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ZFS currently uses ARC buffers which are backed by virtual memory. While functional, there are some major problems with this approach which can be observed on all OpenZFS platforms. ABD was designed to address these issues and includes contributions from OpenZFS developers from multiple platforms. While all OpenZFS platforms will benefit from ABD this functionality is critical for Linux. Unlike the other OpenZFS platforms the Linux kernel discourages extensive use of virtual memory. The provided interfaces are not optimized for frequent allocations from the virtual address space. To maintain good performance a kmem cache is used which contains relatively long lived slabs backed by virtual memory. The downside to the approach is that those slabs can become highly fragmented resulting in an inefficient use of memory. Another issue is that on 32-bit systems the available virtual address space in the kernel is only a small fraction of total system memory. This means the ARC size is highly constrained which hurts performance and make allocating memory difficult and OOMs more likely. ABD is designed to address these issues by using scatter lists of pages for data buffers. This removes the need for slabs which resolves the fragmentation issue. It also allows high memory pages to be allocated which alleviates the virtual address space pressure on 32-bit systems. For metadata buffers, which are small, linear ABDs are allocated from the slab. This is preferable because there are many places in the code which expect to be able to read from a given offset in the buffer. Using linear ABDs means none of that code needs to be modified. The majority of these buffers are allocated with kmalloc so there's minimal impact of the virtual address space. Tested-by: Kash Pande <kash@tripleback.net> Tested-by: kernelOfTruth <kerneloftruth@gmail.com> Tested-by: RageLtMan <rageltman@sempervictus> Tested-by: DHE <git@dehacked.net> Reviewed-by: Chunwei Chen <david.chen@osnexus.com> Reviewed-by: Dan Kimmel <dan.kimmel@delphix.com> Reviewed-by: David Quigley <david.quigley@intel.com> Reviewed-by: Gvozden Neskovic <neskovic@gmail.com> Reviewed-by: Tom Caputi <tcaputi@datto.com> Reviewed-by: Isaac Huang <he.huang@intel.com> Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> Closes #3441 Closes #5135 |
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| .. | ||
| arc_summary | ||
| arcstat | ||
| dbufstat | ||
| fsck_zfs | ||
| mount_zfs | ||
| raidz_test | ||
| vdev_id | ||
| zdb | ||
| zed | ||
| zfs | ||
| zhack | ||
| zinject | ||
| zpios | ||
| zpool | ||
| zstreamdump | ||
| ztest | ||
| zvol_id | ||
| Makefile.am | ||