![]() Rather than having entire disks dedicated to parity, RAIDz vdevs distribute that parity semi-evenly across the disks. RAIDz1, RAIDz2, and RAIDz3 are special varieties of what storage greybeards call "diagonal parity RAID." The 1, 2, and 3 refer to how many parity blocks are allocated to each data stripe. Most vdevs are used for plain storage, but several special support classes of vdev exist as well-including CACHE, LOG, and SPECIAL. Each of these vdev types can offer one of five topologies-single-device, RAIDz1, RAIDz2, RAIDz3, or mirror. Each vdev, in turn, consists of one or more real devices. ![]() vdevĮach zpool consists of one or more vdevs(short for virtual device). Such a mismatched pool will still generally perform as though it were entirely composed of the slowest device present. The utilization awareness mechanism built into modern ZFS write distribution methods can decrease latency and increase throughput during periods of unusually high load-but it should not be mistaken for carte blanche to mix slow rust disks and fast SSDs willy-nilly in the same pool. In more recent versions of ZFS, vdev utilization may also be taken into account-if one vdev is significantly busier than another (ex: due to read load), it may be skipped temporarily for write despite having the highest ratio of free space available. A zpool is not a funny-looking RAID0-it's a funny-looking JBOD, with a complex distribution mechanism subject to change.įor the most part, writes are distributed across available vdevs in accordance to their available free space, so that all vdevs will theoretically become full at the same time. It is a common misconception that ZFS "stripes" writes across the pool-but this is inaccurate. Modern zpools can survive the loss of a CACHE or LOG vdev-though they may lose a small amount of dirty data, if they lose a LOG vdev during a power outage or system crash. Just know up front that in the understated words of OpenZFS developer Matt Ahrens, "it's really complicated."īut before we get to the numbers-and they are coming, I promise!-for all the ways you can shape eight disks' worth of ZFS, we need to talk about how ZFS stores your data on-disk in the first place. Well, today is the day to explore, ZFS-curious readers. In the second of those stories, we even promised a follow-up exploring the performance of various multiple-disk topologies in ZFS, the next-gen filesystem you have heard about because of its appearances everywhere from Apple to Ubuntu.
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