Re: [gentoo-server] what is the best filesystem for a server

[Todd M. Hébert]

Whether the RAID is toast due to the loss of two drives depends on how many drives you're striping the data across and how many parity drives you have.

With RAID 5, if you have 9 drives you end up with one dedicated for parity, and the system should keep running with a loss of 2 drives without incident. (provided that you replace the 2 affected drives before a third fails.)

I believe you can lose three drives out of 9 as long as it's not the parity drive on RAID 5.

On RAID 4, how many drives you can lose depends on whether you are running single or dual parity, and how many you have overall.  You can run 8 data drives plus dual parity.. in this configuration you should be able to lose 2 data drives AND a parity drive (out of a 10-drive configuration) before data becomes endangered. (or 3 data drives.)

The major difference between 4 & 5 is being able to resize the RAID set on the fly.  (adding drives to the RAID pool without having to completely rebuild it.)

The data centre that we're in tests their battery backup once each week, and the diesel generators once a month.  There are two completely discrete power systems (from the cabling that our servers connect to, which are always dual-powered, through the battery backups, and the generators. There is a spare battery backup system that can be manually shunted into place in the even that either of the primaries fails to perform... takes only a few huge switches to change over, and all our networking kit is duplicated, so we can run without incident if we lose either of our power feeds singly.)  We have had zero downtime due to any faults in equipment owned by the data centre since opening in 1999... 3 other data centres within 10 miles of here have had serious outages, including all-day outages, due to faults in their failover.. the guys we lease our space from really know what they're doing.)

--Todd


RijilV wrote:
> 2008/6/9 "Todd M. Hébert" <todd@×××.ie>:
>> We use RAID 1 on servers that are not file servers, RAID 4 or 5 on
>> file-servers (depending on how much need for redundancy we have on a
>
> Doesn't RAID 4 and 5 offer the same level of protection?   I thought
> the only difference was RAID4 had a dedicated parity device whereas
> RAID5 stripes parity information across all devices.  In either case,
> loose two drives and the RAID is toast.
>
>
>> We're in a data centre that isn't likely to have blackouts.  (It can run on
>> batteries for 6 hours, and has diesel generators with 48-hours worth of fuel
>> on-site, as well as an emergency supply-chain for the diesel.)
>
> I too have been in very very high profile very very nice data centers
> with a jabillion hours of battery backup and even more of generator
> power.   One time someone hit the BIG RED BUTTON on the floor where
> our gear was caged and presto - power was gone.  Another time the city
> cut the power mains to the building, and the generator that was
> supposta supply half of our racks someone had left in "manual" mode as
> apposed to automatic, thus taking out a fair number of our servers.  I
> think if you ask any sufficiently large group of people for horor
> stories of power going out in a N+2 redundant power environmnet,
> you'll get way more than you were looking for.
>
>
>> I've not seen problems as described below on 3Ware cards... I believe the
>> configuration for each RAID is is backed-up on each disk in the RAID set.
>>  I've never had a problem with those.  (I have seen a problem with another
>
> I too have only ever had wonderful experiences with 3Ware cards under
> Linux.  I've not had one of their cards fail, but have had backplanes
> fail and bad cables.   Diagonosing and getting your vendor to admit to
> a bad backplane generally requires more than one outtage :(  Now LSI
> cards on the otherhand have caused me some grief.
>
>
>> XFS filesystems no all of the above, just for reference.
>
> FWIW, always have run ext3, unless it was before ext3 was 'stable',
> then it was ext2.
>

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