Tuning SSD RAID for optimal performance

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Checking RAID Consistency

In addition to testing SMART attributes, consistency checks are a further component in optimal RAID operation. Hardware RAID manufacturers also refer to consistency checks as "verification." The mdadm software implements the functionality with the checkarray script. No matter what technology you use, regular consistency checks discover inconsistencies in data or checksums. Therefore, make sure you run regular consistency checks on your system.

1. Mdadm typically sets up a cronjob in /etc/cron.d/mdadm. Every first Sunday of the month, the job starts a consistency check for configured software RAID. But beware – make sure after the check that the associated counter is 0 in sysfs:

$ cat sys/block/md0/md/
mismatch_cnt
0

Although the script checks the RAID array for consistency, it does not perform any corrections itself. Mismatches are indicative of hardware issues for RAID levels 4, 5, and 6. RAID 1 and 10 can also produce mismatches without an error, especially if swap devices are located on them. If an error occurs, incidentally, mdadm sends email only if you enter the correct address in the mdadm.conf file under MailAddr.

2. The MegaRAID Storage Manager (MSM) is the first stop for regular consistency checks for LSI. One clear advantage is that MSM runs on Windows, Linux, and VMware ESXi. The Controller section takes you to the Schedule Consistency Check tab. You can define when and how often to run a check.

3. The Adaptec command-line tool enables consistency checks via the datascrub command. Data scrubbing is often used in the context of consistency checks because it designates the process of error correction. The command

$ arcconf datascrub 1 period 30

sets up a regular check every 30 days.

Conclusions

Current RAID controllers experience no problems with SSD RAID, and performance compares well with Linux software RAID. The choice of RAID technology is therefore mainly a question of the operating system and personal preferences. Linux with recent kernels and mdadm definitely has all the tools you need for SSD RAID on-board. (See the "Checkbox for SSD RAID" box.) One thing is clear: Controller cache and read ahead are not useful for SSD RAID. You can also save yourself the trouble of BBU or flash-based cache protection solutions.

Checkbox for SSD RAID

Always use the latest RAID controller for HWR.
On Linux, use the deadline I/O scheduler for HWR and SWR arrays.
For HWR cache settings, use write through, no read ahead.
Overprovision in the case of high random write requests.
Use SSDs for which the SMART attributes are publicly documented.
Write endurance determines the durability of the SSD; SMART attributes indicate the current status.

Choosing the right RAID level is your responsibility. You need to roll up your sleeves and analyze your applications or systems. Is your I/O primarily write or read? If random reads are the prime focus, RAID 5 is a good choice, while keeping capacity losses low. The classic RAID 1 is not totally out of fashion with SSDs. Solid latencies and 40 percent higher read IOPS than a single SSD speak for themselves. You can expect write IOPS hits in both RAID 5 and RAID 1. If you are thinking of purchasing a large number of SSDs and would like to achieve balanced performance, there is no alternative to RAID 10. Write performance, latency, and read IOPS with RAID 10 are all very much what the admin ordered.

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