JMOR
Cadet
- Joined
- Dec 15, 2020
- Messages
- 2
For a personal computer (Unix) I have to decide between mirroring 2 SSDs or RAIDz1 3 SSDs.
After some reading, I found places that say mirroring is faster than RAIDz1. This doesn't make any sense to me.
Yes, RAIDz1 has to compute the parity info for the third disk. But, CPU times (for computing parity) are orders of magnitude faster than I/O times (for accessing disks). So, that advantage should be close to negligible.
A more impactful consideration is, when writing on disks, RAIDz1 splits the load between two disks, Mirroring does not. One half in one disk, the other half in the other. And this should be done in (or close to) parallel. Which should translate in (or close to) half the time.
This is the whole schtick of RAID 0. Isn't RAID 5 (RAIDz1) equal to RAID 0 plus a third disk for parity?
When READING from disk, mirroring can read in parallel from both disks and have more or less the same performance than RAIDz1.
But WRITING, you have to write the whole thing on each of the mirror disks. I fail to see how that is faster than RAIDz1.
After some reading, I found places that say mirroring is faster than RAIDz1. This doesn't make any sense to me.
Yes, RAIDz1 has to compute the parity info for the third disk. But, CPU times (for computing parity) are orders of magnitude faster than I/O times (for accessing disks). So, that advantage should be close to negligible.
A more impactful consideration is, when writing on disks, RAIDz1 splits the load between two disks, Mirroring does not. One half in one disk, the other half in the other. And this should be done in (or close to) parallel. Which should translate in (or close to) half the time.
This is the whole schtick of RAID 0. Isn't RAID 5 (RAIDz1) equal to RAID 0 plus a third disk for parity?
When READING from disk, mirroring can read in parallel from both disks and have more or less the same performance than RAIDz1.
But WRITING, you have to write the whole thing on each of the mirror disks. I fail to see how that is faster than RAIDz1.