Red Hat Bugzilla – Bug 488071
anaconda raid.py has wrong minimum member count for md raid10
Last modified: 2009-04-12 03:16:06 EDT
Description of problem:
get_raid_min_members lists minimum member count of 4 for md raid10. This is incorrect. Correct minimum for raid10 is 2.
There is one special reason why 2 wuld be better. md raid10 module can do raid1E setup with 3 disks...
What is the rationale for requiring a minimum of two members?
Because 2 is real minimum member count for md raid10. I agree that 3 would kind of make more sense because raid1E configuration would make more sense than raid1 with raid10 module. Especially when different md raid10 layouts are not supported by anaconda. With far layout there is real reason to run raid1 setup with 2 disks using raid10 for better read performance.
I just don't see reason to limit to 3 when 2 is real minimum amount of raid members for md raid10.
It is my understanding that two devices in a RAID10 results in a degraded RAID array, which anaconda explicitly does not support. We have no plans to add support for this, either.
2 disk raid10 setup doesn't have any degraded disks. Try to create one with mdadm if you don't beleave. And same for 3 disks raid10 setup (aka raid1e).
The entire point of RAID10 is to create a stripe across mirrored sets of disks. If you're only using two disks, then you are losing half the purpose of even using RAID10 and might as well be using another RAID level. This is not the kind of configuration that we're looking to support in anaconda.
So far I've been using RAID1, and have been happy with it. Until recently, that is. I did some performance tests and noticed that RAID1 has the same performance as a single disk on sequential I/O. iostat just shows that always one disk at a time is accessed.
I also have a three disk RAID5 array in another PC. And that PC has excelent performance on sequential I/O: 3x the speed of a single disk!
The solution appears to be to use RAID10.f2 (far layout, 2 copies of each block). If I'm well informed RAID10.f2 is there exactly to do this: performance gain. And of course it also supports the other goal of RAID10: striping acros mirrored sets of disks.
In short: I think RAID10 on 2 disks is kept out of anaconda for the wrong reasons:
* RAID10.f2 on 2 disks is NOT a degraded array
* RAID10.f2 on 2 disks delivers a better performance than RAID1
So Chris, could you please reconsider?