23 lines
1.1 KiB
ReStructuredText
23 lines
1.1 KiB
ReStructuredText
.. SPDX-License-Identifier: GPL-2.0
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Bigalloc
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--------
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At the moment, the default size of a block is 4KiB, which is a commonly
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supported page size on most MMU-capable hardware. This is fortunate, as
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ext4 code is not prepared to handle the case where the block size
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exceeds the page size. However, for a filesystem of mostly huge files,
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it is desirable to be able to allocate disk blocks in units of multiple
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blocks to reduce both fragmentation and metadata overhead. The
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`bigalloc <Bigalloc>`__ feature provides exactly this ability. The
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administrator can set a block cluster size at mkfs time (which is stored
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in the s\_log\_cluster\_size field in the superblock); from then on, the
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block bitmaps track clusters, not individual blocks. This means that
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block groups can be several gigabytes in size (instead of just 128MiB);
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however, the minimum allocation unit becomes a cluster, not a block,
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even for directories. TaoBao had a patchset to extend the “use units of
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clusters instead of blocks” to the extent tree, though it is not clear
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where those patches went-- they eventually morphed into “extent tree v2”
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but that code has not landed as of May 2015.
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