Re: [Centos] Promise raid cards - software raid

In my limited experience with linux(i have a firewall using ext3 and a small file server using ext3) power failures have not negatively impacted the file system. The only thing i did wrong was forcing fsck which corrupted things into an unrecoverable state.

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.

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 ?

Thanks in advance to all,

Daniel

----- Original Message -----
*From:* Terrence Martin <mailto:tmartin@physics.ucsd.edu>
*Cc:* CentOS@caosity.org <mailto:CentOS@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@revsharecorp.com>
 >     *To:* dan1 <mailto:dan1@edenpics.com>
 >     *Cc:* CentOS@caosity.org <mailto:CentOS@caosity.org>
<mailto:CentOS@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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