[Centos] Promise raid cards - software raid

Thu Nov 4 19:55:52 UTC 2004
dan1 <dan1 at edenpics.com>

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,


  ----- Original Message ----- 
  From: Terrence Martin
  Cc: 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.


  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>
  >     *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
  >     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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