How well do SSDs work in RAID 1?
March 18, 2010 5:57 AM   Subscribe

Would a pair of Solid State Disks in a RAID 1 perform proportionally worse than a pair of HDDs in a RAID 1? Are there any significant gotchas to running a Windows 7 RAID pair on SSDs?

I'm considering buying a pair of mid-level SSDs to put in a laptop*. The OS would be windows 7 64, which supports the SSD feature 'TRIM'. I am somewhat confused about the way Windows 7 does software RAID; would the TRIM feature continue to be used (i.e. is TRIM, as spoken by windows, used at the physical device level, below the software RAID level?)

This got me wondering about RAID on SSD vs HDD. There are some posts on various forums that seem to indicate that the striping schemes used by RAID controllers/software isn't compatible with the way SSDs store data.

I'm also curious to know if the way that RAID is designed (with spinning disk in mind) is more detrimental when used with SSDs. It seems like some folks are already redesigning things like databases (example here)

So, if a theoretical penalty for 2 HDDs in RAID 1 is 10% performance, would the penalty for 2 SSDs be higher or lower? My guess is that performance would actually be better, as you aren't waiting for 2 spinning disks to seek and write.

*Why? Redundancy and because I can
posted by Danaid to Computers & Internet (9 answers total)
I'm not sure what you're asking in your last question, "So, if a theoretical penalty for 2 HDDs in RAID 1 is 10% performance, would the penalty for 2 SSDs be higher or lower?"

Than what? It would surely be better than two (or any) hard disks, but the 10% performance loss figure presumably means HDD-RAID vs. HDD non-RAID. If you're carrying that meaning through, then I can't see any reason for SSD-RAID not to take a performance hit relative to SSD non-RAID -- there's just more to do.
posted by TruncatedTiller at 6:09 AM on March 18, 2010

What do you mean by performance? Read, write, iops, throughput, latency?
posted by Geckwoistmeinauto at 6:15 AM on March 18, 2010

Response by poster: @Truncated: Thanks, that's exactly what I meant - is the penalty for operating in RAID comparable across the two different disk types.

@Gecko: Combined performance; think the standard day to day load on a personal+work laptop. Office, mp3s, web, light gaming, some video encoding, a fair amount of compiling code and running perl/Python inside cygwin. I know write and throughput performance is where SSDs are somewhat lacking, but the amount of usage that stresses write/throughput would (I think!) be low.
posted by Danaid at 6:36 AM on March 18, 2010

Best answer: You shouldn't expect a performance hit substantially different than the one you'd expect from soft RAID 1; it'll be faster moving to HDs to SSDs, but that's about the extent of it. That said, RAID 1 in software is not good times if you want performance, and why would you throw good money at SSDs if you don't want performance?

If you've got room for two hard drives in your laptop and the extent of your motivation is "because I can", then sure, but your A move there is to put the OS and some choice apps and their swapfiles on the SSD, and all your data and less demanding apps on a the 10x bigger, though slightly slower, HD in the other slot.

RAID is a waste of time and money for personal systems, in my opinion; you still need backups, so just have backups.
posted by mhoye at 6:42 AM on March 18, 2010

I'm with mhoye. Do off-machine backups and leave the SSD alone. the main point of RAID is a "Redundant Array of INEXPENSIVE Disks" because of the lack of spinning parts, there is much less of a need for redundancy, and I don't yet think SSDs count as Inexpensive disks :)
posted by jrishel at 8:07 AM on March 18, 2010

now, if you wanted to do RAID-0....
posted by jrishel at 8:10 AM on March 18, 2010

SSDs work in RAID. Very, very well.
posted by zsazsa at 9:12 AM on March 18, 2010

There are a lot of hardware RAID controllers out there that can't keep up with the IO rate SSDs can support, but I'd expect software RAID to handle it better, since it has access to the full performance of the CPU.

"RAID 1 in software is not good times if you want performance"
What do you mean by this, and what is your evidence? RAID1 is very low overhead. I haven't been able to find benchmarks for windows software raid, but I'd expect writes to be, only slightly slower than a single disk, and reads might actually be faster.

That said, I agree with the suggestion that RAID isn't going to be worthwhile. I'd use a big HDD for data and automated backups of the SSD. As for what I'd put on the SSD. Swap, definitely. Most OS and App code is going to be in RAM cache if its accessed frequently, and unless you are restarting your computer or your apps frequently, it isn't going to have to be loaded from disk very often, so I wouldn't be in a hurry to squander my SSD on that. Simplest though might be to just use it for your boot and app drive and copy data on or off as needed.
posted by Good Brain at 10:04 AM on March 18, 2010

If you're trying to set up a backup system, install the two disks in JBOD mode and just sync one disk to the other on a periodic basis.
posted by Blazecock Pileon at 12:50 PM on March 18, 2010

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