Re: Arguments Pro/Contra Software Raid

Hi Hannes,

Hannes Dorbath a écrit :
> Hi,
>
> I've just had some discussion with colleagues regarding the usage of
> hardware or software raid 1/10 for our linux based database servers.
>
> I myself can't see much reason to spend $500 on high end controller
> cards for a simple Raid 1.

Naa, you can find ATA &| SATA ctrlrs for about EUR30 !

> Any arguments pro or contra would be desirable.
>
> From my experience and what I've read here:
>
> + Hardware Raids might be a bit easier to manage, if you never spend a
> few hours to learn Software Raid Tools.

I'd the same (mostly as you still have to punch a command line for
most of the controlers)

> + There are situations in which Software Raids are faster, as CPU power
> has advanced dramatically in the last years and even high end controller
> cards cannot keep up with that.

Definitely NOT, however if your server doen't have a heavy load, the
software overload can't be noticed (essentially cache managing and
syncing)

For bi-core CPUs, it might be true

> + Using SATA drives is always a bit of risk, as some drives are lying
> about whether they are caching or not.

?? Do you intend to use your server without a UPS ??

> + Using hardware controllers, the array becomes locked to a particular
> vendor. You can't switch controller vendors as the array meta
> information is stored proprietary. In case the Raid is broken to a level
> the controller can't recover automatically this might complicate manual
> recovery by specialists.

?? Do you intend not to make backups ??

> + Even battery backed controllers can't guarantee that data written to
> the drives is consistent after a power outage, neither that the drive
> does not corrupt something during the involuntary shutdown / power
> irregularities. (This is theoretical as any server will be UPS backed)

RAID's "laws":

1- RAID prevents you from loosing data on healthy disks, not from faulty
disks,

1b- So format and reformat your RAID disks (whatever SCSI, ATA, SATA)
several times, with destructive tests (see "-c -c" option from
the mke2fs man) - It will ensure that disks are safe, and also
make a kind of burn test (might turn to... days of formating!),

2- RAID doesn't prevent you from power suply brokeage or electricity
breakdown, so use a (LARGE) UPS,

2b- LARGE UPS because HDs are the components that have the higher power
consomption (a 700VA UPS gives me about 10-12 minutes on a machine
with a XP2200+, 1GB RAM and a 40GB HD, however this fall to......
less than 25 secondes with seven HDs ! all ATA),

2c- Use server box with redudancy power supplies,

3- As for any sensitive data, make regular backups or you'll be as
sitting duck.

Some hardware ctrlrs are able to avoid the loss of a disk if you turn
to have some faulty sectors (by relocating internally them); software
RAID doesn't as sectors *must* be @ the same (linear) addresses.

BUT a hardware controler is about EUR2000 and a (ATA/SATA) 500GB HD
is ~ EUR350.

That means you have to consider:

* The server disponibility (time to change a power supply if no
redudancies, time to exchange a not hotswap HD... In fact, how much
down time you can "afford"),

* The volume of the data (from which depends the size of the backup
device),

* The backup device you'll use (tape or other HDs),

* The load of the server (and the number of simultaneous users =>
Soft|Hard, ATA/SATA|SCSI...),

* The money you can spend in such a server

* And most important, the color of your boss' tie the day you'll
take the decision.

Hope it will help you

Jean-Yves