ESXi 4 Promise SAN. Created 80Gb virtual disk (not thin-provisioned). Guest is SUSE 10.2.
Simply using dd if=/dev/zero of=/test.txt
In performance under Disk, the host performance shows ~75Mb/s write while the guest performance shows 37.5Mb/s (this is in the VSphere client's performance charts). The guest console shows about 40Mb/s.
Now if I delete that file and run the exact same test, it'll run at ~110Mb/s on both the host, guest, and guest console UNTIL it hits the same file size that the file I deleted was and then it goes back to the 75Mb/s -- 37.5Mb/s oddity.
I finally let it write the entire disk. I then deleted the file and rebooted the ESXi server. Everything ran at ~110Mb/s.
Any explanation? The SAN enclosure was fully synchronized. I thought it might have something to do with writing zeros the whole timebut that wouldn't explain the discrepancy between the host performance chart and the guest performance chartso I wrote some actual data (thousands of small files) to the drive, deleted that, and tried again with the ~110Mb/s performance.