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I'm looking at the DELL PowerEdge™ R720 from Hetzner.

This server has a Dell PERC H710 8 Port SAS/SATA 6GBit/s raid controller.

I have the choice between:

  • 3 x 600GB SSD (MLC SATA 3Gb/sec)
  • 6 x 600GB 15k rpm SAS disks

The price between both options are the same.

I need at least 1,2 TB of disk space, more is better.

The server will be used to host ESX 5 development machines. Mostly heavy reads, not that many writes.

Which option would be more performant?

With the 6 x 600GB SAS disks I would also have the possibility to put them in RAID 10 (and of course RAID 5). With the SSD's it will be RAID 5.

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    No, NEVER use RAID 5 with SSDs. And run far away from Hetzner. – Michael Hampton Jun 7 '13 at 8:14
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    @MichaelHampton Can you explain your comment? In this way nobody knows why to do your advise. – Uwe Plonus Jun 7 '13 at 8:15
  • @UwePlonus If you use RAID 5 with SSDs, they are written to continuously and wear out in as little as a few weeks. Goodbye data. And Hetzner... well you can google "Hetzner sucks" for that. – Michael Hampton Jun 7 '13 at 8:18
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    See this answer for why RAID5 isn't so bad after all. @MichaelHampton, it's pure FUD to say SSDs will wear out in weeks in a RAID5. – Daniel Lawson Jun 7 '13 at 8:26
  • @DanielLawson Fair enough, though the write amplification is still a significant issue to be aware of. – Michael Hampton Jun 7 '13 at 8:32
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RAID 10 is better for databases. Even though you'll be heavier on reads, caching and changes to the actual database will still impact the performance of your server.

I would go with the 6x SAN disks. SSDs in RAID 5 is just asking for trouble since you only have a single drive tolerance, and SSDs tend to fail rather suddenly where spinning drives tend to have warning signs.

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Based on the information in your question, it is impossible to say for sure, which option is best. It depends on the specifics of your workload and priorities besides performance. My best guess, from the information, that you provide would be, that the SSD option would outperform the SAS spinning disks. However, RAID-5 has a number of issues, that you should consider, before settling on that version, especially: 1: RAID-5 is quite vulnerable to critical data loss, if a second disk fail during a RAID rebuild. 2: during a RAID rebuild, the system will perform very poor, due to the load of the rebuild. 3: RAID-5 cause write-amplification, where everytime you write user data to your drives, parity must also be written, causing a higher number of total writes, and thus shortening the lifespan of your SSDs.

  • This parrots the thought that all RAID-5 is bad. It is also incorrect. For 600GB volumes the resilver time on replacing a drive on a hardware controller is on the order of a couple of hours, probably closer to an hour for 15K rpm drives. So unless you have either a hot spare or no backups, the 24 hours it takes you to get the replacement drive is of a greater concern than the likely hood of a second drive failure during a resilver operation. – Rowan Hawkins Apr 26 '18 at 17:21

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