I have a cheap 2-bay NAS with a 2TB HDD. To be robust against disk failure, I'm thinking of buying a second 2TB HDD and putting it in RAID1 with Linux mdadm. The file system is ext4.
Will this increase or decrease the performance of the NAS? What about just read or write performance?
There seem to be lots of opinions about this online but no consensus.
So already I've got three different answers: "a fair bit faster", "you wont notice" and "will decrease the performance if anything". (I am interested primarily in read performance.) Wikipedia says "the read performance can go up roughly as a linear multiple of the number of copies". Which one is it?
I've found mounting evidence in support of RAID1 increasing read performance, including the MD manpage:
Changes are written to all devices in parallel. Data is read from any one device. The driver attempts to distribute read requests across all devices to maximise performance.
I also discovered MD's RAID10 with
--layout=f2, which provides redundancy of RAID1 with the read performance of RAID0, and can be used with just two drives. The write performance is reduced however, as a sequential write involves both drives seeking back and forth between distant parts of the drive.
man md for details.