RAID 5 Discouraged ?

[Reinaldo]

We are redoing some of our servers and it seems like raid 5 would be a
good option (price/benefits) however in a couple of posts on this
group as well as on the mongodb site the RAID 5 is greatly discouraged

From the site:

http://www.mongodb.org/display/DOCS/Developer+FAQ#DeveloperFAQ-WhatRAIDshouldIuse%3F
What RAID should I use?
We recommend not using RAID-5, but rather, RAID-10 or the like. Both
will work of course.


Can someone shed some light on why RAID 5 is bad for Mongo DB ?

[Andreas Jung]

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Reinaldo schrieb:

http://www.cyberciti.biz/tips/raid5-vs-raid-10-safety-performance.html

Google "raid 5 vs raid 10" ...

- -aj
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[Alvin Richards]

Bear in mind that not implementations of RAID are the same, esp.
software but there are also many differences between RAID cards and
Storage vendors.

RAID10
- pros : provide protection of a disk failure and stripping so that
you aggregate the IO across the Disks
- cons : needs double the disk capacity

RAID5
- pros : need to dedicate a single disk for parity, so cheaper to
implement that RAID10. Protection against a single disk failure
- cons : depending on the implementation, the parity disk can get very
hot (in terms of IO needs)

RAID6
- pros : need to dedicate a two disks for parity, so cheaper to
implement that RAID10. Protection against a single double failure
- cons : lower chance of parity disk get hot

I have seen great numbers on RAID10 and RAID5... so it comes down to
what you want to protect against and how you want to manage your disk
infra-structure.

-Alvin

On Jan 20, 11:04 am, Andreas Jung <li...@zopyx.com> wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
>
> Reinaldo schrieb:
>
> > We are redoing some of our servers and it seems like raid 5 would be a
> > good option (price/benefits) however in a couple of posts on this
> > group as well as on the mongodb site the RAID 5 is greatly discouraged
>
> > From the site:
>

> >http://www.mongodb.org/display/DOCS/Developer+FAQ#DeveloperFAQ-WhatRA...

> > What RAID should I use?
> > We recommend not using RAID-5, but rather, RAID-10 or the like. Both
> > will work of course.
>
> > Can someone shed some light on why RAID 5 is bad for Mongo DB ?
>
> http://www.cyberciti.biz/tips/raid5-vs-raid-10-safety-performance.html
>
> Google "raid 5 vs raid 10" ...
>
> - -aj
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> Version: GnuPG v1.4.11 (Darwin)

> Comment: Using GnuPG with Mozilla -http://enigmail.mozdev.org/

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>  lists.vcf
> < 1KViewDownload

[Keith Branton]

I suspect the biggest problem with raid 5 would be the performance of the array when running in degraded mode. They are also slower doing writes than raid 10 (though some very high end raid cards with huge cache memory minimize that)

The anecdotal evidence: I worked for a web site that had used raid 5 for their oracle database, and pretty much went down during the rebuild. All the web servers were completely backed up, and waiting for the database. You can bet they got off raid 5 sharpish after that - they went to raid 10.

There's also a fair chance of data loss with raid 5 if you believe this article: http://www.zdnet.com/blog/storage/why-raid-6-stops-working-in-2019/805 "The crux of the problem" section. I'd rather not believe it but have no evidence to the contrary. That is why my main database is 2 replica set nodes, both on raid 10 arrays - 4 copies of the data - and daily offsite backups and incremental local backups. If I had the budget I'd invest in another node right away.

If the risk of some unexpected downtime here and there, and overall performance isn't as important to you as price, though, maybe raid 5 or raid 6 are worth considering - they are certainly cheaper.

@Andreas - your description of Raid 5 sounds more like raid 4. In raid 5 the parity is evenly distributed across the array.

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