I have a Dell R720 Server with PERC H310 hardware raid 5 and 4 x Seagate Cheetah 15K.7 ST3300657SS 300GB hard drives. Windows Server 2008 R2. The reads look good, but the writes are painfully slow.
Running the Atto Disk Benchmark, I'm seeing read speeds of >500 MB/sec at transfer sizes from 128kb to 8192kb.
For writes with transfer sizes from 128kb to 8192kb, I'm seeing read speeds of 20 to 22 MB/sec. About 10 times too slow.
The same machine has a new Samsung SSD. Writes for the SSD are > 450 MB/sec.
Besides checking the drivers, what might cause such bad performance? Where are good places to look? What good additional tests to run?
I was the only user during these tests. No resource hungry processes were running.
Best Answer
The H310 does not have any write cache (compared to the H710, which has 512MB of battery-backed cache), so right off the bat you're not going to get "great" write performance with it. The H310 also doesn't utilize the cache built into the hard drives by default, so everything is strictly "write-through".
You could always enable disk caching in Windows, which uses the server's RAM for write caching... but use caution as unexpected system shutdowns or power loss can lead to data corruption. The same risk exists for using the physical disks' cache - this can be enabled on the Virtual Disk from OMSA with the "Change Policy" option on the virtual disk.
RAID5 has a significant "write penalty" to consider as well. For every single write operation you send to the controller, 4 I/Os must be performed to accomplish it (due to recalculation of parity data). With write-through on the controller, you're getting hit with the write penalty of the RAID type, plus the delay involved in the controller recalculating parity.
The performance stats you're getting look completely normal to me based on the hardware and configuration. If write performance represents a true business need for you, then you should either consider using a different RAID type (e.g. RAID10), consider the risks and benefits of enabling caching on the drives or on the volume from within the OS, or consider upgrading your hardware (e.g. to an H710 controller).