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RAID1 vs RAID5: read performances of a 2-disks array [Resolved]

I'm testing some RAID setups built with mdadm. This is about parallel reads on a 2-disks array.

RAID1 seems to balance the reads by process, some sort of coarse-grained balancing, so reading the whole raid array with a simple dd if=/dev/md0 of=/dev/null bs=1M, according to nmon, basically directs all the read to /dev/sda. When multiple dd commands run in parallel, some read from sda some from sdb.

Curiously, I tested with RAID5, same two disks, same redundancy and 50% disk utilization. One single low-level read with dd is spread across sda and sdb evenly, so every read operation seems to be better parallelized.

Effects on the CPU load seem to be negligible. I also haven't checked write operations.

Does using RAID5 for a 2-disks array make any sense?

By the way, going back and forth between 1 and 5 is as easy as mdadm --grow /dev/md0 --level 1 or 5, no data is moved, only the parallel read pattern changes.


Question Credit: ragazzojp
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Asked February 15, 2019
Posted Under: Network
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