux will see as a regular IDE controller if you don't load drivers), and configure using software RAID under Linux.

You'll get the same speed-up as you would had you installed the Promise drivers, as Linux software RAID is quite capable of doing multipath IO to members of a RAID array.  Less hassle too when you want to update your kernel ...  and all your RAID array details can be found in /proc/mdstat.

Warning:  There is a software driver in the 2.4 kernel called 'ataraid' that purports to provide RAID functionality with Promise controllers.  It works.  Kind of.  The problem is that in the event of a disk failure, you have to replace the drive, reboot the server, and rebuild the array from *within* the Promise BIOS ... making it a horrible solution for production machines.  Avoid at all costs.

In general - the cheap RAID controllers such as Promise won't give you any advantages over Linux software RAID.  If you want proper ATA RAID, spend the money and pick up a 3ware Escalade card or similar.  Otherwise, stick with the OS software RAID.

[FredS]

Thanks for the info

[Jim Gooch]

Re: Mike Kelly

That is the most useful post on the subject I have seen. It confirms exactly what I had determined from the various tests carried out on the NEC Express in question. The Promise ATA/Raid controller simply cannot be used in Raid mode except possibly with a standard RH7.3 install that uses 2.4.18-3 (i.e. that matches the drivers) - I didn't bother to try that because a standard RH install is not going to do what I want without significant extra work. The whole purpose of SME Server being to make extra work redundant and give a standardised easy to admin install in all cases.

One other niggle to report when using the promise controller in ATA mode. SME seemed to have difficulty in installing when the primary HD partition had previously been written as FAT32. No problem though when the primary was originally FAT16 i.e. prepared with a Win98 boot disk and FDISK.