See Important Quotations Explained The novel begins with Lemuel Gulliver recounting the story of his life, beginning with his family history. He is born to a family in Nottinghamshire, the third of five sons. There, under a man named James Bates, he learns mathematics and navigation with the hope of traveling. When his apprenticeship ends, he studies physics at Leyden.
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August Learn how and when to remove this template message ZFS compared to other file systems[ edit ] The management of stored data generally involves two aspects: The management of the individual devices and their presentation as a single device is distinct from the management of the how to write a summary english lesson 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 systemor bit rot events and data corruption which may happen over time, and its complete control of the storage system is used to ensure that every step, whether related to file management or disk management, is verified, confirmed, corrected if needed, and optimized, in a way that storage controller cards and separate volume and file managers cannot achieve.
ZFS also includes a mechanism for snapshots and replicationincluding 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".
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.
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 metadataensuring that 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 blockrather 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. Data can be reconstructed using all of the following: The RAIDZ levels stripe data across only the disks required, for efficiency many RAID systems stripe indiscriminately across all devicesand checksumming allows rebuilding of inconsistent or corrupted data to be minimised to those blocks with defects; Native handling of tiered storage and caching devices, which is usually a volume related task.
Relevant tools are provided at a low level and require external scripts and software for utilization. Native data compression and deduplicationalthough the latter is largely handled in RAM and is memory hungry.
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. 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. 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.
Inadequate RAM — ZFS may use a large amount of memory in many scenarios; Inadequate disk free space — ZFS uses copy on write for data storage; its performance may suffer if the disk pool gets too close to full.
The SLOG device is only used for writing apart from when recovering from a system error. It can often be small for example, in FreeNASthe 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 lifetime of the device.
SLOG is therefore unusual in that its main criteria are pure write functionality, low latency, and loss protection — usually little else matters. 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.
Separate cards can also mislead ZFS about the state of data, for example after a crashor by mis-signalling exactly when data has safely been written, and in some cases this can lead to issues and data loss.
Use of poor quality components — Calomel identify poor quality RAID and network cards as common culprits for low 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.
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 the connection has failed, if there is a delay due to "writing out" a large cache.
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 deduplicationa 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.
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Reputable ZFS commentators such as Oracle  and ixSystems as well as ZFS onlookers and bloggers,   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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