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phunni

Member

From: Bristol, UK

Registered: 2003-08-13

Posts: 787

Best RAID for SSD maximising storage

I need to replace the drives in my desktop and I was thinking this might be a good time to upgrade to SSD and use RAID for redundancy.  I had planned to use RAID 5, since my priority is storage size over performance - I'm assuming that the increased speed of SSD will still outweigh the decreased write performance of RAID 5?

It did, however, occur to me that, since RAID 5 increases the number of writes and that, since SSD has a limited number of writes, that RAID 5 might be a bad idea.  I Googled it and the internet seemed to concur, but simply suggests using RAID 1, or a proprietary version of RAID.

I did discover one discussion that seemed to suggest that the life of a decent SSD meant that worrying about writes wasn't worth it - but it still said RAID 5 was a bad idea, without really explaining why.

So, am I stuck with something like RAID 1 or 10 if I go with SSD drives? Or is there a solution that allows me to maximise storage, but still give me some redundancy?

alphaniner

Member

From: Ancapistan

Registered: 2010-07-12

Posts: 2,810

Re: Best RAID for SSD maximising storage

Everything I've read recently indicates that for brand name, current gen SSDs longevity isn't really a concern. I actually get the impression that such concerns have been overblown for a while now. Just avoid those no name SSDs from China on eBay...

As far as RAID levels go, RAID 5 would result in the fewest number of writes of all the levels with redundancy. AFAICT, the reason folks at that discussion discourage the use of RAID 5 has to do with the 'write hole'. I don't follow the logic that it's more of a concern with SSDs than HDDs though. And RAID 5 is still the most common RAID level used in business/enterprise environments, so it can't be all that dangerous.

I'm a RAID 10 guy, but if RAID 5 is most suitable to your needs then go for it. Maybe look into ZFS and RAID-Z though. If I ever get to the point where RAID 10 isn't suitable anymore that's probably what I'll look into.

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But whether the Constitution really be one thing, or another, this much is certain - that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist.
-Lysander Spooner

falconindy

Developer

From: New York, USA

Registered: 2009-10-22

Posts: 4,111

Website

Re: Best RAID for SSD maximising storage

As far as RAID levels go, RAID 5 would result in the fewest number of writes of all the levels with redundancy. AFAICT, the reason folks at that discussion discourage the use of RAID 5 has to do with the 'write hole'. I don't follow the logic that it's more of a concern with SSDs than HDDs though. And RAID 5 is still the most common RAID level used in business/enterprise environments, so it can't be all that dangerous.

This isn't really accurate... RAID 5 is hugely problematic because the rate of unrecoverable bit errors increase with the size and number of disks involved in the RAID 5 array. During a rebuild, any of these UBEs result in the entire sector being unreadable, destroying real data and parity metadata in the process.

Claiming that RAID5 is "still the most common RAID level used in business/enterprise environments" seems wrong. Citation absolutely needed. If you care about data integrity, you use a mirror (more likely, RAID 10).

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alphaniner

Member

From: Ancapistan

Registered: 2010-07-12

Posts: 2,810

Re: Best RAID for SSD maximising storage

Claiming that RAID5 is "still the most common RAID level used in business/enterprise environments" seems wrong. Citation absolutely needed. If you care about data integrity, you use a mirror (more likely, RAID 10).

It's largely from personal experience working for a company that sells SANs. In seven years I've seen maybe 5 units with anything other than RAID 5 or 6. But I did at least do a quick google first ("most common RAID in enterprise") and a PC mag article from March said:

RAID 5 is by far the most common RAID configuration for business servers and enterprise NAS devices.

I use RAID 10 because I also have the impression "If you care about data integrity, you use a mirror (more likely, RAID 10)." But that really doesn't seem to be the general concensus, and I can't make the case so I don't try.

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But whether the Constitution really be one thing, or another, this much is certain - that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist.
-Lysander Spooner

phunni

Member

From: Bristol, UK

Registered: 2003-08-13

Posts: 787

Re: Best RAID for SSD maximising storage

I could go for RAID 10, but I hate the idea of losing all that space - which is more important to me than performance.  I'd also have to buy an extra disk.

RAID 5 is hugely problematic because the rate of unrecoverable bit errors increase with the size and number of disks involved in the RAID 5 array. During a rebuild, any of these UBEs result in the entire sector being unreadable, destroying real data and parity metadata in the process.

So, the fewer or smaller the disks the lesser the risk?  I'm looking at 3x120gb

edit: Would RAID 6 avoid the UBE error problem?  Again, I'm assuming that the performance loss from RAID 6 would be more than made up for with the increase from using SSD

Last edited by phunni (2014-08-11 16:37:54)

alphaniner

Member

From: Ancapistan

Registered: 2010-07-12

Posts: 2,810

Re: Best RAID for SSD maximising storage

RAID 6 requires at least four drives, and at four drives has the same capacity as a RAID 10.

Last edited by alphaniner (2014-08-11 16:58:46)

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But whether the Constitution really be one thing, or another, this much is certain - that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist.
-Lysander Spooner

phunni

Member

From: Bristol, UK

Registered: 2003-08-13

Posts: 787

Re: Best RAID for SSD maximising storage

RAID 6 requires at least four drives, and at four drives has the same capacity as a RAID 10.

That is an excellent point. So, now I need to decide between RAID 5 in 3 disks or RAID 10 in 4.  Obviously 3 is cheaper, but hardly worth it if there is a significant penalty...

Last edited by phunni (2014-08-11 17:15:33)

alphaniner

Member

From: Ancapistan

Registered: 2010-07-12

Posts: 2,810

Re: Best RAID for SSD maximising storage

Would RAID 6 avoid the UBE error problem?

http://en.wikipedia.org/wiki/RAID#Unrec … ng_rebuild

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But whether the Constitution really be one thing, or another, this much is certain - that it has either authorized such a government as we have had, or has been powerless to prevent it. In either case, it is unfit to exist.
-Lysander Spooner