Re: Possible issue with software RAID 1 in case of disks with different speed

[Andy Smith <andy@strugglers.net>]

Hi,

On Sun, Dec 07, 2025 at 02:41:37PM +0000, Christian Focke-Kiss wrote:
> Conclusion:
> I suspect software RAID 1 has issues if one disk of a three disk RAID 1
> set is significantly slower than the other two disks.

As others have mentioned I suspect your problem is USB. At no point in
the last 25 years have I found it a reliable way to have permanently
attached storage.

Storage with wildly different latency isn't a great setup but I haven't
found MD RAID-1 to have a big problem with it, not to the point of
instability.

write-mostly hasn't tended to have a huge effect for me unless the two
devices are radically different in performance. MD RAID already sends
reads to the device with the lowest amount of pending IO so all else
being equal, if you pair a SSD with a HDD, the SSD will get more reads
because they will complete sooner. Though do note that with RAID-1 a
single sequential read will all come from one device. It is only when
there are multiple threads reading that balancing can take place.

Thanks,
Andy

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