RAID 10 performace with mixed speed disksI

[Tomoki Taniguchi]

I have 4 4TB disks.
2 are 7200RPM and 2 are 5400RPM.
I want to join them as one raid 10 zpool drive.
2 sets of 4TB mirrored drive striped as one pool.
I want to avoid putting the same speed disks together in the mirrored drives because each pair are from the same batch, making them likely to fail near the same period.
(i have had bad experiences with such failures)
how much of a performance hit will I experience if i have 2 sets of pairs each having 1 7200RPM dirve and 1 5400RPM drive and striping the 2 pairs.
The other option is to mirror the 2 7200RPM pair and then mirror the 5400RPM pair and striping the 2 pairs.

If I primarily do reads off of the drive, which layout would yield the better performance?
or would a zraid setup be better?

thank you
Tomoki

 

[Mirfster]

Since you are dealing with just 4 disks, it would basically result in the same space as using Mirrors. This is because RaidZ2 (I personally do not recommend RaidZ1) uses two disks for redundancy. So in the end the net space is still ~ 2 Disks worth.

Some info from the "ZFS Primer" that may provide insight so you can make a decision:

When determining the type of ZFS redundancy to use, consider whether your goal is to maximize disk space or performance:

• RAIDZ1 maximizes disk space and generally performs well when data is written and read in large chunks (128K or more).
• RAIDZ2 offers better data availability and significantly better mean time to data loss (MTTDL) than RAIDZ1.
• A mirror consumes more disk space but generally performs better with small random reads. For better performance, a mirror is strongly favored over any RAIDZ, particularly for large, uncacheable, random read loads.
• Using more than 12 disks per vdev is not recommended. The recommended number of disks per vdev is between 3 and 9. If you have more disks, use multiple vdevs.
• Some older ZFS documentation recommends that a certain number of disks is needed for each type of RAIDZ in order to achieve optimal performance. On systems using LZ4 compression, which is the default for FreeNAS® 9.2.1 and higher, this is no longer true. See ZFS RAIDZ stripe width, or: How I Learned to Stop Worrying and Love RAIDZ for details.

Here are some other links as well:
Getting the most out of ZFS pools!
A Closer Look at ZFS, Vdevs and Performance

 

[Spearfoot]

Yes, just put all four disks in a RAIDZ2 array. That way you can lose any two disks and still not lose your data.

The different rotational speeds probably don't matter as much as the actual I/O transfer rates of the drives. In either case, the array will have the performance of the slowest disk in the pool. But I doubt you'll notice any debilitating performance penalty.

 

[rs225]

Matching the mirrors together will give the better performance. A vdev operates at the speed of the slowest member. Your scrub speeds may also be reduced. But a faster vdev and a slower vdev in the same pool do not have the same effect.

Whether the difference is enough to matter would be up to you to test.

 

[MatthewSteinhoff]

1. It likely doesn't matter.

2. With four drives, I like striped mirrors better than RAIDZ2. You get the same amount of space and better performance at the cost of a bit less redundancy.

Cheers,
Matt

 

[Arwen]

That drive setup is exactly what I have, 2 x 4TB WD Reds, (5400 RPM), and 2 x 4TB
WD Red Pros, (7200 RPM). And in my case I also wanted the models split for extra
redundancy since the Red Pros likely have different failure characteristics than the
plain Reds.

Howerver, I did a RAID-Z2 since that would allow any 2 disks to fail, regardless of
which 2 they were.

If you are going to use 2 x Mirror vDevs, then I would create them with 1 x 7200 RPM
nd 1 x 5400 RPM drive just for that extra redundancy. Performance will be at the minimum
of 5400 RPM drives, but could boost a bit up.

Note that some setups, databases and zVols for virtual machines, prefer Mirrored vDevs.
If you are not doing such, then RAID-Z2 would be the way to go.