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jfly

1 Rookie

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11 Posts

88482

September 4th, 2015 03:00

RAID and TRIM Config for R720xd and SSDs

Hi,

I have:

  • Dell Poweredge R720xd
  • 4 x 800GB Dell branded Intel DC S3700 SSDs
  • 8 x 600GB Dell branded Savvio 10K.6  2.5" HDDs
  • Dell PERC H710P Mini Mono

I would like to configure the server with a RAID 5 array for the SSDs and another RAID 5 array for the HDDs.  

Regarding the SSDs, I read that:

  1. The H710P doesn't pass through TRIM/UNMAP commands from the OS
  2. TRIM is not really needed since the RAID controller has some method to let the drive know that the block is unused
  3. A small percentage of the SSD (~5%) should be left unused.  I believe this is called over provisioning.

Is (3) above really necessary when it comes to RAID with SSDs?  If so, how do I achieve this?  Do I:

only allocate 95% of the available SSD space when creating the RAID volume?

OR

allocate 100% of the available disk space to the RAID volume and then when I create partitions during OS installation, leave 5% unallocated?

Daniel My

12 Elder

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6.2K Posts

September 4th, 2015 11:00

Hello

None of that matters. All of the issues you are concerned about have no bearing on your configuration.

  • The H710P doesn't pass through TRIM/UNMAP commands from the OS
  • TRIM is not really needed since the RAID controller has some method to let the drive know that the block is unused

When you use a hardware RAID controller the operating system has no control over the blocks. Any blocks the operating system thinks it is controlling are virtual, so any TRIM/UMAP the OS is doing is against virtual blocks.

A small percentage of the SSD (~5%) should be left unused.  I believe this is called over provisioning.

Over provisioning happens at a drive level. The SSD manufacturers allocate space for this. If you want to do additional provisioning then some drive manufacturers provide utilities to allow you to manipulate the provisioning, or you can just have unallocated space on the drive. Any unallocated space will automatically be used by the drive, but I don't see a need for this since SSDs are already over provisioned by the manufacturers.

Thanks

jfly

1 Rookie

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11 Posts

September 5th, 2015 05:00

Thanks for the explanation Daniel.

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