SSD rear guard
IOPS, or input-output operations per second, is a popular parameter, good enough to feel exceptionally proud about. Of course, hosting provider but rarely manufacture storage devices, in order that proud mentioned be authentic. Yet these magic numbers are always posted and referred to. In the VPS benchmarks (see links above) on this site, these benchmarks are measured by performing that many input-output operations, each handling 4kB of data (4096 bytes).
Cloudive.com's contacted me to run a series of tests: how would SSD disk behave when placed into RAID of different types. Note that unless RAID controller recognizes SSD and can perform specific commands like TRIM, it's actually doesn't provide any specific advantages when utilizing RAID.
The testing performed has provided to me that well-known SSD Guard is, in fact, of little real use when talking about boosting disks operations I/O. To those not familiar, I cite manufacturer's explanation.
SSD Guard™, unique to MegaRAID, increases the reliability of SSDs by automatically copying data from a drive with potential to fail to a designated spare or newly inserted drive. A predictive failure event notification, or S.M.A.R.T command, automatically initiates this rebuild to preserve the data on an SSD whose health or performance falls below par. This new feature will greatly benefit users employing a RAID 0 configuration due to the added data protection.
When enabled in MSM, SSD Guard will protect any and all logical volumes built using SSD devices (figure 6). In figure 7 and figure 8, we see a MegaRAID adapter with a RAID 0 volume built from two solid state disk drives. Should one of these drives fail, data loss would occur. However, since SSD Guard is enabled, the MegaRAID adapter is actively monitoring the status of both SSDs. Should a failure appear to be eminent, the MegaRAID adapter will automatically begin rebuilding data onto a third SSD hot spare. If a hot spare is not present or not assigned to the RAID 0, MSM will recommend that the user insert a hot spare drive into an available slot. Once the drive is inserted, copyback will begin.
Now let's talk about I/O measurements.