Slow write speeds on RAID 5 setup

[PhoenicianWarrior]

For my RAID speed testings I put in 4 new mushkin reactor 1tb SSD's and I'm using the pegasus r4 RAID box. I added the disks into an array and synchronized them as a logical drive with a RAID 5 level setup. After a few hours when the synchronization was complete I did a test of copying 49.97GB of data to the promise RAID drive and it took 11 minutes and 32 seconds to complete. If my calculations are correct then that would mean the write speed was at about 74Mb/s. I also used a disk speed testing software called Black Magic which gave me write speeds of 666.7MB/s and Read speeds of 793.0MB/s. I do not know why im getting such low write speeds, does anybody know if this is normal or any way to improve the speeds if I somehow tweak with settings?

 

[dgingeri]

Unfortunately, that is a normal thing with RAID 5 and RAID 6. http://rickardnobel.se/raid-5-write-penalty/ It is especially harsh on SSDs, as the RAID parity calculations take far longer than the drives do to write the information. RAID 10 would be better for SSDs, as there are no parity calculations. Since you have 4, you can do RAID 10 pretty easily, but you do only get half the capacity.

 

[Flying Head]

I gave up on redundant drives and am now using Raid O and backups. My backups were really creeping at a snails pace, so I got a used LSI 9565-8i. One drive kept failing after 3 returns to WD, so I gave up on Greens. Now have 4 3GB Reds in Raid 0 backing up to 3 3GB Toshibas (no failures yet) and get 450Mbps read + 450Mbps write = 900Mbps showing on resource Monitor. HD were cheaper than SSD, still a chunk of change (+ greater capacity per buck)! I can not quote where I read that SSD's are really slow overwriting old data because of overhead (trim is supposed to fix that?), and that HD can (maybe) beat them except for read latency. The latest new devices would be faster still. Benchmark software doesn't seem to give real answers, as you witnessed? I only have my system to compare, not a pro tech with 100's of observations.

 

[PhoenicianWarrior]

Unfortunately, that is a normal thing with RAID 5 and RAID 6. http://rickardnobel.se/raid-5-write-penalty/ It is especially harsh on SSDs, as the RAID parity calculations take far longer than the drives do to write the information. RAID 10 would be better for SSDs, as there are no parity calculations. Since you have 4, you can do RAID 10 pretty easily, but you do only get half the capacity.

That may be the case, but would it really drop down to speeds of 74Mb/s? That just seems awfully low. Could there be something on my end causing this?

 

[dgingeri]

Honestly, I'm surprised it's not worse than that.