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Raid 5 write-back caching?

Tags:
  • Hard Drives
  • NAS / RAID
  • Cache
  • Performance
  • Storage
Last response: in Storage
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January 11, 2012 1:02:38 AM

Hey guys,

I want to create a RAID from my 4x2TB Samsung HDDs. Since I don't like the idea of data loss due to a single drive failure, Raid 0 is not an option.

I'm trying to decide between Raid 5 and Raid 10.

Performance bechmarks will show this behaviour:

Raid 10:

- Decent Write Performance (~170MB/s)
- Decent Read Performance (~180MB/s)
- Only 50% Capacity (4TB)

Raid 5:

- Higher Write Performance with write-back caching enabled (~220MB/s)
- Higher Read Performance (~240MB/s)
- Higher Capacity (6TB)
- Poor Write Performance with write-back caching disabled (~35MB/s)


Basically, Raid 5 with write-back caching enabled seems to be to best option for my purpose (Home PC). However the big downside of write-back caching is that in case of a power loss, data might become corrupted. This wouldn't be too much of a problem though, since I'm about to get an overvoltage protection anyway, while this is easily to be combined with a UPS.


Now the big questions are:

What happens to a Raid 5 with write-back caching enabled if I get a BSOD or any other kind of improper shutdowns? Any data loss/corruption?
Without a UPS, assuming there was still data to be written to the HDDs from the cache, can one tell by any chance how critical the according data loss would be - would it only affect the data I (as user) was writing to the disks or could it as well be way more (misc system subroutines trying to index/defrag/whatever) at this point in time?

Thank you for your time reading this.

Any thoughts on this?

More about : raid write back caching

a c 82 G Storage
January 11, 2012 1:47:03 AM

RAID 5 or not, I never implement write-back caching on a system that isn't protected by a UPS. A data loss is very likely to occur if there's a power failure. A data loss while applying an OS update might not be good. Since a RAID isn't a backup, all important data should be backed up to an external drive or online.
January 11, 2012 1:57:41 AM

GhislainG said:
RAID 5 or not, I never implement write-back caching on a system that isn't protected by a UPS. A data loss is very likely to occur if there's a power failure. A data loss while applying an OS update might not be good. Since a RAID isn't a backup, all important data should be backed up to an external drive or online.


I agree with you on that one. As additional information, my OS is operating from a separate 120GB SSD.

However the most important question for me is: what happens during an improper shutdown (while still being supplied with power)? Will the cached data be written to the disks or will it be lost?

Thanks in advance.
Related resources
January 11, 2012 3:06:13 AM

Onboard Controller for Raid5 or discreet card?
Write caching is perfectly safe if you have a discreet card with a battery backup unit.
January 11, 2012 3:42:28 AM

Onboard. I have no intentions on purchasing a discreet card with battery backup unit either.
a c 82 G Storage
January 11, 2012 10:12:29 AM

Frequency44 said:
I agree with you on that one. As additional information, my OS is operating from a separate 120GB SSD.

However the most important question for me is: what happens during an improper shutdown (while still being supplied with power)? Will the cached data be written to the disks or will it be lost?

Thanks in advance.
It will be lost.
January 11, 2012 11:29:59 AM

GhislainG said:
It will be lost.


Thanks for your reply. Any other opinions? Seems a bit pointless to me to get a UPS when a BSOD will just corrupt your data as well :/ 
a c 82 G Storage
January 11, 2012 2:41:49 PM

You shouldn't get BSODs if the system is stable. I have systems that have been running non-stop for several years without a single BSOD.
January 12, 2012 9:22:53 AM

So you would not hesitate to make the integrity of all your files dependent on the stability of your system, meaning you're 100% sure your system won't ever crash. I honestly doubt that ;P
January 12, 2012 10:11:11 AM

Can't have the best of both worlds, You either take the risk or go with a better solution
a c 82 G Storage
January 12, 2012 10:25:31 AM

Frequency44 said:
So you would not hesitate to make the integrity of all your files dependent on the stability of your system, meaning you're 100% sure your system won't ever crash. I honestly doubt that ;P

I don't use write-back caching for OS drives and all my systems are on a UPS. Unlike when a power failure occurs (you have a high percentage of data loss), the system has time to write data if you get a BSOD. I'm more worried about power failures than BSODs. Only one system had several BSODs last year and it didn't cause any data loss (the issue was caused by a bad router, not the system itself).
January 12, 2012 10:38:30 AM

GhislainG said:
I don't use write-back caching for OS drives and all my systems are on a UPS. Unlike when a power failure occurs (you have a high percentage of data loss), the system has time to write data if you get a BSOD. I'm more worried about power failures than BSODs. Only one system had several BSODs last year and it didn't cause any data loss (the issue was caused by a bad router, not the system itself).


Cheers mate, that was the answer I was looking for.

Greetings
!