raid 1 with ssd drives

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I am considering using 2 sata ssd's for a MD raid 1 configuration, and was curious how the performance would be compared to only a single drive. I saw this from the RH storage guide and though it was intriguing, but no further explanation is given: https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Storage_Administration_Guide/ch-ssd.html

Red Hat also warns that 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.

Has anyone had experience trying this?

CO
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SM Guru 3731 points
11 September 2016 6:39 PM

...Two drives would be effective if we think on performance wise and this picture is all about data read/write after it is delivered by file system stack, so masking the OS side here. Since each drive would got its own mechanism of handling data (hardware buffer, data handling technology), performance would be better when we ask to store bunch (mirroring) of data on 2 drives instead of one drive with multiple partitions.

I found this kb article which got some relevant information:- Do RHEL6 Storage Admin Guide comments about using RAID with SSDs apply to hardware RAID?

CO Newbie 12 points
13 September 2016 11:53 PM

Thanks for the link but it still seems to suggest linux software raid is no good with SSD's, which was a surprise.

Looking more closely at the storage guide, I see with EL7 they added this: Discard support for the dm-mirror was added in Red Hat Enterprise Linux 6.1 and as of 7.0 MD supports discards.

So it looks like only MD on LVM support discards properly, but mdadm still doesn't work properly with it on a raw block device. Hope someone that knows could clarify?

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