We have two PE R900, equipped with PERC 4e/DC (SCSI U320) and PERC 6/i (SAS). The SAS PERC 6/i works nicely, while the SCSI array PERC 4e/DC works poorly.
The PERC 4e/DC is connected to a PowerValut 220S unit with 14 Seagate ST3300007LC drives, there are two RAID5 volumes with 7 drives each. These RAID volumes were made into two ext3 file system on the server. Whenever I write large files (e.g. a 10 GB file) to the PERC 4e/DC volumes, it would finish writing fairly quickly ( 90 MB/s), however, the RAID volume would still busy for about 15 minutes afterwards, iowait for the volume would be at about 9% and system load stays high until the iowait goes down below 1%. If I copy more files (using rsync for example), the whole system would be unresponsive for a long time due to high iowait for the volume. Currently, the PERC 4e/DC uses Write-back caching mode.
Does anyone have any suggestion to tune this RAID system so that it would lock up the WHOLE system while there is a lot of writing activities to the SCSI RAID volume. I don't have trouble with the SAS RAID at all.
Thank you for sharing your thoughts.