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First RAID5 ever.

D

DigitalLF

n00b
Joined
Jan 15, 2013
Messages
33
Hello!

I have changed my mind... it will be a RAID6.


I'm now ready to initialize my first RAID6 and to do so i have a few questions.

It will be a RAID6 array and i will use 3x 3TB drives and after the array is built i will transfer data from a 4th 3tb drive and then i will add that drive to the array.

But my question is if you guys/girls think i have selected the right stuff in "MegaRAID Storage Manager" ?

The Raid card is a LSI MegaRAID 9271-8i without a BBU or a CacheVault and the storage will be used for storing Movies and Series Recordings mostly.


Initialization state: Fast Initialization
Strip size: 256KB
Read policy: Always Read Ahead
Write policy: Write Through
I/O policy: Direct IO
Access policy: Read Write
Disk cache policy: Disabled


As i said before its my first time ever using any form of raid so please be gently if i made some obvious mistake.
 
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Im in the process of setting up a large file server too and RAID 5 is what I planned to use as well, but was told I should use RAID 6 instead with FlexRAID, SnapRAID, Drive Pool or Drive Bender. Im going to be testing them all with WHS once the hardware arrives. Jot sure which one I will end up using as of yet.
 
i really want this up and running... onyone got time to say there say about:

Initialization state: Fast Initialization
Strip size: 256KB
Read policy: Always Read Ahead
Write policy: Write Through
I/O policy: Direct IO
Access policy: Read Write
Disk cache policy: Disabled
 
It should work. Those settings are more for performance tuning, so if you're not satisfied with the speed(s) of your array you can tinker with the settings.

RAID5 is kinda frowned on in this forum, though at least you're using a decent hba.

Are you using disks that are on the LSI HCL?

Make sure you have backups if you care about the data.
 
hotcrandel said:
It should work. Those settings are more for performance tuning, so if you're not satisfied with the speed(s) of your array you can tinker with the settings.
Click to expand...

i have no BBU so as far as my understanding goes i cannot use Caching if i care at all about my data.


hotcrandel said:
RAID5 is kinda frowned on in this forum, though at least you're using a decent hba.
Click to expand...

i understand that.. so it is that big of a risk then... hm...

hotcrandel said:
Are you using disks that are on the LSI HCL?
Click to expand...

no they are not on the HCL.

hotcrandel said:
Make sure you have backups if you care about the data.
Click to expand...

well its mostly movies and series.


I'm going back and forth on the Raid 5 vs. Raid 6.. How big could a Raid6 be if i use 3tb drives? Is like 8 drives ok?
 
Write policy: Always Write Back or Write policy: Write Through is the ones I'm mostly concerned about.. i have no clue what that one is about..


but also Read policy: Always Read Ahead


like i said before i have no BBU or a CacheVault.
 
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Regardless of the RAID level, make sure you have a complete backup before doing an online capacity expansion.
 
Honestly i don't have the money. So i will just hope it will go smooth. Its just movies and series..
 
DigitalLF said:
Write policy: Always Write Back or Write policy: Write Through is the ones I'm mostly concerned about.. i have no clue what that one is about..


but also Read policy: Always Read Ahead


like i said before i have no BBU or a CacheVault.
Click to expand...

With no BBU attached to the raid card it will always be in Write Through mode. No need to be concerned really. With a BBU you do gain a tiny bit of Performance.

There is absolutely nothing wrong with using Raid 5.

Just remember minimum disk requirements.
Raid 5 = 3
Raid 6 = 4

Raid 6 is more expensive, you loose 2 drives to parity.

If you use 4 x 3TB disks in Raid 6 you will end up with 6TB of storage. Raid 5 would give you 9TB.

Make sure to have a backup, as Raid is in no way a backup of your critical files. Raid just helps keep you up and running in the event of a physical disk failure.
 
i got a Raid 6 up and running now but i only got 3 disk in it right now. and i just cleaned out the 4th one so I'm adding that one to the Raid 6 in just a few minutes.

feels great to finally gotten around and gotten my self a Raid controller and a bunch of disks..
 
It feels great until drives fall out of the array because they don't support TLER or have another issue.
 
DigitalLF said:
Write policy: Always Write Back or Write policy: Write Through is the ones I'm mostly concerned about.. i have no clue what that one is about..


but also Read policy: Always Read Ahead


like i said before i have no BBU or a CacheVault.
Click to expand...

That has to do with the use of a write cache on the RAID controller. In Write Through mode, data is immediately written to the spindles when the controller receives a write command.

In write back mode, data written to the array is held in a cache on the raid controller. Especially with random writes, there is a HUGE performance benefit to using Write Back mode. The drawback being that the cache is volatile, so if power is lost between when the cache gets written, and when the contents eventually do get written to the spindles, whatever data was stored in the cache is lost.

Ideally, you'd use Write Back With BBU. This is where you have special battery backups for the RAID controllers that keep the volatile cache memory from being lost in the event of a power failure. The batteries cost about $150 apiece, IIRC.

Edit: If what you're using this for is storing movies and TV shows, there's no need to use the BBUs unless you just want to.
 
would a BBU or a CacheVault (super capacitor and NAND Module) be a safety measure against a RAID crash? (yeah i know always have a backup..)
 
DigitalLF said:
would a BBU or a CacheVault (super capacitor and NAND Module) be a safety measure against a RAID crash? (yeah i know always have a backup..)
Click to expand...

No, the battery backup / cache vault holds data that has yet to be written to the array until power returns.

This (being TLER) has to do with the drives falling out of the array, and the array totally breaking during rebuilds.
 
I can't get over 24 MB/s.... whats that about? Raid6 4x3TB SATA3 7200RPM.

JUxJZdA.png
 
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Well, the low write performance is a result of omitting the BBU. Since all data has to be committed to disk directly, you will get really bad random write performance. You can improve it by force enabling write caching, you will however risk data loss in case of a power failure.
 
to do a speed test could i just change

"Write policy: Write Through" & "Disk cache policy: Disabled" to "Write policy: Always Write Back" & "Disk cache policy: Enabled"

I just got series on the array right now so no worries. I'm all about learning. :)
 
Yes, you could try that and it should significantly improve your write speeds. Be aware of the fact that the controller will acknowledge writes as being on stable storage even if they are not. Not even journaling file systems may help you then.
 
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