SAS5IR RAID 1 - No write caching, slow performance?

Question

I just got a poweredge 860 and decided to try to save a few bucks by foregoing SCSI drives and purchased 2x160GB SATA drives.   They came configured as RAID 1 however I'm noticing slow performance and notice that write caching is disabled for these drives.   How do I enable it?  When I go into the Server Administrator >Storage>SAS 5/iR > Virtual Disks - there are NO Tasks Available.   Read Policy = No Read Ahead Write Policy = Write Through   I have the latest firmware/drivers   Firmware: 00.10.49.00.06.12.02.00 Drivers: 1.24.04.00   Do I change this outside of windows?   I read somewhere that by default, write caching is disabled on these drives for safety reasons..however, is there no way to force it on? :smileysad:

dmose · Answer

/bump any feedback?   How do I enable write-caching on this controller?? The fact that the controller doesn't have a battery doesn't concern me, my system is on a UPS and will shutdown gracefully on power failure... we should have the ability to fully control the disk I/O subsystem   Should I contact LSI? Is this a seagate or LSI or DELL issue?? .. :robotindifferent:

thomas_smaistrla · Answer

There is no write caching on the SAS 5iR controller.

dmose · Answer

>http://lists.us.dell.com/pipermail/linux-poweredge/2007-January/029348.html

>

>I was wondering if you had success in enabling write-caching with this

>controller? I'm in a windows x64 environment and can't find anywhere to

>enable this feature. I understand the controller doesn't have a

>battery however the server is on UPS so I'm willing to take the risk

Yes, I enabled write-caching and I understand this setting on this controller actually enable the write cache of the SATA disks.

Dell told me in future firmware/driver versions this will be done automatically, so I went on with this setting to get back to a normal performance.

To enable write caching I used a Linux command line utility (lsiutil) I found in the Linux driver package I downloaded from Dell site.

I don't know about Windows. If you have ne of those Linux live CD on hand you could boot with that and I could send you the utility.

--

giulioo@pobox.com

thomas_smaistrla · Answer

I believe that Dell is referring to the PERC in this statement. When the original PERC 5I and PERC 5E firmware came out it did not have write-back caching enabled by default. Now, when you create any type of array with the latest firmware, it should enable caching by default.   There is no RAM on the SAS 5iR integrated card, therefore there is no write-caching options available. If you look at the features on the PERC it has cache memory and hense the ability to enable caching.

dmose · Answer

Nah they're talking about the SAS5/iR   http://lists.us.dell.com/pipermail/linux-poweredge/2007-January/029348.html   http://www.nabble.com/Dell-SAS5-Performance-Issue-tf3474648.html#a9778750   Basically you're right..however the work around is to enable write caching on the Hard Disk's themselves which increases performance but carries more risk.  My understanding is that if you enable it on the controller, it will enable it for the hard disks as well

dmose · Answer

Dell got back to me..   Download and install the older Raid Manager which gives you access to the VIRTUAL DISK write cache policy       I'm going to run some tests and will let you know the results

dmose · Answer

As a follow up:   PERC5/iR without cache enabled:       PERC5/iR  WITH cache enabled:

mike_tcis · Answer

You say 'older version' of the RAID Manager -- that one is dated 04/2007, and does mention SATA write caching specifically in the rel. notes, so it appears to be a fairly recent version of the util.  Glad to hear it is the solution though.

thomas_smaistrla · Answer

Thanks for all of the information. I ran tests on the SAS 5iR and PERC 5i and came up with very similar performance conclusions.   So that everyone is aware, this option is disabled by default because it is not recommended. If you have a system crash you may have a higher chance of data corruption as the disk cache is not battery protected. Windows may also flag events in the system log that the write cache is enabled and data corruption could occur.

dmose · Answer

The only options are:   1) turn on write caching via the Dell Raid Manager tool I linked to.  This is *risky* because if your system is not on UPS and you experience a power surge you can potentially lose data.  However most will argue that you're in RAID 1 anyways thus the amount of data lose is minimal and next to none if you have a good UPS.   2) Get a new raid controller that has 64mb+ of onboard cache and enable write caching and read ahead on the controller.   3) Switch to RAID 0.

JohnReam · Answer

Thanks dmose -

I think your option #2 is the most safe and smart one.

Can we buy a PERC card and simply swap it in? Such as the PERC 5i, which I guess is one best/ better performer. We have 2 160GB Sata Disks Raid 1. Next question would be, can we swap a new card in and hopefully not have to rebuild the O/S, which is W2K3 sp2?

JohnReam · Answer

Is there any updated information on this topic? I also ran the ATTO Tests and got similar terrible Write Performance!   We just migrated our IIS Web Server to a new Dell Poweredge 840.  Loaded.  But we've noticed the poor performance with the SAS5IR RAID 1.    IIS does do plenty of Disk Writes, mostly the IIS Logs I guess, but I'm sure there is plenty of other disk access writing, temporary and otherwise, such as the virtual memory file.   Is there a solution? Recommendation?     I see that the SAS5IR is a card in the Server.  Can we replace this card with a better one?

dmose · Answer

I'm sure you can recover the raid volume if you change controllers as the parity information is stored on the disk and not on the card itself (i may be wrong about this)   I dont know if you can use a perc5/i ... you need to check with Dell

JohnReam · Answer

Yes, the PERC 5 cards are fully supported in the PowerEdge 840s.

Ok.... so I have to figure out how to get the Disk Images over to PERC, hopefully without having to rebuild the O/S.

How about two Cards in the PC at once? The PERC is a PCI/Exp slot card, and the SAS has its own special slot which I can see. If Two Cards are possible, and since the Raid Cards are enabled during BIOS, the GHOST Images should be able to copied from SAS to PARC drives.... does this sound ok so far?

Then I have to see if the Windows 2K3 image will reconize the new PERC Card, or maybe the dreaded HAL.dll errors will be my next hurdle.... what do you think?

I'm desperate. I recommended the 840, my small company put it into production after many days of O/S building and IIS configuring, and now the boss is looking at me in an unpleasent kind of way.

Help!

Message Edited by JohnReam on 06-22-2007 03:50 PM

JohnReam · Answer

I'd like to report back that I tried dmose's option #1:   1) turn on write caching via the Dell Raid Manager tool I linked to.  This is *risky* because if your system is not on UPS and you experience a power surge you can potentially lose data.  However most will argue that you're in RAID 1 anyways thus the amount of data lose is minimal and next to none if you have a good UPS.   Success.  I installed the Tool v2.08-6 on our Dell Poweredge 840, despite the little detail that the 840 was not listed as being support, I installed it anyway.   The tool started ok and seems to be properly working.  Under the 'Logical' tab, Virtual Disk, Operations Tab, Set Virtual Disk properties, I changed Disk Cache Policy from Disabled to Enabled.    ATTO Disk Benchmarks have improved dramatically for writes.

PowerEdge HDD/SCSI/RAID

SAS5IR RAID 1 - No write caching, slow performance?

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