Software raid VS hardware raid

[Paul Kraus]

On Jan 28, 2013, at 3:43 PM, Artem Kuchin wrote:

> I have to made a decision on choosing a dedicated server.
> The problem i see is that while i can find very affordable and good options they do not
> provide hardware raid or even if they do it is not the best hardware for freebsd.

I prefer SW RAID, specifically ZFS, for two very large reasons:

1) Visibility: From the OS layer you have very good visibility into the health of the RAID set and the underlying drives. All of the lower end HW RAID solutions I have seen require proprietary software to "manage" the RAID configuration, usually from the physical system's BIOS layer. Finding good OS layer software to monitor the RAID and the drives has been very painful. If you don't know you have a failure, then you can't do anything about it and when you have a second failure you lose data. Running a HW RAID system and not being able to issue a simple command from the OS and see the status of the RAID scares me.

2) Error Detection and Correction: HW RAID relies on the drives to report read and write errors. With UNCORRECTABLE error rates of 10^-14 and 10^-15 and LARGE (1 TB plus) drives you are almost guaranteed to statistically run into UNCORRECTABLE errors over the life of a typical drive. ZFS has end to end checksums and can detect a single bad bit from a drive, if the set is redundant it can recreate the correct data and re-write it, effectively correcting the bad data on disk.

NOTE: Larger, more expensive HW RAID systems address both of the above issues, but at a much higher cost in terms of money and management overhead.

DISCLAIMER: I have been managing mission critical, cannot afford to lose it data under ZFS for over 5 years, with no loss of data (even with some horribly unreliable low cost HW RAID systems under the ZFS layer... if we had not used ZFS we would have lost data multiple times).  

--
Paul Kraus
Deputy Technical Director, LoneStarCon 3
Sound Coordinator, Schenectady Light Opera Company