[Centos] Promise raid cards - software raid

Thu Nov 4 20:38:48 UTC 2004
Terrence Martin <tmartin at physics.ucsd.edu>

dan1 wrote:

> Hello, Terrence.
> Thank you for this sharing.
> It is very sad that these cards don't provide cache flush in case of 
> power failure, because it's the main difference I thought between 
> software raid and hardware raid.
> Some people told me that software raid was dangerous because of that 
> fact (no battery when power fails), and hardware raid would solve this 
> problem. In this case, to me there is no main advantage to use 
> hardware raid.

I think that you were not given enough granularity in what is hardware, 
and what is software RAID.

RAID is nothing more than a technique to duplicate data, or generate 
parity from data so that in the event of a single spindle failure 
availability is not interupted and data is preserved at the cost of lost 
redundancy. The most common ways to do raid is OS software RAID, 
Software RAID using a combination of drivers and firmware (promise 
cards), Hardware raid controllers with the parity chips on them (3ware), 
or full on Hardware RAID devices that include a custom OS, controllers 
and a disk chassis. Both hardware raid controllers and hardware raid 
devices can come with batter backups. Whether you need them or not, or 
if they offer anything to you depends on your environment.

Software RAID does not really offer any more or less security than 
hardware in this respect unless you are talking about dedicated RAID 
devices like an Infortrend  fiberchannel attached RAID array or apples 
XServeRAID. These devices can come with memory often 128MB to 1GB with 
batteries that last for several days. However these devices still will 
be stymied by the fact that ATA drives have a write back cache. If the 
disk itself can cache data there is no guarantee that the data is 
written to disk, and no battery on the hardware raid controller is going 
to help.

However a decent UPS on the whole system with a sync command followed by 
a proper OS shutdown will give you the best chance of not loosing any data.

> Then, I am quite interested be anyone having any experience with 
> software raid power failures with ext3. Does the filesystem crash 
> often ? And is it severe ?

I am not sure what you mean about the file system crashing? The file 
system itself does not crash, if the system shutsdown without a proper 
unmount there may be data loss wth any system. Even a batter backup on 
the raid array does not guarantee anything. All it says is that the data 
that the controller got is safe, but the program that generated that 
data may be gone and that itself can cause data corruption. For example 
a complex insert that is only 50% done. No disk battery is going to help 
that. A UPS will though.

I do not recommend ext3 for anything over about 120GB. The delete time 
alone with ext3 vs XFS shows XFS is better designed for larger amounts 
of data, but XFS has many more features that make it superior. Not the 
least of which is that you can tune the FS to fit your hardware. However 
I have recovered disks for all of the journaled file systems with no 
problems that I recall. The important thing is to have a journaled file 
system. Basically a journal just tells the OS what files are fully 
written and which ones are suspect. The suspect ones are deleted. They 
do not protect data but make ensuring file system integrity a lot more 
efficient as anyone who has sat through a 1TB system trying to run an 
FSCK on the file system can attest.

My biggest question is why at this point are you even bothering with 
PATA drives? Compared to SATA drives they are unreliable and poor 
performing for about the same cost.

SATA is a significant step forward (if the drive is native SATA like 
seagates) over PATA and implement many of SCSI features for performance 
and longevity.

Of course you have to make sure your SATA drives are SATA and not PATA 
in SATA clothing. Avoid Western Digital, Some Maxtor , and some Hitachi 
drives. Seagate is a guarantee of native SATA. You also have to make 
sure your controller supports the features. I believe the 3ware 9000 
series does. The 8000 series is emulated SATA I believe.


> Thanks in advance to all,
> Daniel
>     ----- Original Message -----
>     *From:* Terrence Martin <mailto:tmartin at physics.ucsd.edu>
>     *Cc:* CentOS at caosity.org <mailto:CentOS at caosity.org>
>     *Sent:* Thursday, November 04, 2004 7:47 PM
>     *Subject:* Re: [Centos] Promise raid cards
>     Those cards have no battery backup. When the power goes they are off,
>     any cached data is gone. However a power failure in my experience
>     is not
>     a problem, or at least not more of a problem than is to be
>     expected from
>     ATA disks.
>     ATA Disk all suffer from write back caching problem. That is you
>     never
>     really know if data is written to disk. Today many of your ATA drives
>     are coming with 8MB of cache. That is 8MB of data that could be
>     sitting
>     in volatile RAM in the event of a power failure. Even a card with a
>     battery backup will not help you because the disks themselves may
>     hold
>     some important data. If this is a concern get SCSI or a good UPS
>     with a
>     shutdown procedure.
>     You can of course mitigate file system corruption but not data
>     loss with
>     a good journaled FS. We prefer XFS for anything over 100GB and we run
>     into the 10's of Terabytes on some systems.
>     If you system is transaction based (database server) you almost
>     certainly want a UPS on the whole thing with at least 5-10 minutes,
>     preferably 20 minutes of battery life. That should give you time
>     to sync
>     your disks and shut the system down hopefully. You will want to test
>     this of course.
>     Terrence
>     dan1 wrote:
>     > Thank you, Ajay.
>     > 
>     > That's useful to me.
>     > And do you know if the 3ware ATA raid card (7006-2 or 7506-4LP I
>     > suppose) flushes the disks in case of power failure or do they just
>     > forget the buffered datas so that the filesystem crashes
>     afterwards ?
>     > (i.e. do they have a capacitor to hold the datas up to the
>     moment they
>     > are all written). I will have a remote reboot (power failure like)
>     > that I might use quite often and I know this is no problems with
>     ext3
>     > filesystem on 1 IDE disk only (I made some tests), but is it the
>     same
>     > reliability with this 3ware raid card for power failures ?
>     > 
>     > Thanks a lot for sharing your experience !
>     > 
>     > Daniel
>     > 
>     > 
>     >
>     >     ----- Original Message -----
>     >     *From:* Ajay Sharma <mailto:ssharma at revsharecorp.com>
>     >     *To:* dan1 <mailto:dan1 at edenpics.com>
>     >     *Cc:* CentOS at caosity.org <mailto:CentOS at caosity.org>
>     <mailto:CentOS at caosity.org>
>     >     *Sent:* Thursday, November 04, 2004 5:58 PM
>     >     *Subject:* Re: [Centos] Promise raid cards
>     >
>     >     dan1 wrote:
>     >
>     >     > I would like to know if promise RAID cards are compatible with
>     >     CentOS /
>     >     > RHEL ?
>     >     > I have seen that only SATA is supported on the RHEL hardware
>     >     > compatibility list. The other ATA raid cards seems not to be
>     >     compatible.
>     >     > They give source code and promise grants compatibility with
>     >     RedHat 8 and
>     >     > 9 but not RHEL.
>     >     >
>     >     > I would like to know if somebody tried a ATA raid card like
>     >     Fasttrack
>     >     > TX2000, SX4000, Fasttrack 100 TX2, etc..
>     >     > If you could share me your experience it would be great.
>     >     >
>     >     > My provider says that he had some bad experiences about that
>     >     cards and
>     >     > he doesn't allow me to use CentOS on his promise cards he
>     provides
>     >     > (only), so I cannot have RAID on my server... it's a shame..
>     >
>     >     I haven't checked recently, but the last time I played with any
>     >     promise
>     >     controller it was a train wreck.  I then picked up a 3ware ATA
>     >     RAID card
>     >     and never looked back.  It's well supported as the drivers
>     are in the
>     >     main kernel tree since 2.4.  So you install your drives,
>     setup the
>     >     ATA
>     >     raid array in the 3ware bios and when you boot up it's
>     detected as a
>     >     SCSI device.  It's the easiest solution out there, so IMO,
>     it's well
>     >     worth the extra money.
>     >
>     >     --Ajay
>     >
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