ZFS EVIL TUNING GUIDE PDF

Disabling checksum is, of course, a very bad idea. Having file system level checksums enabled can alleviate the need to have application level checksums enabled. In this case, using the ZFS checksum becomes a performance enabler. The checksums are computed asynchronously to most application processing and should normally not be an issue. If a system is close to CPU saturated, the checksum computations might become noticeable. In those cases, do a run with checksums off to verify if checksum calculation is a problem.

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Experimentation does work. However, if you set vm. The vm. The issue of kernel memory exhaustion is a complex one, involving the interaction between disk speeds, application loads and the special caching ZFS does. Faster drives will write the cached data faster but will also fill the caches up faster. Generally, larger and faster drives will need more memory for ZFS. To increase performance, you may increase kern.

On i, keep an eye on vfs. FreeBSD 7. Generic ARC discussion The value for vfs. The high performance solution is to add a SSD. Generally speaking this limits the useful choices to flash based devices. In very large pools the ability to have devices faster than the pool may be problematic.

In smaller pools it may be tempting to use a spinning disk as a dedicated L2ARC device. Generally this will result in lower pool performance and definitely capacity than if it was just placed in the pool. There may be scenarios in lower memory systems where a single 15K SAS disk can improve the performance of a small pool of 5. This is usually the case. If you increase the throttling but the pool disks cannot keep up, you burn CPU needlessly. If you are not this typical use case: say, you are caching streaming workloads, or have several dozens of disks, then you may want to consider tuning the rate.

It can be tuned by setting the following sysctls: vfs. The latter can be used to accelerate the loading of a freshly booted system. Note that the same caveats apply about these sysctls and pool imports as the previous one. A properly tuned L2ARC will increase read performance, but it comes at the price of decreased write performance. The pool essentially magnifies writes by writing them to the pool as well as the L2ARC device. If a heavily used L2ARC device fails the pool will continue to operate with reduced performance.

Be very careful when adding devices to a production pool. By default zpool add stripes vdevs to the pool. Many SSDs benefit from 4K alignment. Using gpart and gnop on L2ARC devices can help with accomplishing this. On Solaris write caches are disabled on drives if partitions are handed to ZFS. Application Issues ZFS is a copy-on-write filesystem. As such metadata from the top of the hierarchy is copied in order to maintain consistency in case of sudden failure, i.

This obviates the need for an fsck-like requirement of ZFS filesystems at boot. However the downside to this is that applications which perform updates in place to large files, e. Reducing the ARC to a minimum can improve performance of applications which maintain their own cache.

Default max is a little over , Playing it safe If numvnodes reaches maxvnode performance substantially decreases. This helps "level out" the throughput rate see "zpool iostat".

Using deduplication is slower than not running it. Deduplication [on 8. Disabling ZIL is not recommended where data consistency is required such as database servers but will not result in file system corruption. ZFS is designed to be used with "raw" drives - i.

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