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I'm trying to understand why one would add a batterypack to a raid card. It seems to me like if power goes down, running just the raid card is going to do little good: without power for HDs and motherboard, writing in-memory data isn't going to work anyway, right?

In addition, doesn't having a UPS facilitate this?

4 Answers4

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It allows the raid card to remember what is in its buffers ( that hasnt been sync'd to disk )

Its very important for people who need high data integrity.. Or to save your DB from certain types of corruption..

(Basically whats on disk, is on disk - so thats safe.. The problem is when the OS thinks its on disk but its actually not and in a RAID card buffer)

When the server starts up again, obviously those buffers get flushed to the disks.. So you have a point in time correlation with your disks and OS..
( otherwise you will just loose information - like a few database records, which you will never know. )

A UPS help sure.. but its not safe enough.. ever decent RAID card should have a BBU (Battery Backed Unit)

Arenstar
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  • +1 I would specify "every add-on RAID Card should have a BBU". Very few integrated RAID cards have them. – Antoine Benkemoun Nov 18 '10 at 11:16
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    Ohh i mean that by "decent".. onboard are never decent :P – Arenstar Nov 18 '10 at 11:17
  • Does not change that every Raid card shold have one ;) Integratd are often pretty crappy. – TomTom Nov 18 '10 at 11:17
  • Ah, that makes sense, it just keeps it in memory until the server comes back up. –  Nov 18 '10 at 11:31
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    Right.. :D + its never a good idea to put all your eggs in one basket.. so you cant/shouldnt just rely on an UPS.. – Arenstar Nov 18 '10 at 11:32
  • Still though, only after the UPS runs out of juice you'd wish you had a battery pack, right? This is not for a server but a workstation, so it will rarely be unattended. –  Nov 18 '10 at 11:35
  • True True.. What type of RAID do you have? – Arenstar Nov 18 '10 at 11:37
  • I'm in the market. Probably a LSI 9260-8i or Areca ARC-1680x. –  Nov 18 '10 at 11:38
  • Oh i know this card (LSI 9260-8i).. its standard in my old servers.. :D – Arenstar Nov 18 '10 at 11:39
  • They only started selling it here in december 2009, so I'm guessing those servers aren't THAT old. –  Nov 18 '10 at 11:41
  • I just finished working for this client.. thats what i mean by "old".."they arent my servers anymore" – Arenstar Nov 18 '10 at 11:43
  • +1 - It is 100% worth having evening if its not probable your UPS will fail. I've seen UPS's without warning simply die on power loss as the battery was aged. – JamesK Nov 18 '10 at 11:50
  • I see. So you recommend this card? It'll have to handle all sorts of IO, lots of small files and huge files (10-30GB) as well. Stuff like remote management is not important obviously. –  Nov 18 '10 at 11:52
  • Ah, just in it for the creds, are we ;) Sure. –  Nov 18 '10 at 11:52
  • @James.. really??? ive not had "that much" experience with UPS's ( mostly the datacenters take care of them)- What a ridiculous piece of equipment to see an UPS die instantly... :O – Arenstar Nov 18 '10 at 11:53
  • Well no no.. im try to help you all firstly.. :D but i only started my account 12 days ago, and seeing how far i can get before i go on holidays next week.. hehe :P – Arenstar Nov 18 '10 at 11:55
  • You should get extra creds for the entertainment value of your creative headgear. –  Nov 18 '10 at 11:57
  • Oh right.. hehehe.. Creative headgear came from me drunk, at a girls birthday party, that pink thing was wrapped around her present.. :P – Arenstar Nov 18 '10 at 12:00
  • You think i could get a badge for that :D – Arenstar Nov 18 '10 at 12:00
  • Jake.. just a point.. The cards you mentioned dont have a BBU do they? – Arenstar Nov 18 '10 at 13:25
  • Not standard, no. –  Nov 18 '10 at 17:44
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Whilst I agree with Arenstar I've recently moved from battery-backed cache based controllers to flash-backed ones. This eradicates the urgency in moving the controller and any risk of accidentally disconnecting the battery during the movement. They seem to be about the same price roughly and actually have more cache anyway.

Chopper3
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Most RAID controllers that support Write caching, will not enable it without a battery backup pack. Imagine the damage a large 64 Megs of cached writes, not written to disk would do to a volume.

Without write caching, RAID5 controllers write performance drop by a factor of 5-10 times. (We had a Dell PERC 3 (The LSI, not Adaptec ones) that would write sustained at about 8 GB/hour with write cache off, but at 70-90 GB/hour with write caching on.

geoffc
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    I do believe in using the batteries when available, but am not overly concerned if a server doesn't have one. In practice, I've noticed that the cached writes have a very short life in the buffer. They make it to disk surprisingly quick even on our heavily utilized servers. It also doesn't solve the issue of the writes/processes that were only partially supplied to the card from the app & OS. Does it help, yes, it will help minimize one particular case of data corruption. However, there's still a LOT of other places for it to go wrong during a power outage. – Brian Knoblauch Nov 18 '10 at 15:12
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    Ah but some RAID controllers REQUIRE a battery pack before they will even enable write caching. So of course, that is a different animal. – geoffc Nov 18 '10 at 16:12
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A raid battery pack is a necessity depending of the cache configuration of your Raid array.

If you happen to use Write-back (when the controller informs the Os the data write was successful while still in cache, in opposition to write-through when the controller waits for the data to be on disk), you could lose crucial data should power fail, because all cached data would be lost.

You could still lose cached data if the controller itself fails though.

Berzemus
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