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Write-Back Caching on PERC 5/i
Hi Everyone,
I have an SBS 2003 R2 box. It's a Dell box with 2 X Xeon 2.0 GHz, 4GB RAM, Power Edge RAID Card 5/i with a RAID 1 on a logical SAS 500GB HDD. System Volume C: is 150GB, Data Volume E: is 350GB. Both Exchange databases reside on the DATA volume.
The PERC 5/i controller has a battery backup. It's default setting is "Write-back" cache with a battery backup. The controllers documentation Notes: If Write-back is enable and the system quickly turned off and then on, the controller may pause as the system flushes cache memory.
The other setting available is "Write-Through caching", the controller sends a data transfer completion signal to the host when the disk subsystem has received all the data in a transaction.
I've read Microsoft Documentation which states when an Exchange 2003 storage groups database is modified, several steps occur. First the database is read into the database cache, and then the time stamp on the page is updated. Next, the log record is created, stating what is about to be done to the database. This occurs in the log cache buffer.
Then the page is modified and a connection is created between these two entries so that the page cannot be written to disk without the log entry being written to disk first. This step guarantees that a modification to the database on disk is updated.
Hence, there is a legitimate concern over the write-back caching that can be enabled on a log file disk controller. Essentially, "Write-Back Caching" means that the hardware reports back to ESE a successful disk write even though the information is held in the disk buffer on the controller to be written to disk at a later time.
Question: Will changing the Write policy setting to "Write Through" have a significant impact on performance in an SBS box while still offering the intended goal of any avoidance in corrupting any intended exchange databases? Any experience with this situation would by humbly helpful. There is no comparison to experience verses inexperience. Thank you in advance.
Joey
mattfez
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Posted July 24th, 2010 09:00
Hi Joey,
My experience with PERC cards is limited to FreeBSD systems only, but I think that the principles are the same. The default install of FreeBSD disables the controller cache and not knowing this I carried on happily. Then one day I was doing some tweaking and found that I had not been using the cache all this time, so being inquisitive, I turned it on. Diskinfo showed a marked improvement in performance of both the seek, read and write times.
Basically, if your application (Exchange 2003) suggests that write caching is a bad idea, then you should probably disable it to be safe. However doing so will made a significant difference on disk I/O.
Matt