>> Was my raidz2 performance comment above correct? >> That the write speed is that of the slowest disk? >> That is what I believe I have >> read.
> You are > sort-of-correct that its the write speed of the > slowest disk. My experience is not in line with that statement. RAIDZ will write a complete stripe plus parity (RAIDZ2 -> two parities, etc.). The write speed of the entire stripe will be brought down to that of the slowest disk, but only for its portion of the stripe. In the case of a 5 spindle RAIDZ2, 1/3 of the stripe will be written to each of three disks and parity info on the other two disks. The throughput would be 3x the slowest disk for read or write. > Mirrored drives will be faster, especially for > random I/O. But you sacrifice storage for that > performance boost. Is that really true? Even after glancing at the code, I don't know if zfs overlaps mirror reads across devices. Watching my rpool mirror leads me to believe that it does not. If true, then mirror reads would be no faster than a single disk. Mirror writes are no faster than the slowest disk. As a somewhat related rant, there seems to be confusion about mirror IOPS vs. RAIDZ[123] IOPS. Assuming mirror reads are not overlapped, then a mirror vdev will read and write at roughly the same throughput and IOPS as a single disk (ignoring bus and cpu constraints). Also ignoring bus and cpu constraints, a RAIDZ[123] vdev will read and write at roughly the same throughput of a single disk, multiplied by the number of data drives: three in the config being discussed. Also, a RAIDZ[123] vdev will have IOPS performance similar to that of a single disk. A stack of mirror vdevs will, of course, perform much better than a single mirror vdev in terms of throughput and IOPS. A stack of RAIDZ[123] vdevs will also perform much better than a single RAIDZ[123] vdev in terms of throughput and IOPS. RAIDZ tends to have more CPU overhead and provides more flexibility in choosing the optimal data to redundancy ratio. Many read IOPS problems can be mitigated by L2ARC, even a set of small, fast disk drives. Many write IOPS problems can be mitigated by ZIL. My anecdotal conclusions backed by zero science, Marty -- This message posted from opensolaris.org _______________________________________________ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
