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I am preparing PCs built with 64GB RAM on the Gigabyte AORUS X299-GAMING-3 motherboard, ordered for Cent OS 7 use, with RAID-1 on a pair of Intel 545 Series SSDSC2KW512G8X1 SSDs. The configuration is without a PCIe RAID hardware controller so I will use Intel's RST.

Since CentOS stopped documenting v7, I went to Red Hat docs and was able to find https://access.redhat.com/solutions/2801341 a warning which concerns me:

Software RAID levels 1, 4, 5, and 6 are not recommended for use on SSDs. During the initialization stage of these RAID levels, some RAID management utilities (such as mdadm) write to all of the blocks on the storage device to ensure that checksums operate properly. This will cause the performance of the SSD to degrade quickly.

Will I also face this problem with RAID-1 with Intel's RST RAID?

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RST/TRIM Research

I have no first hand experience with this, but I was able to find this article which discusses TRIM as being part of the issue/concern when setting up SSDs in a RAID-1 configuration.

You may notice that the read/write speeds of your Solid State Drives (SSD's) in a RAID 1 configuration degrade over time. If the operating system is reinstalled or the system is re-imaged the problem is temporarily resolved.

...

The process of garbage collection involves reading and rewriting data to the flash memory. This means that a new write from the host will first require a read of the whole block, a write of the parts of the block which still include valid data, and then a write of the new data. This can significantly reduce the performance of the system over time. If you need to confirm that TRIM is the cause of write speed degradation on an SSD in a RAID 1, you can re-image the drive (outside of the RAID volume) and re-test the write speed. Expected writes speeds for all SSD's vary dependent on the model.

... In Linux, fstrim provides this functionality, readying the drive for new data to be written and extends the life of the drive over the long term. Since trimming SSDs is not automatic on the Linux distributions that I have used, it is imperative that it be scheduled or the performance of the SSD will degrade over time.

The article then goes on to show how to do this:

$ mount -t ext4 -o discard /dev/sda2 /mnt
$ sudo fstrim / -v

This trimming can then be incorporated into a CRON job using this approach. Searching more led me to the Arch Linux's Wiki on SSDs. There the fstrim.service is mentioned.

The util-linux package provides fstrim.service and fstrim.timer systemd unit files. Enabling the timer will activate the service weekly. The service executes fstrim(8) on all mounted filesystems on devices that support the discard operation.

Assuming that these services are working, I would assume that the trimming is being properly done against any SSD's on the system, that support TRIM.

MD RAID TRIM Support

To try and put more definitive nails in the coffin, I searched for "raid linux trim support" which led me to this AU Q&A titled: How to set up SSD raid and TRIM support?

The answers there are a bit dated but still included several useful clues on how to proceed. Looking to see if MD RAID supported DISCARD & TRIM support led me to the next tip.

MD RAID does support DISCARD/TRIM

The SU Q&A titled: Implementing Linux fstrim on SSD with software md-raid where Michael Hampton's answer states this:

Patches for mdraid RAID 1 and 10 support for TRIM went up about a year ago.

He then goes on to show how you can confirm support for this:

You can test your kernel's support by mounting the filesystem with the discard option (which enables automatic TRIM). If it is supported, you'll see a message in syslog or dmesg like the following:

EXT4-fs (md1): re-mounted. Opts: discard,data=ordered

Otherwise you would get this:

EXT4-fs warning (device md1): ext4_issue_discard:2619: discard not supported, disabling

You don't need to reboot to see if it's working:

mount /dir -o remount 

Conclusions

So it would appear that the warning is dated, prior to these patches and is no longer applicable, assuming your SSD support TRIM/DISCARD.

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