ZFSGuru: 8 DIsk ST32000542AS raidz2 upgrade with ST2000DL003 Drive?

S

spankit

Limp Gawd
Joined
Oct 18, 2010
Messages
262
Current Setup:
8x ST32000542AS single RAIDZ2

I'm running ZFSGuru 0.1.7. I will be looking at upgrading my server to handle more drives in the near future. Would there be any issue in running the new Seagate green drives in conjunction with the the old LP drives? I had a look at the specs and the main difference is that the new one's have double the cache and are 6Gbps compatible. The new setup will go from 8 to 24 drives in two raidz2's.

Any input?
 
ST32000542AS = Seagate Barracuda® Green SATA 3Gb/s 2TB Hard Drive, a 5900rpm drive with 4 platters of 500GB each, so comparable to WD20EADS 2TB drive. But a little bit slower, as i see they are rated for 95MB/s max throughput. The Samsung F4 is max 140MB/s which is almost 50% higher, but has some performance loss due to 4K sectors but that comparison is still beneficial to the F4.

The disadvantage of your current drives is that they are susceptible to corruption/bit rot since they use 512-byte sectors in combination with high density platters. You definitely need RAID-Z2 if you have 512-byte sector drives, and that offers less protection then you may think. One complete failure + one other disk with BER that is what you can fix. If you have two failures then it could be that some files will be corrupted when rebuilding as your remaining disks may not be able to read all sectors. 4K sector disks would have less problems with this Bit-Error-Rate (BER) issue.

If you want 24 drives you could keep your existing 8 drives in RAIDZ2, and add two other RAID-Z2's of your new disk, so three RAID-Z2 consisting of 8 disks each (3*8=24).

Alternatives would be if you buy Samsung F4 drive instead, you keep the 8 drives, then have 10 drives in RAID-Z2 or 2x6 drives in RAID-Z2. Or have 2x 10 Samsung F4 in RAID-Z2 but that would leave you with 20 + 8 = 28 disks, more than you can store in your casing i believe.

Either way, do go RAID-Z2 and 10 disks in RAID-Z2 would be optimal for the Samsung F4. For 512-byte sector HDDs you have the advantage of more flexibility as for example a 9-disk RAID-Z2 would work just as well too, whereas the Samsung F4 and other 4K disks would prefer special combinations like:
RAID-Z: 3, 5 or 9 disks
RAID-Z2: 6 or 10 disks

The added cache and 6Gbps interface are kind of meaningless though, don't expect any performance gain from those features. HDD performance primarily is about platter density and spindle speed. All the rest are details.
 
I'm looking to loose minimal drive space. So I was hoping to make two separate raidz2's. So I would essentially buy another 16 drives. Build a 12 drive raidz2, transfer the files from the existing 8 disk raidz2 to the new 12 and then scrap the 8 and build a second 12 disk raidz. Unless performance is really terrible then I guess I could just build 2*10 disk raidz2's and then use the remaining space in the case for other things if performance is really that terrible. The reason I was looking at the ST2000DL003 drives instead of the F4 is simply about my budget for the project.

How terrible would a 12 Disk raidz2 run with 4k sector drives?
 
Buying 2x10 Samsung F4 is not an option? That would be ideal for storage capacity (least space wasted while still having good redundancy) but it would get you with 28 drives instead of 24.

Not sure about the price of ST2000DL003, but the Samsung F4 is one of the cheapest drive on the planet, is it not? I mean it sells for 69 euro over here, which i guess could be translated as 75 dollars. That's a steal for the latest 2TB with 666GB platters, achieving 140MB/s at very low idle power consumption; great for a NAS!

Their 4K sectors do mean you may run into performance issues on non-optimal configurations, though. If you cannot do the 2x10 disk RAID-Z2 and want a 12-disk RAID-Z2 instead, then you could consider the sectorsize override feature. If you can make 2x10 work for you then this would be the optimal configuration with least amount of wasted space.

Your benchmarks probably will suggest way over 100MB/s performance, but keep in mind this is when your pool is empty and thus the 'maximum performance' of your pool; not the average performance after having filled your pool and with some fragmentation over time. So generally, you want your pool to perform at least twice as good as your target performance. So 100MB/s over gigabit would mean a minimum of 200MB/s write performance in benchmarks.

A 10-disk RAID-Z2 kan peform like 600MB/s or so, the 12-disk RAID-Z2 probably also has sufficient throughput, but i'm worried that the performance degradation will be worse as you fill your pool.

Either way, before you commit real data to your disks, do run benchmarks so you know how your hardware performs under simulated conditions.
 
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