A raidZ vdev cannot be converted to or from a mirror, although additional vdevs can always be added to expand storage capacity (which can be any kind including raidZ). A device in any vdev can be marked for removal, and zfs will de-allocate data from it to allow it to be removed or replaced. Of note, the devices in a vdev do not have to be the same size, but zfs may not use the full capacity of all disks in a vdev, if some are larger than other. This only applies to devices within a single vdev. As vdevs are independent, zfs does not care if different vdevs have different sizes or are built from different devices. Also as a vdev cannot be shrunk in size, it is common to set aside a small amount of unused space (for example 1-2gb on a multi-tb disk so that if a disk needs replacing, it is possible to allow for slight manufacturing variances and. Cache devices edit In addition to devices used for main data storage, zfs also allows and manages devices used for caching purposes.
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If data redundancy is required (so that data is protected against physical device failure then this is ensured by the user when they organize devices into vdevs, either by using a mirrored vdev or a raidZ vdev. Data on a single device vdev may be lost if the device develops a fault. Data on a mirrored or raidZ vdev will only be lost if enough disks fail at the same time (or before the system has resilvered any replacements due to recent disk failures). A zfs vdev will continue to function in service short if it is capable of providing at least one copy of the data stored on it, although it may become slower due to error fixing and resilvering, as part of its self-repair and data integrity processes. However zfs is designed to not become unreasonably slow due to self-repair (unless directed to do so by an administrator) since one of its goals is to be capable of uninterrupted continual use even during self checking and self repair. Since zfs device redundancy is at vdev level, this also means that if a pool is stored across several vdevs, and one of these vdevs completely fails, then the entire pool content will be lost. This is similar to other raid and redundancy systems, which require the data to be stored or capable of reconstruction from enough other devices to ensure data is unlikely to be lost due to physical devices failing. Therefore, it is intended that vdevs should be made of either mirrored devices or a raidZ array of devices, with sufficient redundancy, for important data, so that zfs can automatically limit and where possible avoid data loss if a device fails. Backups and replication are also an expected part of data protection. Vdevs can be manipulated while in active use. A single disk can have additional devices added to create a mirrored vdev, and a mirrored vdev can have physical devices added or removed to leave a larger or smaller number of mirrored devices, or a single device.
Therefore, biography it is easiest to describe zfs physical storage by looking at vdevs. Each vdev can be one of: a single device, or multiple devices in a mirrored configuration, or multiple devices in a zfs raid raidZ configuration. Each vdev acts as an independent unit of redundant storage. Devices might not be in a vdev if they are unused spare disks, disks formatted with non-zfs filing systems, offline disks, or cache devices. The physical structure of a pool is defined by configuring as many vdevs of any type, and adding them to the pool. Zfs exposes and manages the individual disks within the system, as well as the vdevs, pools, datasets and volumes into which they are organized. Within any pool, data is automatically distributed by zfs across all vdevs making up the pool. Each vdev that the user defines, is completely independent from every other vdev, so different types of vdev can be mixed arbitrarily in a single zfs system.
No attempts made to identify/resolve issues using zfs tools - zfs exposes performance data for much of its inner operations, allowing troubleshooting of performance issues with precision. It may, in some cases, need these tools to be used, to gain the best performance it can provide. Terminology and storage structure edit because zfs acts as both volume manager and file system, the terminology and layout of zfs storage covers two aspects: How physical devices such as hard drives are writings organized into vdev s (virtual devices - zfs's fundamental "blocks" of redundant. Zfs commands allow examination of the physical storage in terms of devices, vdevs they are organized into, data pools stored across those vdevs, and in various other ways. Various commands expose in-depth statistics of zfs' internal status and internal performance/statistics data, to allow settings to be optimized. Physical storage structure: devices and virtual devices edit The physical devices used by zfs (such as hard drives (HDDs) and ssds ) are organized into vdev s virtual devices before being used to store data. The vdev is a fundamental part of zfs, and the main method by which zfs ensures redundancy against physical device failure. Zfs stores the data in a pool striped across all the vdevs allocated to that pool, for efficiency, and each vdev must have sufficient disks to maintain the integrity of the data stored on that vdev. If a vdev were to become unreadable (due to disk errors or otherwise) then the entire pool will also fail.
A memory cache that would be appropriate for the former, can cause timeout errors and start-stop issues as data caches are flushed - because the time permitted for a response is likely to be much shorter on these kinds of connections, the client may believe. Similarly, many settings allow the balance between network latency (smoothness) and throughput to be modified; inappropriate caches or settings can cause "freezing slowness and "burstiness or even connection timeouts. Inappropriate use of deduplication - zfs supports deduplication, a space-saving technique. But deduplication in zfs typically requires very large or extreme amounts of ram to cache the entirety of the pools's deduplication data which can require tens or hundreds of Gigabytes of ram. This is because zfs performs deduplication encoding on the fly as data is written. It also places a very heavy load on the cpu, which must calculate and compare data for every block to be written to disk. Therefore, as a rule, deduplication requires a system to be designed and specified from the outset to handle the extra workload involved. Performance can be heavily impacted - often unacceptably so - if the deduplication capability is enabled without sufficient testing, and without balancing impact and expected benefits. Reputable zfs commentators such as Oracle 18 and ixSystems, 19 as well as zfs onlookers and bloggers, 20 21 strongly recommend this facility not be used in most cases, since it can often result in impact performance and resource usage, without significant benefit in return.
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Slog is therefore unusual in that its main criteria are pure write functionality, low latency, and thesis loss protection usually little else matters. Lack of suitable caches, or misdesigned/suboptimally configured caches — for example, zfs can cache read data in ram arc or a separate device l2arc in some cases adding extra arc is needed, in other cases adding extra L2arc is needed, and in some situations adding. Use of hardware raid cards, perhaps in the mistaken belief that these will 'help' zfs. While routine for other filing systems, zfs handles raid natively, and is designed to work with a raw and unmodified low level view of storage devices, so it can fully use its functionality. A separate raid card may leave zfs less efficient and reliable. For example, zfs checksums all data, but most raid cards will not do this as effectively, or for cached data.
Separate cards can also mislead zfs about the state of data, for example after a crash, or by mis-signalling exactly when data has safely been written, and in some cases this can lead to issues and data loss. Separate cards can also slow down the system, sometimes greatly, by adding cas latency to every data read/write operation, or by undertaking full rebuilds of damaged arrays where zfs would have only needed to do minor repairs of a few seconds. Use of poor quality components calomel identify poor quality raid and network cards as common culprits for low performance. 16 developer Jeff Bonwick also identifies inadequate quality hard drives, that misleadingly state data has been written before the data is actually written in order to appear faster than they are. 17 poor configuration/tuning zfs options allow for a wide range of tuning, and mis-tuning can affect performance. For example, suitable memory caching parameters for file shares on nfs are likely to be different from those required for block access shares using iscsi and Fiber Channel.
Highly tunable many internal parameters can be configured for optimal functionality. Can be used for high availability clusters and computing, although not fully designed for this use. Inappropriately specified systems edit Unlike many file systems, zfs is intended to work towards specific aims. Its primary targets are enterprise data management and commercial environments. If the system or its configuration are poorly matched to zfs, then zfs may underperform significantly. In their 2017 zfs benchmarks, zfs developers Calomel stated that: 16 "On mailing lists and forums there are posts which state zfs is slow and unresponsive.
We have shown in the previous section you can get incredible speeds out of the file system if you understand the limitations of your hardware and how to properly setup your raid. We suspect that many of the objectors of zfs have setup their zfs system using slow or otherwise substandard I/O subsystems." Common system design failures include: Inadequate ram — zfs may use a large amount of memory in many scenarios; Inadequate disk free space —. Around 70 is a recommended limit for good performance. Above a certain percentage, typically set to around 80, zfs switches to a space-conserving rather than speed-oriented approach, and performance plumments as it focuses on preserving working space on the volume; no efficient dedicated slog device, when synchronous writing is prominent — this is notably. The slog device is only used for writing apart from when recovering from a system error. It can often be small (for example, in Freenas, the slog device only needs to store the largest amount of data likely to be written in about 10 seconds (or the size of two 'transaction groups although it can be made larger to allow longer.
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Data can be reconstructed using all of the following: error detection and correction checksums write stored in each block's parent block; multiple copies of data (including checksums) held on thesis the disk; write intentions logged on the slog (ZIL) for writes that should have occurred but did. Native handling of standard raid levels and additional zfs raid layouts raidz. The raidz levels stripe data across only the disks required, for efficiency (many raid systems stripe indiscriminately across all devices and checksumming allows rebuilding of inconsistent or corrupted data to be minimised to those blocks with defects; Native handling of tiered storage and caching devices. Because it also understands the file system, it can use file-related knowledge to inform, integrate and optimize its tiered storage handling which a separate device cannot; Native handling of snapshots and backup/ replication which can be made efficient by integrating the volume and file handling. Relevant tools are provided at a low level and require external scripts and software for utilization. Native data compression and deduplication, although the latter is largely handled in ram and is memory hungry. Efficient rebuilding of raid arrays — a raid controller often has to rebuild an entire disk, but zfs can combine disk and file knowledge to limit any rebuilding to data which is actually missing or corrupt, greatly speeding up rebuilding; Ability to identify data that. For example, synchronous writes which are capable of slowing down the storage system can be converted to asynchronous writes by being written to a fast separate caching device, known as the slog (sometimes called the zil zfs intent Log).
15 Snapshots can also be cloned to form new independent file systems. Summary of key differentiating features edit Examples of features specific to zfs include: Designed for long term storage of data, and indefinitely scaled datastore sizes with zero data loss, and high configurability. Hierarchical checksumming of all data and metadata, ensuring that god the entire storage system can be verified on use, and confirmed to be correctly stored, or remedied if corrupt. Checksums are stored with a block's parent block, rather than with the block itself. This contrasts with many file systems where checksums (if held) are stored with the data so that if the data is lost or corrupt, the checksum is also likely to be lost or incorrect. Can store a user-specified number of copies of data or metadata, or selected types of data, to improve the ability to recover from data corruption of important files and structures. Automatic rollback of recent changes to the file system and data, in some circumstances, in the event of an error or inconsistency. Automated and (usually) silent self-healing of data inconsistencies and write failure when detected, for all errors where the data is capable of reconstruction.
an ntfs-formatted drive of their data, and ntfs is not necessarily aware of the manipulations that may be required (such as reading from/writing to the cache drive or rebuilding the raid array if a disk. The management of the individual devices and their presentation as a single device is distinct from the management of the files held on that apparent device. Zfs is unusual, because unlike most other storage systems, it unifies both of these roles and acts as both the volume manager and the file system. Therefore, it has complete knowledge of both the physical disks and volumes (including their condition and status, their logical arrangement into volumes, and also of all the files stored on them). Zfs is designed to ensure (subject to suitable hardware ) that data stored on disks cannot be lost due to physical errors or misprocessing by the hardware or operating system, or bit rot events and data corruption which may happen over time, and its complete. Zfs also includes a mechanism for snapshots and replication, including snapshot cloning ; the former is described by the Freebsd documentation as one of its "most powerful features having features that "even other file systems with snapshot functionality lack". 15 Very large numbers of snapshots can be taken, without degrading performance, allowing snapshots to be used prior to risky system operations and software changes, or an entire production live file system to be fully snapshotted several times an hour, in order to mitigate data. Snapshots can be rolled back "live" or previous file system states can be viewed, even on very large file systems, leading to savings in comparison to formal backup and restore processes.
Solaris, with a team led by the cto of Sun's storage business unit and Sun Fellow. 6 7 In 2005, the bulk of Solaris, including zfs, was licensed as open-source software under the common development and Distribution License (cddl as the OpenSolaris project. Zfs became a standard feature of Solaris In 2010, Oracle stopped releasing updated source code for new OpenSolaris and zfs development, effectively reverting Oracle's zfs to closed source. In response, the illumos project was founded, to maintain and enhance the existing open source solaris, and in 2013 Openzfs was founded to coordinate the development of open source zfs. 8 9 10 Openzfs maintains and manages the core zfs code, while organizations using zfs maintain the specific code and validation processes required for zfs to integrate within their systems. Openzfs is widely used in Unix-like systems. In 2017, one analyst described zfs as "the only proven Open source data-validating enterprise file system". 14 better source needed contents overview and design goals edit zfs compared to other file systems edit The management of stored data generally involves two aspects: the physical volume management of one or plan more block storage devices such as hard drives and sd cards and their organization.
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This article is about the sun Microsystems filesystem. For summary other uses, see. Zfs is a combined file system and logical volume manager designed by, sun Microsystems and now when? registered as a trademark of, oracle corporation ;. Zfs is scalable, and includes extensive protection against data corruption, support for high storage capacities, efficient data compression, integration of the concepts of filesystem and volume management, snapshots and copy-on-write clones, continuous integrity checking and automatic repair, raid-z, native, nFSv4, acls, and can be very. The two main implementations, by Oracle and by the. Openzfs project, are extremely similar, making zfs widely available within. The zfs name stands for nothing - briefly assigned the backronym zettabyte file system it is no longer considered an initialism. 5, originally, zfs was proprietary, closed-source software developed internally by sun as part.