On 04/17/2013 02:08 AM, Edward Ned Harvey (openindiana) wrote: >> From: Sašo Kiselkov [mailto:skiselkov...@gmail.com] >> >> If you are IOPS constrained, then yes, raid-zn will be slower, simply >> because any read needs to hit all data drives in the stripe. > > Saso, I would expect you to know the answer to this question, probably: > I have heard that raidz is more similar to raid-1e than raid-5. > Meaning, when you write data to raidz, it doesn't get striped across > all devices in the raidz vdev... Rather, two copies of the data get > written to any of the available devices in the raidz. Can you confirm?
No, this is not what happens. Raid-Z indeed does stripe data across all leaf vdevs (minus parity) and does so by splitting the logical block up into equally sized portions. A block *can* take up less than the full stripe width for very small blocks or for very wide stripes, both of which should be a rare occurrence. 4k sectored devices change this calculation quite dramatically, which is why I wouldn't recommend using them in pools unless you understand how your workload and raidz geometry will interact and take note of it. See vdev_raidz_map_alloc here: https://github.com/illumos/illumos-gate/blob/master/usr/src/uts/common/fs/zfs/vdev_raidz.c#L434-L554 for all the fine details (the above description is grossly oversimplified). > If the behavior is to stripe across all the devices in the raidz, > then the raidz iops really can't exceed that of a single device, > because you have to wait for every device to respond before you > have a complete block of data. But if it's more like raid-1e and > individual devices can read independently of each other, then at > least theoretically, the raidz with n-devices in it could return > iops performance on-par with n-times a single disk. As a general rule of thumb, raidz has the IOPS of a single drive. This is not exactly news: https://blogs.oracle.com/roch/entry/when_to_and_not_to Cheers, -- Saso _______________________________________________ OpenIndiana-discuss mailing list OpenIndiana-discuss@openindiana.org http://openindiana.org/mailman/listinfo/openindiana-discuss